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HEALTH AND SAFETY LABORATORYAn agency of the Health and Safety Executive

Broad Lane, Sheffield, S3 7HQTelephone: 0114 289 2000Facsimile: 0114 289 2500

Project Leader: G. P. White

Engineering Control Group

Safety of Timber Stacks - Stability of Sawn Timber

G. P. White, BEng (Hons) AMIMechE andP. K. Swift, I.Eng, FIMM

ME/99/25

Mr E. Marshall FOD, Midlands RegionMr A. Hodkinson FOD, Midlands RegionDr A Jones HSL, Director of operationsMr P HeyesMr B RoebuckMr C WilsonDr W GearyMr I KerrLIS (10 copies)


Available to the public.

HSL Report Approval Mr P HeyesDate of issue 14 February 2000Job number JS2000499Registry file SME/919/068/1998AmiPro File K:\support\sup1998\tmbrstak\pswift\rp.sam

Distribution

© Crown copyright 2000

Summary

Objectives

The Health and Safety Executive provide guidance for the timber processing industry for the safestorage and transport of timber. The guidance is being updated to take account of current practiceand to ensure that it is founded on sound basic engineering principles. The Health and SafetyLaboratory was asked to assist with this process by carrying out surveys, tests, and research into anumber of different areas. Earlier work, White, 1998, investigated available band materials, bandingmaterial properties and the forces involved in banding of typical packs of sawn and planed-squareedge timber (P.S.E.). Swift and Roebuck, 1998, reported findings from a survey of a number oftimber yards involved in harvesting, storing, transporting and sawing of both softwood andhardwood logs. The latter also investigated factors which affect the stability of stored stacks of logswhilst considering other associated risks.

The objectives for this part of the project were to :-

w carry out visits to 10 organisations where stacked sawn timber was being stored;

w identify good and bad practice for storing stacked sawn timber;

w identify factors which affect the stability of sawn timber stacks;

w analyse the affects of each stability factor on the stability of sawn timber stacks;

w examine systems of work for good and bad practice in stacking and de-stacking ofsawn timber stacks;

w produce recommendations for limiting the height of stacks of various types of sawntimber, where certain risks may not be entirely eliminated;

w recommend safe systems of work for stacking and de-stacking sawn timber stacks.

Main Findings

A total of 14 premises were visited where stacked sawn timber products were being stored.

Measurements were made of pack and stack geometries to assess, for example, maximum likelypack offsets and the degree of lozenging. These were used as a basis for stability calculations andfor stack modelling.

For packs which are banded to a high quality, on hard standing or stable ground conditions whererisks from impacts and other external forces are low, the stability calculations showed that it isreasonable to expect stacks with a height to base ratio of up to 4 : 1 to remain stable.

The following factors were identified as potentially affecting stability of timber stacks and need to betaken into consideration when assessing the risk of stacks collapsing:

w ground conditions, including slopes, unevenness, load carrying capacity anddrainage. Obstacles and contamination may also be a problem;


w internal stability of individual packs, including quality of banding and quality andnumber of separating sticks. Internal stability may deteriorate every time a pack ismoved. Out of squareness, internal spaces and non-uniform timbers in the samepack are factors which may lead to internal collapse of a pack. Banding ispotentially important to pack stability. The wrong type of banding, incorrect bandtension, band application out of square or damaged bands will increase the risk ofpacks being internally unstable;

w layout of the storage area, including stack and aisle spacing, truck turning space andallowable stack heights;

w length, cross-section, uniformity and positioning of bearers or chocks andprocedures for discarding and replacing poor or damaged bearers;

w systems of work usually involved the use of front or side loading lift trucks.Accuracy of positioning of packs within a stack and stack heights need to be knownand stepping down of stacks in front of taller stacks to reduce wind loading wascommon practice;

w stock yard or warehouse management can have a big effect on risk, includingrestriction of pedestrian access, training of truck drivers, housekeeping, goodlighting, stack condition monitoring, etc.

Main Recommendations

Pack quality

Good packs will make good stacks. Packing the same type of timber together and removing internalair space will improve pack stability. Use of suitable sticks to bind layers of timber together will aidstability. Broken or loose banding, lozenging, balling and internal collapse of packs are all signs ofpoor practice in producing packs. Out of shape or collapsing packs should be quickly identified,removed from stacks, using a safe system of work, and placed to one side for rectification.Collapsed and partially collapsed packs should not be transported or placed in stacks.

Packs should not contain varying lengths of timber which could provide a means for climbing thestack or which could be a danger to persons or vehicles in the vicinity of the stack.

Stack heights/quality

Based on the findings of this project a maximum height to base ratio of 4 : 1 is recommended forstacks on hard standing or with stable ground conditions where risks from impacts and otherexternal forces are low. For external stacks which are subject to wind loading this ratio should bereduced to at least 3 : 1.

Stacks may well remain stable even with noticeably out of shape packs. However, to remain stable,the packs have to stay intact and not be subjected to any extraneous forces such as wind loads orforces produced by unstable ground conditions. When these can not be eliminated then the height tobase ratio of the stack should be reduced further e.g. 3 : 1 indoors, 2 : 1 outdoors. The surroundingsmay also limit the height of stacks e.g. sprinkler systems.


Packs should be square or rectangular in cross section where possible. Centres of gravity of eachpack should be directly over the centre of the bottom pack. Larger heavier packs should be stackedat the base of the stack. Packs should not be placed in such a manner as to bridge across 2 stacksor packs at the same level.

Banding quality

Banding should be in good condition and placed as close as possible to columns of sticks within thepack. If bands or clips, buckles etc. are damaged, then they should be replaced. If this necessitatesthe removal of a pack/s from a stack, this should be done using a safe system of work. Regularchecks on banding should reveal recurring problems which in turn should result in a review ofbanding methods. Eye protection should be used when any banding material is cut.

Ground conditions

Storage areas should ideally be virtually flat. Any slope should not exceed 2°. A slight slope alongthe length of the stack will allow water to drain off. Concrete, asphalt and hard standing are the bestground for standing stacks. The ground surface needs to be strong enough to avoid cracking/breakup of surface under load or with wear.

Bearers and separating sticks

Bearers and separating sticks need to be square, uniform, in sound condition and in plentiful supply.Too often unsuitable bearers were seen by the authors. The cross section of bearers should be of asufficient size to allow access for the forks of a fork lift truck (FLT). The length of bearers and sticksshould be equal to the width of the packs in the stack. Sufficient numbers of bearers and sticksshould be positioned along the length of the pack to prevent sagging of the timber.

Yard management

The layout of the yard should be planned, monitored, maintained and provide safe access andegress for FLTs to each stack. The layout should take account of prevailing wind directions andmicro-climate issues.

Good yard management will reduce the risk of pedestrians being in the stacking area and controlvehicle movements to reduce the risk of impacts with timber stacks

Promote good housekeeping, ensure satisfactory lighting and ensure proper training of FLT driversand other personnel.

When travelling with a load, never travel with the mast fully elevated, keep the load as low aspossible to the ground. Side loading FLTs are recommended as they tend to give better visibility andgreater clearance whilst manoeuvring. Audible warnings e.g. horns, reversing alarms etc. will help toreduce risks to personnel.

If possible plan operations such that packs of banded timber are transported a minimum number oftimes. Storage areas should be kept clean, tidy and free of debris such as bearers, sticks etc. Debrisshould also be removed from the top of packs and stacks.

Monitoring should identify stacks where stability is suspect and there should be a low risk procedurefor de-stacking and making safe such stacks.


Workers should be encouraged to look for and report any operational problems or defects such aspotholes, bad drainage, poor stacks etc. Workers should also be encouraged to discuss ways inwhich risks can be reduced.

Strategic positioning of mirrors around the site and/or on vehicles will improve visibility by reducingor eliminating blind spots. High visibility clothing should be worn. Extra care should be taken at sheddoorways, gangways and roadsides etc. Relevant signs should be used to inform persons of rules,risks and operational procedures.

The storage area should be adequately fenced to keep out children and the general public. Personsnot normally involved in transport or the running of the yard should be directed by clearly markedsigns to report to a reception which should preferably be located at the entrance to the yard.

Climbing stacks of sawn timber or gaining access by way of the forks of an FLT should beprohibited. If access to tops of stacks is essential a safe means should be provided, for example asecured or footed ladder. In these cases checks should be made to ensure the stack is stable. Inaddition, the area surrounding the access route to the stack should be coned off whilst work iscarried out.

Sections of timber should only be taken from packs which have been removed from a stack andplaced at ground level. They should never be removed from stacks at height.

The forks of an FLT should not protrude beyond the pack being handled to avoid striking anystacks to the rear. Adequate spacing of stacks will help avoid this.

De-stacking should be carried out from the top down, tier by tier. Care must be taken to ensure thestability of the main body of the stack and to check for signs of movement.

To avoid leaving isolated stacks, a practice of stepping down stacks in front of tall stacks wasobserved. This was carried out under a system of work. This practice shields the taller centralstacks and reduces the wind loading. This practice may be useful in exposed sites, but could result inproblems when access to the centre stacks is required, because at some time during de-stacking,the central stack/s may be exposed to the prevailing wind.

Stacks should not lean on adjacent stacks. This was observed frequently during this project. Duringde-stacking, forces will be exerted on the adjacent stack/s and may cause a collapse.

Anyone using this information should consider the importance of each point raised and assess itsimportance with respect to their own individual situation. This will enable them to produce a safecode of practice for employees to follow within their timber yard and set up safe systems of work tominimise the risk of a collapse.


Contents

22 7 CONCLUSIONS

21 6 SYSTEMS OF WORK

18 5.8 F8 - Yard Management

17 5.7 F7 - External forces

14 5.6 F6 - Stacking/Stacks

14 5.5 F5 - Separating/drying sticks

13 5.4 F4 - Banding

11 5.3 F3 - Packs

10 5.2 F2 - Bearers (or chocks)

9 5.1 F1 - Ground

9 5 FINDINGS

7 4.1 Results of stability modelling

6 4 STABILITY MODELLING

6 3.4 Tip line movement

5 3.3 Lozenging of packs

5 3.2 Angle of packs

5 3.1 Maximum Pack Height to Base Ratio

3 3 SUMMARY OF SITE OBSERVATIONS AND MEASUREMENTS

3 2.3 Board Materials

3 2.2 Storage practices in the hardwood Industry

2 2.1 Storage practices in the softwood Industry

2 2 THE SAWN TIMBER INDUSTRY

2 1.4 Literature search

1 1.3 Project specific visits

1 1.2 Objectives

1 1.1 Preliminary visits

1 1 INTRODUCTION

Page No.


1 INTRODUCTION

The Health and Safety Executive provides guidance for the timber processing industry for the safestorage and transport of timber. The guidance is being updated to take account of current practiceand to ensure that it is founded on sound basic engineering principles. The Health and SafetyLaboratory was asked to assist with this process by carrying out surveys, tests, and research into anumber of different areas. Earlier work, White, 1998, investigated available band materials, bandingmaterial properties and the forces involved in banding of typical packs of sawn and planed-squareedge timber (P.S.E.).

1.1 Preliminary visits

Swift and Roebuck, 1998, reported findings from a survey of a number of timber yards involved inharvesting, storing, transporting and sawing of both softwood and hardwood logs. The latter alsoinvestigated factors which affect the stability of stored stacks of logs whilst considering otherassociated risks. Advantage was taken during site visits for the log stacking project to carry outpreliminary examinations, discussions with workers and to photograph various stacks of sawntimber. This gave an initial impression of some of the good/bad practice and stability factors whichare present within the industry.

1.2 Objectives

The objectives for this part of the project were to :-

w carry out visits to approximately 10 organisations where stacked sawn timber wasbeing stored;

w identify good and bad practice for storing stacked sawn timber;

w identify factors which affect the stability of sawn timber stacks;

w analyse the affect of each stability factor on the stability of sawn timber stacks;

w examine systems of work for good and bad practice in stacking and de-stacking ofsawn timber stacks;

w produce recommendations for limiting the height of stacks of various types of sawntimber, where certain risks may not be entirely eliminated;

w recommend safe systems of work for stacking and de-stacking sawn timber stacks.

Many of the factors which affect the stability of a stack of sawn timber were listed in the earlystages of the project from personal input and from discussions with colleagues. These tended to bepoints which were obviously important, (e.g. having a solid/firm, level and even ground surface).

1.3 Project specific visits

More detailed observations, discussions and measurements were obtained in later visits which werecarried out specifically for this project. The measurements which were taken were used in stabilitycalculations, the results of which were intended to be used for recommending height to base ratiosfor different types of sawn timber stacks.


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1.4 Literature search

Some relevant previously published material was already available, but in order to determine if morewas readily available, a literature search was requested. This was carried out by staff from the HSLInformation Centre. Many of the references requested from the search were unavailable or obsolete.A search on the Internet confirmed that the most readily available information is contained in presentHSE guidance, HSG 172, 1997; HSW 47, 1975; and information sheets such as WIS (2), 1998,which appear to be produced from extracts of the guidance.

Useful information was obtained from three safe codes of practice developed by individualcompanies, one was obtained from a premises during a previous incident investigation and two fromsite visits for the present project. These gave a general overview of good/bad practice and stabilitymatters considered as important by organisations within the industry. An incident report from a SiteSafety Officer regarding a fatality due to a stack collapse also provided useful information.

The sites visited, both preliminary and specifically for this project were selected to give a generaloverview of the industry and are listed in Appendix 1. For reasons of confidentiality, they have notbeen have not been specifically identified within the text of the report. For the same reason, thesources for the codes of practice and incident report are also not named.

2 THE SAWN TIMBER INDUSTRY

To give a brief overview, the following is a summary of initial observations of the sawn timberindustry and gives an impression of when, where and how different sawn timber products are storedand transported.

2.1 Storage practices in the softwood Industry

Sawn softwood is generally used for fencing, sheds, pallets and similar applications. The sawntimber is not usually kiln dried and can be described as green timber. The period of time betweenharvesting to final sawn product may only be a few days. The resulting sawn timber is likely to havea high moisture content when it is banded into packs for transportation or storage. Individual packsusually contain sawn timber lengths which are identical or similar in cross section, as shown in Figure1. Softwood packs are usually transported from the sawmill soon after production. Hence, eachpack is only likely to be moved twice, within the grounds of the sawmill, to and from a short termstorage area. Some of the softwood packs will be stored and dried prior to treatment withpreservatives. Hence, these packs will see an increased number of movements within the grounds ofthe sawmill.

Production of timber for other applications such as the construction industry may require additionalprocessing and checking. For example, this type of timber may require kiln drying and strengthtesting. It is likely to be banded and transported a number of times within the grounds of the sawmill.During these stages banding is used to help maintain the integrity of each pack, to prevent damageand to reduce distortion.

Home produced softwood timber packs are usually banded with a synthetic type of bandingmaterial such as woven or stranded polyester, or fibrous polypropylene. Steel is also used, but it isless common. The use of ratchet straps as a means of banding has also been observed, Figure 2.


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Higher grades of softwood construction timber are often imported in a variety of sections asrequired by the industry, usually in steel banded packs.

2.2 Storage practices in the hardwood Industry

The volume of timber which passes through hardwood mills is generally much lower than insoftwood mills. The problems of stacking are very similar to those in the softwood industry. Themajor difference is the density of the wood and hence, the mass per pack of timber. This willinfluence the type of banding used, if any is considered necessary. Most, if not all hard wood is kilndried to reduce moisture, although the initial reduction will be done by natural air drying in opensheds or out in the open. The drying process means that the timber can be transported a number oftimes, possibly unbanded, within the grounds of the sawmill.

A significant amount of sawn and banded hardwood timber packs are imported into the U.K. fromother countries, particularly from South America. This timber is usually banded with steel banding.Sawmill employees have described how packs are often found to have collapsed entirely, within thetransportation containers, at some time during the transportation process. In these cases the timberhas to be sorted and the packs re-assembled and re-banded.

2.3 Board Materials

Large board materials also tend to be banded in packs for transportation and storage. Because ofthe high value of these processed materials they are usually stored in enclosed warehouses andprotected from the elements. Smaller board materials such as door blanks and worktops are oftenstored horizontally, on shelf racking, or vertically, in toast racking.

3 SUMMARY OF SITE OBSERVATIONS AND MEASUREMENTS

During the visits to sites where sawn stacked timber was being stored, a wide variety of types andsections of stacked sawn timber were observed and recorded. Many measurements and notes weretaken. Combinations of factors can cause geometric changes in the shape of packs such as lozengingand balling of packs. In reality it is not possible to isolate each factor and quantify its individual effecton the stability of a stack.

In initial visits where measurements were taken, the geometry of many packs of sawn timberproducts were recorded. In later visits similar pack measurements were recorded. Hence, to avoidamassing large amounts of repeated data, only extreme or new problems were measured andrecorded.

Table 1 is a summary of the geometric factors which were measured and recorded.


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Table 1 - A summary of the geometric properties measured on site.

* Problem attributable to or exaggerated by poor pack or stack assembly - therefore not highlighted as max.

** Free standing stacks of individual pieces of sawn timber (not a stack of packs)

Maximum values

------60---0.50.22Boardpacks

120---1001002.40.95

------30---0.21.05

------2001050.2up to 1.2

640--------- 6.8*---Softwoodpacks

200Slight100--- 1.00.95

60---200100 2.01.25

------------ 5.0*1.08

------------20.81

------120---0.50.81

------------20.82

------5095---2**Hardwoodpacks

200 (Combination)120---------

---70------0.5---

------------0.23---

---200------0.60.41

---280------0.50.91

100---------------

---17020---0.3up to 0.53

---22030---0.3up to 0.93

Offsetbearers

BallingOffsetpacks

Tip line movement (mm)Lozenging

(Degrees)

Angle of packs

(Degrees)

Max. pack heightto base ratio

Type

Factor & MagnitudeTimber

The geometric factors listed will affect the stability of a stack. The following sections describe theseand the assumptions which were used in the stability calculations.

3.1 Maximum Pack Height to Base Ratio

In general, the measured packs of softwood and hardwood timber were assembled to be square orrectangular in cross section. The data shown in Table 1 shows that some packs were taller than they


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were wide. This was taken into account for the stability calculations. The maximum pack height tobase ratios which were used in the calculations were :-

Pack Type Height to base ratio

w Hardwood packs 1.08

w Softwood packs 1.25

w Board Material 0.22

3.2 Angle of packs

The site visits revealed that the slope of the majority of sawn timber stacks was 2o or less. Thismagnitude of angle is visibly noticeable, especially on stacks with height to base ratios of 3 or more,and would be likely to be picked up by stack monitoring procedures . As described previously,angling of packs, up the height of the stack, due to compression on one side of the stack was notgenerally observed. Where this was observed it was attributable to poor pack assembly of the typeillustrated in Figure 3. Although the maximum slope of any stacks of board materials which wereobserved was only 0.5o, due to the storage areas being inside a concrete floored warehouse, aconstant angle of 2o was used in the stability calculations, as this is known to be common fordrainage purposes.

3.3 Lozenging of packs

Lozenging of packs has two effects on stability, as illustrated in Figures 4 and 5. In a lozenge shapedpack the centre of gravity moves and an "effective" offset is introduced for the upper packs. Thistype of deformation appears to occur mainly in packs of regular shaped timber. The maximumamount of lozenging which was observed, for each type of timber, is shown below :-

Pack Type Lozenging (o)

w Hardwood packs 95

w Softwood packs 105

w Board Material0

The lozenging of 95o for hardwood packs is small and was not generally seen, with balling seemingto be the more common way for hardwood packs and packs of irregular sectional timber to go outof shape. For this reason lozenging of these types of packs was not taken into account in thecalculations.

Packs of board materials were only observed lozenged along their lengths, which, due to the generaldimensions of the packs, will not have a great influence on stack stability.

The maximum lozenge angle of 105o, for softwood packs, was taken into account in the stabilitycalculations.

3.4 Tip line movement

The accuracy with which placing of bearers or packs occurs was found to be typically 200 mm or less. It was often found that one bearer was offset more than the other/s. To cater for


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offsets in the calculations it was assumed that all packs within a stack were offset, relative to thepack directly below, by 200 mm. Although the problem of offset bearers was not seen with boardmaterials it could occur and hence, the same 200 mm offset was used in the calculations. An offsetof 200 mm will obviously be more critical on narrow packs as it will represent a bigger proportionof the overall width. For this reason a width of 1 m was assumed for each type of timber pack in thestability calculations, as this was typically the minimum for packs of sawn timber.

Balling of packs appeared to occur regularly with irregular section timber and hardwood bandedpacks. Balling of packs is when the corners of the pack are pulled inwards by the banding and themid-sides tend to bulge outwards, so that rather than having square sides, the section of the pack isball shaped. The maximum balling which was measured effectively reduced the width of the pack by2x280 mm (280 mm from each side of the pack). This value was used in the calculations forhardwood sawn timber packs. Again, it must be noted that this amount of balling will have a greatereffect on narrower packs as the 2x280 mm will be a greater proportion of the overall width of thepack.

Figure 6 is a summary of all the geometric quantities listed above, which were taken into account inthe stability calculations and illustrates how geometric factors such as lozenging and balling weremeasured.

4 STABILITY MODELLING

After seeing and measuring the above stacking factors on site, modelling work to develop stabilityequations was carried out at HSL in Sheffield. Before developing stability equations it was importantto understand how stacks of packs of banded timber are likely to act during a collapse. It was notedthat in small scale tests using blocks of wood to represent a stack of packs of banded sawn timberon an incline, that the base pack of a stack tended to remain in contact with the surface on which thestack was assembled and that the upper packs tend to pivot, as one body, about a tipping point atthe top corner of the base pack, as shown in Figures 7 and 8. Figure 8 shows a series of framestaken from a computer simulation of a collapse of a stack of packs of sawn timber on a slope witheach pack in the stack offset. Observations from previous incident reports and photographs confirmthat this appears to be the common mode of collapse.

Equations were developed to investigate the stability of simple stacks of packs of sawn timber. Anumber of assumptions were made in order for the equations to be developed. It was assumed that:-

w the effect from bearers on the height of a stack will generally not be significant andwould complicate any stability calculations, so they were not taken into account inthe calculations;

w the packs remain securely banded and acted as solid blocks;

w the Centre of Gravity (C of G) of each pack was assumed to be at its geometriccentre;

w the slope of the stack, offsets due to poor stacking and out of shape packs wouldbe catered for.


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Taking the above into account, the geometry shown in Appendix 2 was used to develop formulaefor calculating the stability of a stack. The formulae are also listed in Appendix 2.

When the calculations had been used on each pack in the stack, the moments about the tipping pointwere summed. If the total moment was positive the stack was assumed to be stable, if the total wasnegative the stack was assumed to be unstable.

The above formulae were inputted into a computerised mathematical solver called TKSolver.Known values of pack width (a), pack height (b), pack offset (x), lozenge angle ( ) and stack slope ( ) were input into the TKSolver model. The calculated results were all identicalb hto those from a full scale accurately drawn diagram of the known dimensions produced onAutoCAD R14. This gave confidence that the formulae had been developed correctly.

The formulae were initially developed such that the angle of each upper pack to the horizontal couldbe varied. This was intended to allow for likely compression of base packs. This provision wasfound to be unnecessary at a later stage in the project as no significant compression of this type wasobserved. When angles were observed they were attributable to poor pack or stack assembly.

4.1 Results of stability modelling

The graphs shown in Figures 9 to 11 were created by using the data given above in the stabilitycalculations (Appendix 2), with an assumed pack weight of 1 unit. For each graph, each linerepresents the static stability of a stack made from packs with sets of assumed geometric quantities.For example, the solid red line on Figure 9 represents the stability of stacks of 1m2 softwood packson a 2o slope, which are all lozenge shaped to 105o and are each offset within the stack by 200 mm.The portion of the line which lies in the green upper section of the graph represents the numbers ofpacks which could be stacked and be statically stable, i.e. they would not collapse unless additionalforces were applied. When the lines cross into the red lower portion of the graph the stacksrepresented would be statically unstable, i.e. they would collapse without any additional forcesapplied.

The magnitude of each of the geometric factor used in the stability calculations is significant. Ifslopes, offsets, or out of shape packs of this magnitude occurred in any pack within a stack then theinspection system should pick up the problem and provide a safe method for correcting it. It can beconcluded that other less visible problems, such as short/offset bearers, bad stacking, impacts andwind loading are the more likely reasons for stack collapses.

Table 2 is a summary of the maximum number of packs which could be stacked, using the datarepresented in Figures 9 to 11.

The calculations showed that if a slope of 2 degrees were considered solely, high stable stacks werepredicted without risk of collapse. When the slope was combined with another geometric factor thestable height to base ratio generally reduced to around 4. Considering 2 geometric factors combinedwith the slope reduced this further to around 2.


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Table 2 - summary of the data represented in Figures 9 to 11

0.9 : 14Slope = 2o, Offset = 200 mm

28.8 : 1131Slope = 2o1 x 0.22

4 : 14Slope = 2o, Offset = 200 mm

29 : 129Slope = 2o1 x 1

BoardMaterials

2.2 : 12Slope = 2o, Offset = 200 mm,Balling = 280 mm

13 : 112Slope = 2o, Balling = 280 mm

4.3 : 14Slope = 2o, Offset = 200 mm

29.2 : 127Slope = 2o

1.08 x 1

2 : 12Slope = 2o, Offset = 200 mm,Balling = 280 mm

13 : 113Slope = 2o, Balling = 280 mm


29 : 129Slope = 2o

1 x 1

Hardwood

2.5 : 12Slope = 2o, Offset = 200 mm,Lozenge = 105o

3.8 : 13Slope = 2o, Lozenge = 105o


28.8 : 123Slope = 2o

1.25 x 1

2 : 12Slope = 2o, Offset = 200 mmLozenge = 105o

4 : 14Slope = 2o

Lozenge = 105o


29 : 129Slope = 2o

1 x 1

Softwood

Stack HeightStack Width

Ratio

Max. No. ofpacks before

unstable.

Geometric ParametersPackHeight x Width

(m)

SawnTimberPacks

The data given in Table 2 errs on the very cautious side. It is very unlikely that the geometricquantities considered would all occur together for each pack in a stack of sawn timber. Asmentioned previously, if one pack in a stack had one or more of these problems the safe system ofwork for the yard should identify and safely rectify it. Nethertheless the data does show thatintroducing an offset or a lozenge to each pack in the stack does have a dramatic effect on themaximum stable height to base ratio for that stack. Balling of packs, on its own, has less of an effect,but, in this case, other factors such as the lost integrity of the packs could contribute more to acollapse.


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5 FINDINGS

The following is a list of identified stability factors. Listed under each main heading are a number ofareas which need to be considered by organisations that store stacks of sawn timber products.

5.1 F1 - Ground

The ground on which stacks are assembled is a very important factor for stability. The followingpoints need to be considered when selecting and preparing any ground area for stacks of sawntimber :-

(a) Slope. The ground on which timber stacks are stored should be level. The overallslope of the ground is an important factor. It is generally accepted that the maximumpermissible slope of a stack should not exceed 2o .

(b) Unevenness. Well maintained asphalt, tarmac and concrete surfaces will generallybe fairly flat. Unevenness of the ground is more likely on uncovered or loosesurfaces. Uneven ground should be rectified by covering with a suitable hardstanding surface. Uneven ground may result in movement of the tip-line with thestack being less stable than it appears.

(c) Material. Stacks should be assembled on well prepared, stable ground. Thecondition and strength of unprepared ground materials will vary with environmentalconditions and may result in the collapse of a stack.

Stacks were often observed on well prepared concrete, asphalt and hard standingground. Occasionally stacks were also observed on poorly prepared ground (Figure12).

(d) Strength. Account must be taken of the load carrying capacity of the ground. Theground may, at any one time, be subjected to the weight of the stack, machine andpayload.

Figure 13 shows a concrete platform which has cracked under the weight of thehardwood stacks standing on it. The result of this failure is an uneven surface withthe stacks sloped along their lengths at an angle of 4.5o . In the same yard an asphaltsurface was observed which had sunk, under the weight of the stacks, introducingan angle of 4.5o across the width of the stack(Figure 14). This type of problem is a major issue for larger/heavier hardwoodstacks.

(e) Obstacles. Stacks should not be assembled on top of obstacles such as loosebearers or pallet material etc. This may move the tip-line or cause the stack to leanand become unstable. Good yard management should ensure that the stacking areais clear of obstacles.

(f) Contamination. Any contamination such as oil or grease etc., in the area where astack is to be assembled, should be removed. Contamination of this type may


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reduce the friction between bearers and the ground and may contribute to thecollapse of a stack.

(g) Drainage. The stacking area should be adequately drained to avoid deterioration ofbearers, stacks or the ground itself.

5.2 F2 - Bearers (or chocks)

Bearers are used to raise the lowest pack off ground level and to support higher packs whilstcreating a gap between packs for access to stack/de-stack, usually with a fork lift truck (FLT). Thefactors which need to be taken into account when considering stability are listed below :-

(a) Shape. The bearers should be straight, uniform, identical in cross section andpreferably in length. This will assist in keeping each pack level. The cross sectionshould be of a sufficient size to allow access for the forks of a FLT.

They should preferably be of a square cross section. If bearers are rectangular incross section the long edge should be horizontal ( Ý8, Ü 3 ). This is the moststable position and if external forces act on the packs they will be less likely to rollthe bearers. This will also assist in minimising the height of the stack.

The length of the bearers should be equal to the width of the packs in the stack.Bearers should not protrude from the sides of a stack to a degree which mightencourage climbing of the stack or where they may easily be impacted by passingvehicles or be a danger to personnel. Short bearers will affect the position of thetip-line, as will offset bearers, see (d) below.

(b) Condition. The bearers should be of sound material and in good condition. Theyshould not be damaged, rotten, have nails/screws protruding and should preferablybe made from one piece of timber. A broken bearer may indicate a more seriousproblem. Bearers were often observed made up from a number of pieces of timber.An example of this is shown in Figure 15. These bearers are made from offcuts fromthe board material contained in the packs. This has the benefit that the length of thebearers is exactly the same as the width of the pack. A disadvantage is that theoffcuts making up each bearer were not bonded or secured together, resulting insome of them collapsing. If bearers are made up from a number of pieces of timber,they should be bonded or securely fastened together.

(c) Positioning. The bearers should be positioned such that the weight of anysupported pack is carried satisfactorily without significant sagging. Packs of timbershould not overhang the ends of bearers, as the banding will be subjected toadditional forces from the weight of the overhanging portion of the pack. Bearersmay be banded as part of a pack and this may contribute to the stability of thestack.

Short and offset bearers were observed in both softwood and hardwood stacks ofsawn timber as shown in Figures 16 and 17. This can have a noticeable effect on thestability of the stack as the effect is to move the tip-line inwards from the edge of the


10

stack to the point where the upper pack touches the end of the short or offsetbearer, as illustrated in Figure 18. The additional tension in the band material, due tothe overhanging portion of the pack can result in failure of the band or collapsing ofthe upper pack as shown in Figure 16.

Discussions with one yard employee revealed that there was a misunderstandingover the positioning of some bearers in packs of pallet timber. This resulted in theheight to base ratio of a stack being inadvertently raised. It was observed that somepallet timber packs had bearers which were banded as part of the pack. Whenobserved from the side the bearers were well inset from the edge of the pack(Figure 19). This effectively moved the tip-line inwards from the edge of the stack.Because the stack was 3 packs high the employee took this to be equivalent to therecommendations in HSG172. The shape of the packs and the misunderstandingover the bearer positions meant that the actual height to base ratio was visiblyaround 5 or 6 : 1.

(d) Binders. Bearers can pass from one stack to adjacent stacks and act as binders,tying adjacent stacks by friction. This will have a beneficial effect in terms of staticstability. Problems may arise when de-stacking and the stability of the stacks mustbe monitored closely during stacking/de-stacking and personnel must be well clear.

5.3 F3 - Packs

The individual packs of sawn timber are the building blocks of a stack of timber. Hence, the factorsassociated with them are important and are listed below :-

(a) Weight and dimensions. The weight and dimensions of the packs making a stackwill be an important factor. As stack height increases so does the pressure on thebase packs. Banding may come loose and base packs may lose their integrity.

Partially planked rectangular hardwood packs were observed as shown in Figure20. This type of pack can contain large spaces between adjacent planks. The widthof the pack is determined by the planks which butt up to one another, mainly at thetop or bottom of the pack.

(b) Gaps/air spaces. Ensuring that individual timbers are tightly packed and bandedtogether in each pack will assist in creating a stack which is as stable as possible.Spaces/gaps between timbers can be minimised before banding by using a jig toassemble the pack and by shaking down or impacting the pack.Over a period of time, during transport and under external loads the packs can losetheir shape and collapse if spaces/gaps are present between timbers.Large gaps were frequently observed within some hardwood packs, usually thosecontaining different sections of timber from planked logs, Figure 21. Many of thesepacks could be seen to be badly out of shape and collapsing.

Softwood and hardwood packs were observed where insufficient timbers had been


11

assembled on the top or bottom rows of packs. This can result in the tip-line beingmoved inwards from the edge of the pack or angling of the upper packs within thestack, as illustrated in Figure 3.

(c) Uniformity of timbers. The uniformity of individual timber lengths contained in apack will also contribute to pack stability. Where possible, packs should containtimber of the same cross-section and length. This will assist in obtaining arectangular tight pack. Packs should always be rectangular in cross section.

Packs should not contain varying lengths of timber which could provide foot holdsfor climbing of the stack or which could be a danger to persons or vehicles in thevicinity of the stack. If access to stacks is essential a safe means should be providedwith safety lines and harnesses.

(d) Out of squareness. Out of square or lozenge-shaped packs will affect the stabilityof the stack and will cause an increase in the tension in the band material. Figure 4 isa schematic showing the effect of lozenge shaped packs. A lozenge shaped packwill move the centre of gravity of that pack and introduce an "effective" offset of thecentre of gravity of upper packs. This type of deformation appears to occur mainlyin packs of regular shaped timber as shown in Figure 5. Separating/drying sticks canintroduce planes of lowered coefficient of friction. Packs can also ball or roll andhave the effect of moving the tip-line inwards from the edge of the pack to wherethe balled pack touches the bearer, as shown in Figure 22. In this case the width ofthe pack is effectively smaller than it appears. This type of deformation was oftenobserved in banded hardwood packs containing noticeable gaps between thetimbers, Figure 23.

Out of shape packs should be quickly identified, removed from stacks, using a safesystem of work, and placed to one side for rectification. Measurements of typicalmagnitudes of lozenging and balling of packs were taken during the site visits andused in the stability calculations shown earlier in this report.

Banded packs of non-rectangular or non-square timber sections can presentproblems with the integrity and shape of the pack. The pack of triangular sectiontimber shown in Figure 24 has what may be described as vertical bearers banded aspart of the pack with steel banding. The vertical bearers help to maintain the shapeof the pack. Without these bearers the pack would be likely to lose its shape asshown in Figure 25.

(e) Collapsed. Collapsed and partially collapsed packs should not be transported orplaced in stacks. If observed within a stack they should be removed, using a safesystem of work, re-assembled and re-banded. In many hardwood packs, largedistortion of the timber sections was observed to an extent where bands appear tohave broken or the pack had gone badly out of shape as illustrated previously.

5.4 F4 - Banding


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Banding of packs is beneficial in holding packs square, preventing timbers from warping andkeeping the integrity of the pack. The number of bands which are required to obtain a solid tightpack over the required storage period depends on many factors such as band material, packdimensions, the type of timber, the storage conditions etc. This information is only likely to be gainedthrough individual experience within particular situations. As discussed in White, 1998, in order toavoid banding becoming loose during transit due to shakedown, spaces between individual pieces ofbanded timber packs should be minimised. Those shown in Figure 1 are generally well assembled,but spaces can be seen between boards, Figure 26. These spaces are likely to be a direct result ofinadequate compacting of the pack when it was banded. The banding factors which should beconsidered are listed below :-

(a) Positioning. The banding should be placed as close as possible to the columns ofsticks. This will assist in holding the pack squarely and firmly together.

Poor positioning of the banding was observed, such as banding passing over theends of the columns of sticks as shown in Figure 27. Any movement of this packwould be likely to dislodge the band and result in loosening of the bands.

Non-squarely applied bands were also frequently observed. This type of poorapplication is likely to result in the banding coming loose as it squares up. a properlysquared band has a smaller length than one not applied squarely.

(b) Looseness. If the band material becomes loose due to shrinkage of the timbers oris damaged, the individual timbers in packs can become loose when disturbed, asseen by the lower packs to the left and right of the photograph in Figure 28. InFigure 29, only the centre band of the nearest pack of timber is tight and thebanding on the upper pack is broken. Other problems can also be seen in Figure29, including a collapsed pack of timber at the top left of the photograph. Damagedsteel banding has failed to maintain the shape of the pack shown in Figure 30. Thestability of stacks can be compromised when several packs are stored on top of aninsecure pack as shown to right of centre of Figure 31. The photograph in Figure 32shows another example of this problem for a stack of home produced timber.

Many employees said that packs are often received where some or all of thebanding has broken. These packs need to be re-assembled and re-banded using asafe system of work before they can be stacked.

Tight, well applied banding was frequently observed during the site visits.

Loose banding, or no banding at all, was also frequently observed. With heavierunbanded hardwood packs, the opinion within the industry was that if the timberwas going to move it would move and any banding would be unlikely to restrain themovement. If the banding is already broken, this can be taken as a sign that thetimber has already moved and that the stability of the stack may be suspect.


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(c) Damage. If bands or clips, buckles etc. are visibly damaged they are likely to havebeen weakened and may fail. Where damage of this type is observed, the damagedcomponent should be replaced as soon as possible. If this necessitates the removalof a pack/s from a stack, this should be done using a safe system of work. Withrepeated failures, the supplier may be able to give appropriate advice.

(d) Properties. It is good practice to take account of band properties and what is likelyto happen to the banded pack, White, 1999. As far as is reasonably practical thebanding operation should be carried out when the timber is dry and unlikely tochange shape due to moisture changes.

(e) Quality. Regular checks on banded packs will reveal any problems with thebanding. If persistent banding problems arise, the user should make appropriatechanges to try and correct the problem (e.g. by using additional bands or bychanging the type of band).

Eye protection should always be used when any banding material is cut.

5.5 F5 - Separating/drying sticks

The main purpose of separating/drying sticks is to allow air circulation between rows of timbers.This allows treated or freshly cut timber to dry evenly. Inclusion of sticks also improves the overallstability of the stack as it aids the production of a square tight pack and prevents packs from rollingor balling. Separating sticks can introduce a plane of lower friction which can result in lozenging. Thefactors which could affect the stability of a stack are listed below :-

(a) Shape. The sticks should be of a uniform cross section (preferably square) and thesame length. For the same reasons given for the bearer dimensions, the length of thesticks should equal the width of the stack. Sticks should not protrude from the sidesof packs to an extent where they are a danger to persons or where they could becaught by passing vehicles etc.

(b) Condition. The sticks should be of a good quality with no damage etc.

(c) Positioning. There should be enough columns of sticks within the pack to preventsagging of timbers. The sticks should be positioned above one another and withsufficient numbers to prevent sagging between sticks. This knowledge is only likelyto be obtained through experience.

5.6 F6 - Stacking/Stacks

Observations of a number of sawmills and storage facilities revealed that banded packs are generallytransported, when required, using either a front or side loading fork lift truck (FLT). In general, theforward visibility afforded by side loading FLTs meant that drivers preferred this method oftransportation.

The number of times that timber is transported is likely to have a bearing on the quality of thestacking. Where timber is likely to be stored for long periods of time, the packs and the stacking arelikely to be of a higher quality than when the timber is stored for short periods of time.


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The actual process of stacking/de-stacking will introduce risks to personnel and external forces tostacks. Safe systems of work should ensure that these risks are minimised, for example, personnelshould be excluded from areas where a collapse could occur as a result of the stacking/de-stackingprocess.

As the height of a stack is increased the height of its overall centre of gravity (C of G) also increases,hence, the higher the risk of collapse. In addition to the height of the stack the quality and care withwhich a stack is made is one of the most important risk factors :

(a) De-stacking. De-stacking should be carried out from the top down, tier by tier.Care must be taken to ensure the stability of the main body of the stack and tocheck for signs of movement. Individual pieces of timber must only be removedfrom a pack when it has been lowered to the ground.

Only one pack should be moved by FLT at any one time.

Doors, smaller panels, veneer etc. were observed stored on cantilever racking. Thiswill tend to be a safer method than stacking due to the relatively small width ofpacks of such items.

(b) Offsets. The centres of gravity of the stacked packs should be positioned directlyabove one another. Packs should not be offset within a stack such that theyprotrude. Protruding packs are more likely to be struck by FLTs or may encourageclimbing of the stack. Offset packs were observed frequently. An example is shownin Figure 33. The same figure also shows a collapsing pack, loose banding andmade up bearers.

(c) Logical stacking. When it is necessary to stack packs which are different in sizeand mass, the smaller/lighter packs should be placed on top of the larger/heavierpacks with their centre of gravities in line, Figure 34. Equal size packs should bestacked such that their edges are square along all 4 sides. Packs should not beplaced in such a manner as to bridge across 2 stacks or packs at the same level.This practice will impart loads to adjacent stacks, particularly duringstacking/de-stacking operations.

To avoid leaving isolated stacks, a practice of stepping down stacks in front of tallerstacks was observed. This was carried out under a system of work. This practiceshields the taller central stacks and reduces the wind loading. This practice may beuseful in exposed sites, but could result in problems when access to the centrestacks is required, because at some time during de-stacking, the central stack/s maybe exposed to the prevailing wind.

Stacks should not be assembled such that they lean on adjacent stacks. This wasobserved frequently. During de-stacking , forces will be exerted on the adjacentstack and may cause a collapse.


15

(d) Slope. It is good practice for stacks to have a slight slope along their length in orderto allow water to run off. This will reduce moisture absorption and the associatedproblems.

At one site where stacks were created manually piece by piece, it was observedthat if the stack began to lean or become unlevel, a good system of work meant thatthis was corrected by assembling slightly thicker timbers on the low side of the stackto re-level it.

(e) Height to base ratio. This project was intended to provide some scientific basis,from measurements and stability calculations, for recommending height to baseratios for different types of sawn timber stacks. Good practice generally limited thisratio to 4 : 1 indoors and 3 : 1 outdoors with 4 : 1 also being used for shieldedstacks outdoors.

It is good practice to consider the area surrounding the storage area. If persons andvehicles frequently pass certain areas there may be a higher risk of an impact orinjury in a collapse. In these circumstances the height should be reduced. In low riskareas it may be more appropriate, with regular monitoring, to allow higher stacks. Ina warehouse containing sprinklers it was observed that the height of the stacks werelimited such that the sprinklers would be effective in the event of a fire.

If base packs of particular types of timber frequently begin to collapse or becomemis-shaped without impacts or other external factors, this may indicate that the basepacks are not capable of carrying the load satisfactorily. Therefore, the stack heightshould be reduced for the particular type of wood or stacking method.

HSW47, 1975, gives a recommendation on the very cautious side in that individualstacks of sawn timber should not exceed a height of one-and-a-half times the facewidth.

Stacks were regularly observed where the height to base ratio was at least 5 : 1, Figure 35. In some circumstances this was being carried out with theknowledge of the yard workers. In other circumstances a misunderstanding of theactual width of the stack by employees meant that the ratio was higher than realised.A height to base ratio of 4 : 1 does not mean that packs can be stacked 4 high. Thedimensions of the pack, the positioning of the bearers etc. all have to be considered.

(f) Cross bracing. The base will effectively be increased if cross bearers (binders) areused to tie adjacent stacks together, but as mentioned in Section 5.2 (d), this couldlead to problems when de-stacking and is not recommended.

(g) External supports. If external supports are used, it may be safe to stack to agreater height. This was not, however, seen during this project.

5.7 F7 - External forces


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External forces are an important factor which must be taken into account when considering thestability of stacks of sawn timber. If any combination of the other factors means that a stack is closeto collapsing, an additional external force could initiate the collapse. The following may result inforces being applied to the stack :-

(a) Wind. Wind forces will act on external stacks, particularly in an open storage yard.It is good practice to plan the layout of the yard such that as small a cross section ofthe stack as possible is facing the prevailing wind direction. Micro-climate issuesmust also be considered i.e. the geometry of the land or the layout of buildings orstacks may cause channelling of the wind.

Partial wrapping of timber packs is becoming increasingly common for protectionfrom the elements during storage periods. This type of sheeting was often observedto be loose and billowing in the wind. This may result in additional overturningmoments particularly from loose sheeting on the uppermost packs. The higher thestack the bigger the overturning moment. The integrity of sheeting needs to bechecked as part of any inspection regime

(b) Impacts. Discussions with a number of workers revealed that impacts fromvehicles is a major problem and one of the main causes of stack collapse.

Stacks should be arranged such that enough clearance is allowed for vehicles tosafely manoeuvre and avoid impacts to stacks. If a stack does receive an impact,the safe system of work should ensure that it is immediately inspected. If it isconsidered unsafe, the system of work should ensure that the stack is made safe andthat personnel are prohibited from the area whilst this is being done.

During site visits evidence of impacts to stacks was noted, an example of this beingshown in Figure 36.

(c) Personnel. Interference from personnel in other ways, (e.g. pickers taking timbersfrom packs within the stack), is likely to result in forces being applied to the stack.

No one should climb up sides of stacks of sawn timber or gain access by way of theforks of an FLT. It is good practice to use signs to warn personnel of the dangers ofclimbing stacks.

Where access to stacks is essential it should be by way of a secured or footedladder. In these cases checks should be made to ensure the stack is stable. Inaddition, the area surrounding the access route to the stack should be coned offwhilst work is carried out.

At one site it was observed that the stacking process required employees to workstanding on top of stacks of timber up to 12 ft high without any safety harness orprotection from a fall. In addition to being an unsafe system of work, this will impartforces to the stack which could contribute towards a collapse. In the event of a


17

collapse, the risk of serious injury or a fatality would be very high. The dangerwould be increased in damp or icy conditions.

Evidence was often observed of timber sections having been "picked" from packswithin stacks of timber. This often meant that bands had been cut or had comeloose, leaving the stack in a dangerous condition. Picking of sections of timber frompacks within stacks should not occur, it will introduce forces which may cause astack to collapse. Sections of timber should only be taken from packs which havebeen removed from a stack and placed at ground level.

(d) Stacking and de-stacking. The act of stacking and de-stacking will introduceexternal forces to the stack.

FLT drivers should be careful that the forks of the truck do not protrude too far andstrike any stacks to the rear of the one being worked on. This could cause acollapse of a stack/s adjacent to the one being worked on and would be a danger tothe FLT driver and any personnel in the surrounding area, Figure 37. Rather thanplacing the emphasis on the more important issue of safety, one organisation'ssystem of work placed the emphasis on not damaging the timber.

An incident report by one site safety officer, on a fatality, alleged that a stack behindthe one which was being de-stacked collapsed. This suggests that the rear stackmay have received an impact or may have been leaning on the front stack andcollapsed when one of the packs from the front stack was removed. FLT operatorsor their supervisors must assess the condition of stacks of timber and ensure thatpersonnel are clear of all surrounding stacks whilst these operations are carried out.

(e) Environment. Moisture, sun, wind, ice and other environmental considerations canaffect the stability of stacks particularly over a long time period and should be takeninto account as far as reasonably practicable. Regular monitoring of stacks couldreveal where problems occur.

5.8 F8 - Yard Management

Many of the points raised in the previous sections can be considered as good or bad yardmanagement. Yard management is an extremely important factor for reducing risks.

(a) Transport Operations. Control and regulation of transport operations is essentialand will reduce the risk of accidents and collisions with people and stacks of sawntimber.

(b) Audible warnings. FLT drivers were observed using the vehicle horn as a warningto pedestrians and other vehicles when emerging from between rows of stacks oftimber or blind corners. Reversing should be avoided whenever possible, ifnecessary consider reversing alarms.

(c) Transport Rules. Any transport rules/procedures for the site, should be given tothe appropriate people (e.g. FLT/lorry drivers).


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Only trained, authorised drivers should use FLTs and other site vehicles.

When travelling with a load, never travel with the mast fully elevated, keep the loadas low as possible to the ground.

For transporting packs, workers expressed their preference for side loading FLTs.These were said to give better visibility and greater clearance whilst manoeuvring.

If possible, plan operations such that packs of banded timber are transported aminimum number of times. This will reduce the risk of collisions and of the bandingbeing damaged.

(d) Maintenance. FLT drivers should check machines and report any defects. FaultyFLTs should not be used under any circumstances.

(e) Housekeeping. Storage areas should be kept clean, tidy and free of debris such asbearers, sticks etc. Debris should also be removed from the top of packs andstacks.

Clean and tidy storage areas were observed, as were untidy yards containing debris(see Figure 32).

(f) Lighting. It is a legal requirement that the storage area and any access/egress pointsare well lit.

(g) Reporting. It is good practice to encourage yard workers to look for and reportany operational problems, any defects such as potholes, bad drainage, poor stacksetc. Workers should also be encouraged to discuss ways in which risks can bereduced.

At one organisation workers were issued with yearly diaries. These contained asummary of the rules for the yard and were available such that any problems orconcerns could be recorded and reported to the foreman/yard manager.

(h) Monitoring. A safe system of work should ensure regular monitoring of thecondition of stacks, particularly where cross winds, impacts or other disturbancesare likely to occur. If there are any doubts over stack stability or condition theseshould be rectified immediately.

(i) Yard Layout. Good yard layout should provide safe access and egress for FLTs toeach stack. These should be maintained and kept free of obstructions.The layout of the yard should be planned, monitored and maintained.

If possible the stacking areas and gangways should be clearly marked. This wasobserved at one site.

If possible a one way system could be used with appropriate speed limits.


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The storage area should be adequately fenced to keep out children and the generalpublic. Visitors to the yard should be directed by clearly marked signs to report to areception which should preferably be located near to the entrance to the yard.

Road aisles/gangways should be of an adequate width to enable transport activitiesto take place. It should not be necessary for the forks of an FLT to be raised abovethe height of adjacent stacks in order to manoeuvre into position. One open-sided warehouse was observed which was poorly laid out. Thegangways were too narrow and front loading FLTs had to travel with the forksraised such that the load would clear the surrounding stacks of timber.

(j) Visibility. The use of mirrors positioned around the site or on vehicles may improvevisibility by reducing or eliminating blind spots.

High visibility clothing should be worn by workers in the vicinity of moving vehicles.

Good working practice was observed in some sites where high visibility waistcoats,helmets, gloves, safety boots, hearing protection etc. were worn routinely byworkers. At other sites workers were seen in ordinary clothing without anyprotective or high visibility clothing.

Drivers and pedestrians should take extra care at shed doorways, gangways androadsides etc.

(k) Training. FLT drivers should be trained and authorised. Yard workers should alsobe trained and informed of site rules and regulations.

(l) Signs. Relevant signs should be used to inform persons of rules, risks andoperational procedures.

At one site signs were observed instructing persons not to climb stacks of boardmaterial.

Table 3 is a summary of the above.


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Table 3 - Summary of the factors identified as likely to affect the stability of sawn timber stacks

(a) Transport operations, (b) Audible warnings, (c) Transport rules, (d)Maintenance, (e) Housekeeping, (f) Lighting, (g) Reporting,(h) Monitoring, (i) Yard layout, (j) Visibility, (k) Training, (l) Signs

Yardmanagement

F8

(a) Wind, (b) Impacts, (c) Personnel, (d) Stacking and de-stacking,(e) Environment

External forcesF7

(a) De-stacking, (b) Offsets, (c) Logical stacking, (d) Slope, (e) Height to base ratio, (f) Cross-bracing, (g) External Supports

Stacking/stacks

F6

(a) Shape, (b) Condition, (c) PositioningSeparating/drying sticks

F5

(a) Positioning, (b) Looseness, (c) Damage, (d) Properties,(e) Quality

BandingF4

(a) Weight and dimensions, (b) Gaps/air spaces, (c) Uniformity oftimbers, (d) Out of squareness, (e) Collapsed

PacksF3

(a) Shape, (b) Condition, (c) Positioning, (d) BindersBearersF2

(a) Slope, (b) Unevenness, (c) Material, (d) Strength, (e) Obstacles, (f)Contamination, (g) Drainage

GroundF1

Sub-factorDescriptionFactor

All of the above points are considered as relevant. Some may be of great importance in specifictimber yards and of little importance in others. Anyone using this information should consider theimportance of each point raised and assess its importance with respect to their own individualsituation. This will enable them to produce a safe code of practice for employees to follow withintheir timber yard and minimise the risks of a collapse.

Whilst obtaining and compiling the above information general safety issues regarding lorry loadswere also considered. This information is listed in Appendix 3.

6 SYSTEMS OF WORK

Everyone involved in stacking operations should be familiar with simple safe systems of work forforeseeable tasks. These systems should cover the following important areas:

a) training, competence, and authorisation of truck drivers carrying out the stacking;

b) integrity assessment of the packs being stacked and a method for reassembling collapsed orpartially collapsed packs;

c) ground conditions assessment;

d) bearer selection;

e) maximum height to base ratios internally and externally;


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f) layout rules, including guidance on aisle widths;

g) responsibility for the storage area;

h) monitoring and reporting of dangerous stacks.

i) site rules e.g. specific things to avoid, including riding on the forks, travelling with the forkselevated, bridging of stacks and leaving bearers on the top of stacks.

7 CONCLUSIONS

A total of 14 premises were visited where stacked sawn timber products were being stored.

Measurements were made of pack and stack geometries to assess, for example, maximum likelypack offsets and the degree of lozenging. These were used as a basis for stability calculations andfor stack modelling.

For packs which are banded to a high quality, on hard standing or with stable ground conditionswhere risks from impacts and other external forces are low, the stability calculations showed that itis reasonable to expect stacks with a height to base ratio of up to 4 : 1 to remain stable. For external stacks which are subject to wind loading a more suitable heightto base ratio would be 3 : 1.

Stacks may well remain stable even with noticeably out of shape packs. However to remain stablethe packs have to stay intact and not be subjected to any extraneous forces such as wind loads orforces produced by unstable ground conditions. When these cannot be eliminated then the height tobase ratio of the stack should be reduced further e.g. 3 : 1 indoors, 2 : 1 outdoors. The surroundingsmay also limit the height of stacks e.g. sprinkler systems.

The following factors were identified as potentially affecting stability of timber stacks and need to betaken into consideration when assessing the risk of stacks collapsing:

w ground conditions, including slopes, unevenness, load carrying capacity anddrainage;

w internal stability of individual packs, including quality of banding and quality andnumber of separating sticks. Internal stability may deteriorate every time a pack ismoved;

w layout of the storage area, including stack and aisle spacing, truck turning space andallowable stack heights;

w length, cross-section, uniformity and positioning of bearers or chocks andprocedures for discarding and replacing poor or damaged bearers;

w systems of work usually involved the use of front or side loading lift trucks.Accuracy of positioning and stack heights need to be known and stepping down toreduce wind loading was common practice;

Stock yard or warehouse management will have a big effect on risk, including restriction ofpedestrian access, training of truck drivers, housekeeping, good lighting, stack condition monitoring,etc.


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All of the above points are considered as relevant. Some may be of great importance in specifictimber yards and of little importance in others.


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REFERENCES

White, 1998, Stability of sawn timber stacks - Banding of sawn timber packs, ME/98/21

Swift and Roebuck, 1998, Stability of stacked logs, HSL report No. ME/98/25

Norton 1999, Kickback on double circular rip saws, HSL report No. ME/99/06

HSW 47, 1975, Safety in the stacking of materials, Health and Safety Executive

HS(G) 172, 1997, Sawmilling - A run of the mill business?,Health and Safety Executive

WIS (2), 1998, Woodworking Information Sheet 2, Health and Safety Executive


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LIST OF FIGURES

Figure No. Description

1 Identical section softwood timber in banded packs.2 Use of ratchet straps for banding.3 Poor pack assembly resulting in movement of the tip-line or angling of upper packs. 4 The effect of out of shape packs on stability - lozenge shaped packs.5 Examples of out of shape packs - lozenge shaped packs.6 Summary of pack geometry considerations for stack stability calculations.7 Schematic showing tip-line and tip-point for a stack of sawn timber packs.8 Computer simulation of a collapse of a stack of sawn timber packs.9 Stability of stacks of sawn timber - geometric considerations for softwood

packs.10 Stability of stacks of sawn timber - geometric considerations for hardwood packs or packs of irregular section timbers.11 Stability of stacks of sawn timber - geometric considerations for board materials.12 Stacks of banded sawn timber packs on unprepared ground.13 Failure of concrete platform resulting in a 4.5o slope along the length of the stack.14 Sinkage of asphalt surface resulting in a 4.5o slope across the width of the

stack.15 An example of bearers made up from a number of sections of timber.16 Short/offset bearer in a stack of softwood packs resulting in a collapse of the overhanging portion of the upper pack.17 Short/offset bearer in a stack of hardwood packs.18 Schematic showing the movement of the tip-line and tip-point due to short/offset bearers19 A stack with a height to base ratio of 5 or 6 : 1 due to bearer positions (NB. misunderstanding of present guidance!).20 Banded partially planked rectangular hardwood packs.21 Large gaps between timber sections within a banded hardwood pack.22 The effect of out of shape packs on stability - balled packs.23 A close-up showing the effect of balling in a hardwood pack containing large gaps between sections of timber.24 Triangular section timber packs banded with "vertical" bearers.25 Collapse of a pack of triangular section timbers pack banded without "vertical" bearers.26 Spaces/gaps between timber sections.27 Poor application of banding.28 Collapsing packs due to loose banding.29 Examples of poor and broken banding.30 Collapsing steel banded pack.31 Stacking on top of an insecure pack.


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32 Stacking on top of an insecure pack.33 A stack containing an offset pack, a collapsing pack, loose banding and made up bearers.34 Lighter/smaller packs stacked on top of larger/heavier packs.35 A stack with a height to base ratio of approximately 5 : 1.36 Evidence of an impact to a stack of sawn hardwood timber.37 Schematic showing the forks of an FLT protruding too far and striking an

adjacent stack.


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APPENDIX 1 - Stacking of sawn timber - site visits

Log stacking/preliminary stacking of sawn timber site visits

Lockerbie Sawmill, Lockerbie, July 1998R. Carruthers Timber, Ecclefechen, July 1998James Jones, Heathfield, July 1998James Jones, Heathall, July 1998Blairquann Sawmill, Blairquann, July 1998Adam Wilson, Ayr, July 1998Adam Wilson, Ayr, July 1998

Stacking of sawn timber site visits

Linnel Bros. Ltd., Silverstone, August 1998Glenmere Timber Co. Ltd., Market Harborough, August 1998J. T. Ellis & Co. Ltd., Huddersfield, April 1999ABP (Port Safety Officer), Hull Docks, April 1999Global, Hull Docks, April 1999Nidd Valley Sawmills,Pateley Bridge,May 1999NCS, Hull Docks, May 1999


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APPENDIX 2 - Stability equations

Pack 2 :-

.................................................................................(1)e = b2.sin(180−b)

...................................................(2)c = e2 + ( a2 )2 − (e.a.cos(180 − b))

..........................................................................................(3)d = b − 90 + h

...................................................................(4)c = arccosc2+ a

22−e2

c.a

............................................................................................(5)a = b − d − c

...........................................................................................(6)y = x.cos(h)

......................................................................................(7)z = c.sin(a) − y

...............................................................(8)d = c2 + a2 − (2.c.a.cos(c))

.........................................................................................(9)f = 2.e.sin(d)

Moment of pack 2 about tipping point = W.z

Moment of pack 3 about tipping point = W.(z− y − f)

Moment of pack 4 about tipping point = W.(z− 2.y − 2.f)

Moment of pack 5 about tipping point = W.(z− 3.y − 3.f)

.......etc.


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APPENDIX 3 - General safety information regarding lorry loads

Road transport rules mean that lorry drivers are responsible for ensuring that their load is safe andsecure.

In one site visit a lorry was observed where one timber pack had been placed bridging betweenstacks which were already 2 packs high. This uppermost pack was only secured using a singleratchet strap.

It is bad practice on curtain sided lorries to rely on the curtain to hold the load in place. In the pastthis practice has resulted in fatalities to individuals who have released the curtain for unloading.

A lorry was observed being loaded with steel banded packs of sawn timber. Some of the steelbanding was seen to break during the loading process. This could have caused the packs to collapseor the band itself could have been a danger to anyone standing in the vicinity. Packs should beloaded carefully, any damage to banding should be noted, those packs with broken banding shouldbe made secure and workers should wear safety clothing/spectacles. FLT drivers should know thelocation of the lorry driver at all times. The need for employees to climb onto vehicles should bereduced as far as is reasonably possible.

FLT drivers highlighted how the upright poles on curtain sided lorries can hamper access whenunloading or unloading with a FLT. Care must be taken to avoid parts of the lorry whilst loading orunloading.

On loads arriving by lorry, bearers are often missing or too small. This causes problems whenattempting to unload using a FLT. When loading, the FLT driver should consider how easily thepacks can be removed when the load reaches its destination.

In open sided lorries, the act of sheeting of sawn timber loads was observed as a dangerousoperation for the lorry driver. The driver is often required to climb on top of the load without safetyharnesses or restraints and manoeuvre heavy sheeting, often in windy conditions. One lorry driverdescribed how he fell and sustained serious injuries whilst carrying out this operation.

A lorry was observed being loaded with packs of timber. When an upper pack was being loaded itwas noted that a bearer had been left resting on top of the pack. This bearer meant that there wasinsufficient clearance between the load and the top of the lorry and the loose bearer caught on theroof of the lorry. The bearer could have easily been dislodged and may have struck one of theworkers who were standing close by. Packs should always be checked for loose debris prior toloading.


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