multinail hub - basic timber truss mechanics

Multinail Australia

Multinail Hub - Basic Timber Truss Mechanics25 May 2020

Basic Timber Truss Mechanics

Basic Timber Truss Mechanics

It is best to think of a truss as a large beam with each member being in eithertension or compression. The panels in both the top chords and bottom chordsare simply beams fixed between the panel points.

You can calculate the forces applied to the truss as well as the reactions thatthe truss applies at the support points and can then determine the tie-downsrequired to cater for uplift.

The following truss examples show the compression and tension in differentmembers for various trusses.

Figure A1-02-04-01

Figure A1-02-04-02

Failure types

Failure of a truss generally means failure of an individual member; howeverany timber member can fail in a building. An individual member can also fail ifits deflection is too large.

Failure type Affected member

Deflection Chords Floor joists Rafters

Bending Chords Floor joists Beams

Tension Webs Chords

Compression Webs Chords Studs

Shear Bottom chords Beams

Due to continual product improvement Multinail Australia Pty Ltd. reserves the right to change the product/s depicted - both in description and specification.This document has to be read in conjunction with Multinail’s Technical Manual.

Modifying timber members because of failures

You can make the following changes to modify the truss based on failuremodes. The following lists appear in order of likely effectiveness (i.e. the firstchange type in the list is more effective than the remaining change types):

Failure type Correction

Deflection Decrease length of panel

Increase size of member (depth, thickness)

Increase grade

Tension Increase grade


Compression Decrease length - add ties


Increase grade

Shear Increase grade


Decrease length of panel

Load durations

When you first apply a small load to a timber member, the timber member deforms elastically. If you maintain the load, an increase in deformation occurs gradually withtime and reduces strength.

This increase in deformation is called “creep” and depends on the size of the applied load, temperature, humidity, etc.

Under a sustained load, the deformation increases for a period of time and then will gradually stables itself.

For example, an unseasoned beam may creep up to 3 times its initial deflection under load. A seasoned beam subjected to sustained loading only creeps to abouttwice its initial elastic deflection.

Because of creep, you must make allowance for the fact a piece of timber loaded for a long period may deflect further after being loaded.

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Multinail Australia

Multinail Hub - Design Loads25 May 2020

Design Loads

Design loads

The following details contain the basic dead, live and wind loads used for alltruss design. The loads are used in as many combinations as required toachieve the most adverse loads on a particular truss.

Dead loads are those loads considered to be applied to a truss system for theduration of the life of the structure. They include the weight of roof sheeting andpurlins, ceiling material and battens, wind bracing, insulation, self-weight of thetruss, hot water tanks, walls, etc.

Loads are considered in two major combinations:

1. Maximum dead load value - used for calculations involving all deadand live load combinations and for wind load (acting down on thetruss) which is an additional gravity load.

2. Minimum dead load value - used in combination with wind loadcausing maximum possible uplift on the structure, thus achieving thelargest stress reversal in the truss members.

The following tables show examples of loads used for truss details.

NOTE:

This information is subject to changes according to Code requirements.

Tiles = Approximately 55kg/m²

Sheet Roof = Approximately 12kg/m²

Plaster = Approximately 10kg/m²

Live loads

Top chord live loadsFor non-trafficable roofs (from AS1170 – Part 1 Table 3.2).

For roofs of houses 0.25kPa i sapploed and a 1.1kN point load.

For non-residential roofd where the area supported by the truss exceeds 14m²,a value of 0.25KPa live load is applied over the plan area of the roof.

If the supported area is less than 14m², the value of live load is taken as:

= 1.8 + 0.12kPa(Supported Area)

The supported area is usually the product of the truss span and spacing.

Bottom chord live loads

(From AS1170 – Part 1 Section 3.7.3)

The load is assumed as that of a man standing in the centre of a particularpanel of the truss bottom chord. The value is taken as 1.4kN where the internalheight of the truss exceeds 1200mm and 0.9kN for height less than 1200mm.


Wind load

1. Design wind velocity VDesign wind velocity V is determined from AS1170 Part 2.

For other areas, the following regional wind velocities can be used in timberstructures:

Region A - 45m/sec;Region B - 57m/sec;Region C - 66m/sec;Region D - 80m/sec

2. Design wind velocity VzV = V x M(z, cat) x Ms x Mt x Md

Where:

M(Z, cat) = Terrain-Height Multiplier (Clause 4.2)

Ms = Shielding Multiplier (Clause 4.3)

Mt = Topographic Multiplier (Clause 4.4)

Mi = Direction Multiplier (Clause 3.3)

Alternative for houses AS4055 can be used and Table 2.1A and 2.1B give:

Wind Class Design Wind Speed

N1 34

N2 40

N3 50

N4 61

C1 50

C2 61

des

des

des R

3. Wind pressure (P) Wind Pressure (P) = 0.0006 x V ² x Cfig (kPa)

4. ExamplesAssuming a house with an eaves height of less than 5 metres and in aCategory 2 Region C area.

The Wind Load is calculated as follows:

Regional Wind Velocity = 57.0 (m/sec)M(z, cat) = 0.91Ms = 1.0Mt = 1.0Design Wind Velocity = 57 x 0.91 x 1 x 1 = 52(m/sec)Wind Pressure (P) = 52² x 0.0006 x CfigP = 1.6224 (kPa) x Cfig

Other design criteria

Roof Span - 10 metresRoofing - SheetingCeiling - PlasterboardRoof Pitch - 15°Truss Spacing - 900mmTimber - Green HardwoodWeb Configuration - A TypeThe Wind Uplift force of each truss at support is calculated asfollows:Wind Uplift = P x Spacing x Cfig x Span/2

des

NOTE: This information is provided as a guide only.

Wind load calculation changes occur continuously and you must carefully consult the relevant Codes and other sources before undertaking this task.

Multinail’s Truss Design Software performs these calculations automatically, based on the latest Code refinements and “best practice’” design criteria.


Multinail Australia

Multinail Hub - Design Properties25 May 2020

Design Properties

Introduction

The Australian Standard AS1720.1 “Timber Structures Code” outlines thedesign properties of timbers for bending, tension, shear and compression.

Multinail software checks that truss member stresses do not exceed theallowable values and, if required, larger members or higher strength timber areconsidered to help ensure stresses do not exceed allowable values.

Stress levels in nailplates are checked against the allowable tooth pickup andsteel strength values that Multinail has determined over the years with differenttimbers.

Tension

A member in tension is subject to tensile stress (e.g. tow rope or chain).

Tension Stress (in a member)= P/A in MPaWhere: P = Load in NewtonsA = Area in Square mm

Figure A1-02-02-01

Compression

A member in compression is tending to buckle or crush. Long compressionmembers buckle and are weaker than short ones which crush. The allowablecompression stress for a particular timber depends on the “slenderness ratio”which is the greater of length/width or length/depth.

Figure A1-02-02-02

Bending

Beams are subject to bending stress (e.g. scaffold plank, diving board, etc.).The actual bending stress f = Bending/Section Modulus.

The Section Modulus (Z) is the resistance of a beam section to bending stress.This property depends upon size and cross sectional shape and for auniformly rectangular shaped beam:Z = bd² 6

Where:

b = width in mmd = depth in mm

b

NOTE:‘Z’ depends on the square of ‘d’, so doubling the depth increasesthe strength of the beam four times.

Figure A1-02-02-01

For example:

Increasing 125 x 38 to 150 x 38 gives a sectional modulus of 142500mmwhich is stronger than a 125 x 50 which has a section modulus of 130208mm3.

The depth is simply more important than the width. Deep beams require morecareful lateral restraint.

The Bending Moment depends on the load and the length of the beam.

For example:

Consider a simply supported beam carrying a Point Load ‘P’ at midspan.

3

Figure A1-02-02-04

By doubling the length (L) or load (P), you double the bending moment.

Deflection

During the analysis process when designing a truss, a number of deflectioncalculations are made to determine:

A) Chord inter-panel deflection

The actual deflection of the timber chord between panel points is calculatedand compared to the allowable deflection by the Australian Standard or stricterlimits that may be applied.

B) Joint deflection

Each joint within the truss is checked for vertical and horizontal deflection. In aflat (i.e. horizontal) bottom chord truss, there is no horizontal deflection in thebottom chord panel points.

The deflection calculated in the bottom chord panel points is used to calculatecamber built into the truss during manufacture.

NOTE:

For trusses without horizontal bottom chords, the horizontaldeflection is very important as it may cause the supportingstructure to deflect outwards.

Care must be taken in applying the truss loads, fixing the trussto the bearing point and maybe even design of the supportingstructure, to resist these loads.

Figure A1-02-02-05

By doubling the length (L) or load (P), you double the bending moment.


Camber

A vertical displacement which is built into a truss to compensate for theanticipated deflection due to applied loads. All trusses spanning relativelylarge distances are cambered.

Camber is normally applied to the bottom chord joints during the trussmanufacturing. It varies depending on the truss type and span, roof pitch androof loads. Trusses with large camber need to be handled and installedproperly and carefully on site.

Camber helps to resist loads. The amount of camber is calculated by Roof toresist the load of applied roofing and ceiling materials. Eventually, thedesigned truss with camber will flatten out to provide straight chords once it hasbeen fully loaded. Therefore, the amount of camber can represent the trusslong-term deflection.

Figure A1-02-02-10

Truss Action

A truss is like a large beam with each member in tension or compression, the chords acting as beams between the panel points as well as carrying axial load. At anyjoint, the sum of the forces acting must be zero (otherwise motion would occur) - this enables the forces to be determined.

For example:

Figure A1-02-02-01

By measuring (or scaling) or by using simple trigonometry The forces are foundto be 37.3 kN tension in the bottom chord and 38.6 kN compression in the topchord.

Figure A1-02-02-07

The following diagrams show typical Tension (T) or Compression (C) forces in the modulus of a truss under uniformly distributed gravity loads.

Figure A1-02-04-01 Figure A1-02-02-08 Figure A1-02-04-02

Figure A1-02-02-09


Multinail Australia

Multinail Hub - Mechanical Properties of Timber25 May 2020

Mechanical Properties of Timber

Mechanical propertiest

Mechanical properties of Timber describes the way Timber behaves under applied forces.Different timber species have certain characteristics that affect the strengths of the timber and limit the potential uses for the timber.Mechanical properties consider the actual engineering terms and values that form the Grading Systems, Joint Groups and Strength Groups that are used in ourindustry.Before we discuss mechanical properties, you need to understand how timber works.

Forces and Loads

A force is a measure of a load applied to an object in a certain direction. Forexample, if you consider the tiles on roof trusses, we know the load and weknow the direction – therefore we can calculate the force.

Timber estimators work with the following loads:

Dead Load (DL) - permanent or long term loads including weights offixed materials (e.g. tiles to top chord, plasterboard ceiling tobottom chord, hot water systems, air-conditioning units, flooring,etc.).Studs, wall plates and lintels have been sized in accordance withthe relevant Tables from AS1684.Wind Load (WL) - environmental loads due to the wind pressureapplied to a structure.Service Loads (SL) - Solar Hot Water Systems (SHWS). AirConditioning, etc.

When you add these types of loads to a Timber Structure, the engineers andSoftware program analysis the forces applied to the Timber members.Depending on whether the timber moves (i.e. bends or deflects) you can takeactions to rectify, increase sizes or grades, increase thicknesses, decreasespans, add additional supports, etc.

When you understand the stresses and strains on the timber, you can alsodevelop methods to withstand the forces to make the structure sound.

A mass of 1kg is approximately equal to 10 N, or 100kg is equivalent to 1kN.

Building plans show units of force as a kilonewton (kN) and show units ofstress as kilonewtons per square meter (kN/m2). This combination of units arealso commonly know as a Megapascal (MPa).

Engineers refer to the following four main stresses and strains:

Compression;Tension;Shear; andBending

Also considered is the deflection of individual members as well as the overallcomponent such as the Truss and Lintel etc.

Compression

When a load is applied which tends to shorten or crush the body, the body is incompression. The shortening is known as compressive strain and the stress iscompressive stress.Compression strength relates to the ability of a section to support a given loadacting across that section. Compression occurs in columns, studs and stumpswhere a member is holding up a load. It can also occur in some trussmembers.

Figure A1-01-01-01

Tension

Tension occurs when the load applied tends to lengthen or stretch a body. Thelengthening is known as tensile strain and the stress produced is tensile stress.

Tension strength is the ability of a member to withstand an axial tension load.The bottom cord of a roof truss is a typical example of a timber membersubjected to tension forces.

Figure A1-01-01-02

Shear

Shear occurs when a load tends to move one part of a body over an adjacentpart. The amount of movement along the shear plane is the shear strain andthe stress produced is the shear stress.The shear strength of timber is gauged by its ability to oppose forces acting inopposite directions in a section, tending to cause a sliding action of one partover another.An example of shear forces acting on a section of timber is the case of a boltedjoint in the bottom cord of a truss. Shear stress is also present where a beamrests on a wall. In this situation, the shear is perpendicular to the grain.

Figure A1-01-01-03

Bending

Bending commonly occurs in structural members such as Beams and Joists. ABeam supported at both ends and loaded in the middle tends to benddownwards

When this occurs, the top part of the beam tends to shorten and undergoescompressive stresses; while the bottom part tends to stretch and undergoestensile stresses.

Both stresses decrease toward a neutral axis, near the centre of the beam,where no bending stress exists and where shear stress is at its maximum. Thisis the reason for requiring higher Grades of Timber and better joints on theoutside rather than at the centre of laminated beams.

Many species of timber have been tested and rated against these four testsand using this data, Timber Engineers have been able to equate values tovarious timber groups. Timber Groups are various species of timber that arerated in the same level of testing values.

There are two types of values gained from the testing – Strength Group andJoint Group. The Strength Group refers to how strong the Timber is in relationto the density and the above 4 stresses and strains placed on the Timber. Thefollowing table shows the strength group for dry timber only - green timber hasa similar table.

Note – Softwoods are usually grouped in SD5 to SD7; hardwoods usually

Figure A1-01-01-04


found in groups SD1 to SD4.

Property Strength Group

SD1 SD2 SD3 SD4 SD5 SD6 SD7

Bearing perpendicular to grain 26 23 19 17 13 10 8.6

Bearing parallel to grain 76 67 59 51 40 30 23

The Joint Group refers to how well the Timber performs with fixings.

The following table shows the range of values for joint groups for seasoned timber:

Property Joint Group

JD1 JD2 JD3 JD4 JD5 JD6

Air dry density between 10% - 15% (kg/m3) 940 750 - 935 600 - 745 480 - 595 380 - 475 300 - 375

The timber strength relates to the tests performed and the species grouping. To calculate the actual Stress Grade (i.e. how strong the timber is), you must review thenatural characteristics and grade the timber based on the number of defects found. The Timber Grades range from 1 (being the strongest) to 4 (being the weakest).Timber below a Grade 4 are not classed as Structural Timbers.

The following table shows the possible stress grades and strength groups for each timber group:

Strength Group Stress Grade

Grade 1 Grade 2 Grade 3 Grade 4

SD1 F34 F27 F22 F17

SD2 F34 F27 F22 F17

SD3 F27 F22 F17 F14

SD4 F22 F17 F14 F11

SD5 F17 F14 F11 F8

SD6 F14 F11 F8 F7

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Multinail Australia

Multinail Hub - Roof Truss Numbering System25 May 2020


Roof Truss Numbering System

Multinail uses a simple, flexible and very versatile system ofmember and joint numbers to identify all members andconnectors.

HL - Left heel HR - Right heelTO - Always allocated to the apexB0 - Always allocated to the joint immediately below the apex jointT1R - First Joint to the right of apexT2R - Second Joint to the right of apexT1L - First Joint to the left of apexT2L - Second Joint to the left of apexTCR - Top Chord right of apexTCL - Top Chord left of apex

Figure A1-04-01-01

Figure A1-04-01-02

A variation to this Numbering System occurs when the Top Chord contains asplice.

The Top Chord is then allocated two denotations:

TC1-R - If the upper Top Chord right is equal length to the left side then it islabelled as TC1

TC2-R - If the lower Top Chord right is equal length to the left side then it islabelled as TC2

If the Top Chord contains three members, than the next Top Chord would bemarked TC3-R, etc.

Bottom Chords

For the bottom chord, the numbers and markings are similar. If the Truss doesnot have a B0 joint, then the joints are marked as B1R, etc. and B1L from animaginary line from the T0 connector. Hence, the more joints, the morenumbers to each side of this imaginary line.

With Standard Trusses, there are normally only one splice joint per bottomchord and the size and stress grade of each member is the same, thus thebottom chords are numbered BC1 and BC2 as it is not critical to which side ofthe truss it is positioned.

For Trusses with multiple Bottom Chords such as Cathedral Trusses in whichthere may be up to five Bottom Chords (each one may be a different size). Themembers are numbered from the left hand side to of the Truss and are markedas BC1, BC2....BC5. This numbering reflects the manner in which the Trussdrawing is developed.

Splices

When a chord is spliced between panel points, it is marked as LTS1 (being thefirst splice in the top chord on the left hand side of the TO position). Similarly toRTS1.

When the chord is spliced at a panel point, the joint is marked as LTS2 orLBS3 relevant to the joint number in the top or bottom chord.

Webs

Webs (the internal truss members) are marked according to their position inrelation to the apex joint T0 and the vertical web under this joint, or theimaginary line from the T0 position.

The webs to the left of the T0 are marked W1L, W2L, etc. and webs to theright of this line are marked W1R, W2R, etc.

Note that it is possible that there may be more webs on one side than the otherof the T0 position.


Multinail Australia

Multinail Hub - Roof Truss Terminology25 May 2020

Roof Truss Terminology

Figure A1-03-01-01

Truss

A prefabricated, engineered building component which functions as astructural support member.

Member

Any element (chord or web) of a truss.

Apex

The point on a truss at which the top chords meet.

Axial force

A force (either compression or tension) that acts along the length of a trussmember. Measured in newtons (N) or kilo newtons (kN)

Axial stress

A measure of the intensity of an axial force at a point along a member,calculated by dividing the axial force at that point by the cross-sectional area ofthe member. Measured in mega pascals (MPa).

Battens

Structural members which are fixed perpendicular to the Top Chords of aTruss to support the roofing material or to the Bottom Chords to support theceiling material and to restrain the Truss from buckling.

Bending moment

A measure of the intensity of the combined forces acting on a member; i.e. thereaction of a member to forces applied perpendicular to it (including theperpendicular components of applied forces). The maximum bending momentis generally towards the centre of a simple beam member.

Bending stress

A measure of the intensity of the combined bending forces acting on amember, calculated by dividing the bending forces acting on a member, arecalculated by dividing the bending member by the section modulus of themember.

Bottom chord

The member which defines the bottom edge of the truss. Usually horizontal,this member carries a combined tension and bending stress under normalgravity loads.

Butt joint

A joint perpendicular to the length of two members joined at their ends.

Camber

A vertical displacement which is built into a Truss to compensate for theanticipated deflection due to applied loads. All trusses spanning relativelylarge distances are cambered.

Cantilever

Where the support point of the Truss is moved to an internal position along theBottom Chord of the Trusses.

Combined stress

The combined axial and bending stresses which act on a membersimultaneously; i.e. the combination of compression & bending stresses in aTop Chord or tension & bending stresses in a Bottom Chord which typicallyoccur under normal gravity loads.

Concentrated or Point Load

A load applied at a specific point; i.e. a load arising from a man standing on theTruss.

Cut-off

The term used to describe a truss which is based on a standard shape but cutshort of the full span.

Dead Load

The weight of all the permanent loads applied to member of a truss; i.e. Theweight of the member itself, Purlins, Roofing Ceilings, Tiles etc.

Deflection

The linear movement of a point on a member as a result of the application of aload or a combination of loads. A measure of the deformation of a beam underthe load.

Eaves Overhang

The extension of the top chord beyond the end of the truss to form the eaves ofa roofing structure.

Heel

A point on a truss where the top and bottom chords join.

Hip Joint

The joint between the sloping and horizontal top chords of a Truncated Truss.

Laminated Beam or Truss

Two or more Members or Trusses mechanically fastened to act as a compositeunit. Lamination allows the achievement of the increased strength without theuse of a solid, larger section Timber.

Lateral Tie

A member connected at right angles to a chord or web member of a truss torestrain the member.

Live Load

Temporary loads applied to the truss during maintenance by workers andduring constructions.

Load Duration Factor

The percentage increase in the stress allowed in a member based upon thelength of time that the load causing the stress is on the member. (The shorterthe duration of the load, the higher the Load Duration Coefficient). (k )2

Manufacturing Details

Drawings which contain the data for truss fabrication and approval by localbuilding authorities. (Produced automatically by the software used by MultinailFabricators.)

Mid-panel Splice

A splice in a member (at a specified distance from a panel point).

Mitre Cut

A cut in one or more member made at an angle to a Plane of the Truss. I.e.The Top or Bottom Chords of a Creeper Truss are mitered at 45 degrees atthe end of the Truss where they meet the Hip Truss.

Overhang

The clear extension of a chord beyond the main structure of a Truss. (See alsoEaves Overhang)

Panel

The chord segment of a Truss, usually the Top or Bottom Chord between twopanel points.

Panel Point

The connection point between a Chord and Web.

Panel Point Splice

A Splice joint in a Chord and Web.

Pitch

The angular slope of a Roof or Ceiling. Also the angular slope of the Top orBottom Chords of a Truss which form and/or follow the line of a Roof orCeiling.


Plumb cut

A vertical cut. A plumb cut is perpendicular to a horizontal member. All splicesare plumb cut.

Purlin

A structural member fixed perpendicular to the Top Chord of a Truss tosupport the roofing.

Span

The distance between the outer edges of the loadbearing walls supporting theTrusses.

Splice Joint

The point at which the Top or Bottom Chords are joined (at or between panelpoints) to form a single Truss Member.

Support Reactions

Those forces (usually resolved into horizontal and vertical components) .whichare provided by the Truss supports and are equal and opposite to the sum ofthe applied forces.

Top Chords

Generally the sloping of members to a Truss which define its top edge. Undernormal gravity loads, these members usually carry a combined compressionand bending stress.

Truncated Girder Station

The position of a Truncated Girder. Defined in terms of its distance from theend wall.

Webs

Members which join the Top and Bottom Chords, together with them form aTruss by which structural loads are transferred to the Truss support.

Wind Loads

Wind loads are the forces applied to the Roof Trusses by the virtue of windblowing on the structure, typically (not always) upwards.


Multinail Australia

Multinail Hub - Truss Design & Analysis25 May 2020

Truss Design & Analysis

Introduction

This section provides a brief introduction to the techniques for Truss design; itis not intended as a comprehensive guide.

The design of the Truss members can begin immediately after determining theanticipated loadings (i.e. Dead Load, Live Load and Wind Load) .

Truss Analysis

For Truss shapes, where members and joints form a fully triangulated system(i.e. statically determinant Trusses), Truss analysis makes the followingassumptions;

Chords are continuous members for bending moment, shear anddeflection calculations. Negative moments at joint (nodes) areasevaluated and these moments are used to calculate the shear anddeflection values at any point along the chord, for distributed andconcentrated loads.

Member forces can be calculated using a “pure” Truss (i.e. allmembers pin-jointed).Total Truss deflection used to evaluate Truss camber and to limitoverall deflections can be calculated using the system of virtualwork. Again, the members are considered as pin-ended and a unitload is placed at the required point of deflection.

Truss loading combination and load duration

The following load combinations are used when designing all Trusses:

SW + DL: permanent durationSW + DL + SLL: short term live load combinationSW + DL + MLL: medium term live load combinationSW + DL + WL: extremely short duration

Where:SW = Self Weight (timber Trusses)DL = Dead Loads (tiles, plaster)SLL, MLL = Live Loads (people, snow)WL = Wind Loads

Each member in the Truss is checked for strength under all three combinationsof loadings. Dead loads plus live loads and dead loads plus wind loads mayconstitute several separate combinations in order to have checked the worstpossible combination.

Load Duration

The limit state stress in a timber member is depends on the load duration factor(k1). For a combination of loads, the selected load duration factor is the factorcorresponding to the shortest duration load in the combination.

For Trusses designed according to AS1720.1-2010, the duration of a loadconsidered to act on a Truss is of major importance for dead loads only; theload is considered permanent and thus factor k1 at 0.57 is used. For dead andwind load combinations, the wind load duration is considered as gusts ofextremely short duration and k1 of 1.0 (for timber) is used. For dead and liveload combinations several load cases may have to be checked due to differingload durations. In general, live loads are taken as applicable for up to 5 dayswith a k1 of 0.94 (for timber). Live loads on overhangs are applicable for up to5 hours with a k1 of 0.97. Either may be critical.

Design of Truss Members

Truss webs are designed for axial forces and chords, for axial forces plusbending moments and checked for shear and deflection between webjunctions.

Webs

Tension webs are checked for slenderness and the nett cross-sectional area isused to evaluate the tension stress. The cross-sectional area is taken as theproduct of the actual member depth and the thickness.

Compression webs are also checked for slenderness. Effective length is usedfor buckling of the web in the plane of the Truss and out of the Truss plane.


Chords

Tension chords are designed for strength and stiffness and must withstandcombined tension and bending. The slenderness of the member is checked astension webs and also as a beam. The shear of the member is also checkedbut is usually critical only on heavily loaded members (e.g. girder Trusses).

The stiffness criteria is to limit the deflection of a chord between the panelpoints. The long term deflection is calculated for dead loads only, as theinstantaneous deflection under this load multiplied by duration factor (fromAS1720) is determined by the moisture condition of the timber. The limit on thisdeflection is (panel length) ÷300.

Live load and wind load deflections are calculated separately withoutconsideration of the dead load deflection. Deflection is limited for live and windloads to overcome damage to cladding materials and to reduce unsightly bowsin the roof or ceiling.

Compression chords are also designed for strength and stiffness. The strengthof compression chords depends largely on the lateral restraint conditions of thechords. The combined compression and bending stresses in the members arechecked using the index equation in AS1720. Shear stress is also checked asfor tension chords.

Deflections are checked as for tension chords and designed for similarallowable values.

Modification factors used in design

The following is a typical calculation for bending strength. The capacity inbending (ɸM) of unnotched beams, for strength limit state, shall satisfy:

(ɸM) ≥ M*where:(ɸM) = ɸk[f’ Z]and,M* = design action effect in bendingɸ = capacity factork = k x k …. x k and is the cumulative effectiveof the appropriate modification factors f’ = characteristic strength in bendingZ = section modulus of beam about the axis of bending (bd2/6)

b

1 4 12

b

Standard and complex design

Both standard Truss designs and complex Truss designs can be generated byMultinail Fabricators or Multinail Engineers.

When a complex design is generated by the Fabricator for a quotation job, it isstandard practice for a copy of the input and output to be checked by anengineer - either an independent consultant or a Multinail Engineer - beforemanufacturing the Truss.

For large projects (e.g. hospitals, schools, offices, etc.) the entire project isinitially analysed and an overall Truss and bracing layout completed. EachTruss is then individually analysed, designed, drawn to scale, costed andpresented with full cutting and jig layout dimensions to ensure accurate anduniform manufacture.

Trusses are usually analysed and designed for dead, live and wind loads;however the analysis and design may be extended to include concentratedpoint loads as required. Trusses can also be analysed and designed for snowload, impact loads, moving loads, seismic loads, etc.

If the drawing specifies the purpose of the structure and the anticipated loads,all the loads will be considered during Truss analysis and design and clearlyitemised on the drawing. Computations can also be supplied if required.


Multinail Australia

Multinail Hub - Truss Shapes25 May 2020

Truss ShapesThese diagrams indicate the approximate shapes of the Trusses mostcommonly used. The choice of the Truss shape for a particular applicationdepends upon the loading and span requirements. General spans mentionedare for 90mm for the Top and Bottom Chords

Kingpost Truss

For spans approximately 4m. Used primarily inhouse and for garageconstructions.

Figure A1-05-01-01

Queenpost Truss

For spans approximately 6m. Used mainly for house construction.

Figure A1-05-01-02

A-Truss

For spans approximately 9m. This is the most commonly used Truss for bothdomestic and commercial applications.

Figure A1-05-01-03

B-Truss

For spans approximately 13m. Used primarily in residential and smallercommercial buildings, this Truss is generally preferred to the A-Truss for largerspans since it offers greater strength (additional web members) at lower costdue to the reduction in size of top and bottom chord timber.

Figure A1-05-01-04

C-Truss

For spans approximately 16m. Used principally for commercial and industrialbuildings. This can be constructed with lower strength Timbers.

Figure A1-05-01-05

Half A-Truss

For spans up to 6m. Used for residential construction where the Trusses mayform a decorative feature.

Figure A1-05-01-06

Half B-Truss

For spans up to 9m. Uses are similar to those for the A-Truss.

Figure A1-05-01-07

Half C-Truss

For spans up to 11m. Uses are similar to those for the A-Truss but tends to bepreferred to the half A-Truss for reasons given under C-Truss.

Figure A1-05-01-08

Truncated Truss

Spans depend on depth. There are two types of Truncated Trusses; theTruncated Girder Truss and the Standard Truncated Truss. Together theyfacilitate Hip Roofc constructions.

Figure A1-05-01-09

Hip Truss

A half Truss with an extended Top Chord which is used to form a Hip Ridge ofa Hip Roof.

Figure A1-05-01-10

Jack Truss

Similar to the half A-Truss but with an extended mitered Top Chord whichoverlies a Truncated Truss to meet the extended Top Chord of a Hip Truss.

Figure A1-05-01-11

Creeper Truss

Similar to a Jack Truss but mitre cut to intersect the Hip Truss between theouter wall and the Truncated Girder.

Figure A1-05-01-12

Girder Truss

Special Trusses of any standard Truss shape used to support other Trusseswhich meet at the right angel. (The standard shape is maintained but GirderTruss members are generally larger in both size and stress grade) Girders canbe any shape and carry Trusses at any angle.

Figure A1-05-01-13

Scissor Truss

Not a standard Truss design but often used to achieve special vaulted ceilingeffects, sometimes with relatively wide spans.

Figure A1-05-01-14

Half Scissor Truss

As its name implies, one-half of the Standard Scissor Truss used for industrial,commercial and residential buildings.

Figure A1-05-01-15

Pitched Warren

Generally used for industrial or commercial buildings to achieve low pitch withhigh strength over large spans.

Figure A1-05-01-16

Dual Pitch Truss

Non-standard Trusses used to achieve special architectural effects. The leftTop Chord is a different Pitch to the right Top Chord.

Figure A1-05-01-17

Cut-off Truss

Any Truss, the shape of which forms part of a standard Truss. As the nameimplies, the cut-off Truss has a shorter span than that of the standard Truss onwhich it is based; it is terminated by a vertical member along the line of the‘cut-off’.

Figure A1-05-01-18

Bowstring Truss

Used for large span industrial and commercial buildings including aircrafthangars, where the roof profile is curved. Bowstrings can be used everywheredomestic included.

Figure A1-05-01-19

Howe Truss

For spans up to 12m. Used mainly for applications which involve high loadingof the Bottom Chord (in preference to the A-Truss).

Figure A1-05-01-20

Pratt Truss

For spans up to 12m. May be used in preference to Howe Truss forcircumstances of a high Bottom Chord loading.

Figure A1-05-01-21

Fan Fink Truss

For spans up to 9m. Used mainly for applications which involve high loading ofthe Top Chord (e.g. where the Truss is exposed and the Ceiling load is carriedon the Top Chord).

Figure A1-05-01-22

For spans up to 16m. Used for the same reasons as the Howe Truss (inpreference to the B-Truss).

Figure A1-05-01-23

Parallel Chorded

As its name implies, the Top and Bottom chord are parallel. Used for both floorand roof applications.

Figure A1-05-01-24

Attic Truss

Special purpose Truss to simplify an Attic Construction.

Figure A1-05-01-25


Portal Frame

A standard commercial and industrial design for wide spans.

Figure A1-05-01-26

Inverted Cantilever

Used to achieve special architectural effects in Churches, Restaurants, Motelsetc.

Figure A1-05-01-27

Cathedral Truss

A non-standard Truss used mainly in residential buildings to achieve a vaultedceiling effect.

Figure A1-05-01-28

Bell Truss

A Standard Truss used to achieve a Bell-shaped roof-line. E.g. 4 Top Chordsat different pitches

Figure A1-05-01-29


multinail hub - basic timber truss mechanics

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