learning from failures in timber structures€¦ · why should we learn from previous failures /...
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Learning from failures gin timber structures
Dr. Eva Frühwald, Div. of Structural Engineering, [email protected]
OutlineOutline
• Failure – error ? • Why learning from failures ?Why learning from failures ?• Research project• Explicit cases• What can we learn from failures?• What can we learn from failures?
Definition of failure (1)Definition of failure (1)
• Failure is a deviation from the status quo.
• Failure is not meeting target expectations.
• Failure is any secondary defect.
Definition of failure (2)Definition of failure (2)Ulti t li it t t S i bilit li it t tUltimate limit state• Risk for human life,
negative effect on
Serviceability limit state• No consequence for
safety of structurenegative effect on safety and performance of structure
safety of structure
– Direct collapse of structures
– Vibrating floors– Excessive deformations
– Local cracking– Crushing
degradation
Excessive deformations– Moisture movements– Mould and fungi
– degradation
Why should we learn from previous failures / collapses ?
Hypothesis: All failures are caused by human errorsAll failures are caused by human errors.
• Errors of knowledge (inadequate training in relation to tasks)
• Errors of performance (non-professional performance, carelessness)
• Errors of intent (consciously taking short-cuts and risks to save time/money)
Should be possible to prevent failures
[Kaminetzky]
Learning from failuresLearning from failures
Education and training are the only effective ways to minimize failures.yThe training of an engineer, architect or contractor should provide an understanding notcontractor should provide an understanding not only of the best solutions that may be adopted, but also of practices that should be avoided. (Kaminetzky, 1991)
Previous studies: common failure causesPrevious studies: common failure causes• Concrete
– material qualityq y– work execution– structural design and detailing (joints, openings, supports,…)
• Steel– insufficient temporary bracing during construction– errors in design / construction mainly of connections and details– deficient welding– excessive flexibility and non redundant designexcessive flexibility and non redundant design– Vibration induced failures– stability type failures– fatigue and brittle failure
i d– corrosion damage• Timber
– inadequate behavior of jointseffects of moisture exposure (imposed strains shrinkage)– effects of moisture exposure (imposed strains, shrinkage)
– poor durability performance– inadequate bracing of structural system– inadequate performance of material and products– inadequate appreciation of load
Previous studies: common failure causesPrevious studies: common failure causes• Concrete
– material qualityq y– work execution– structural design and detailing (joints, openings, supports,…)
• Steel– insufficient temporary bracing during construction– errors in design / construction mainly of connections and details– deficient welding– excessive flexibility and non redundant designexcessive flexibility and non redundant design– Vibration induced failures– stability type failures– fatigue and brittle failure
i d– corrosion damage• Timber
– inadequate behavior of jointseffects of moisture exposure (imposed strains shrinkage)– effects of moisture exposure (imposed strains, shrinkage)
– poor durability performance– inadequate bracing of structural system– inadequate performance of material and products– inadequate appreciation of load
Level of safety in timber structuresLevel of safety in timber structures
• Is the level of safety adequate for timber structures compared to other materials?
Implementation of Eurocode 5 (national application rules)– Implementation of Eurocode 5 (national application rules)– Spectacular collapses of timber structures in Europe
Swedish-Finnish project 2005-2007• Research questions investigating collapsed buildingsg g g
– Reasons for failure?– Which type of components are most prone to failure?– Which failure modes are most frequent?– What can be done to avoid or reduce failures?
Survey of failure casesy(Swedish-Finnish survey)
• 4 partners
• 127 cases lit t– literature
– own investigations
• Only cases implying risk for human lives are included (ULS)!included (ULS)!
reportreport1. Introduction2 E i f i2. Experiences from previous
failure investigations3. Present survey of failure y
cases - methodology4. Results and interpretation
of the information collectedof the information collected5. How can we learn from
previous failures6. Summary and conclusions
Annexeexempel
Annexe– Overview failure cases
and classificationand classification – 127 cases, 1-2 pages /
case
Classification of error types causing failureClassification of error types causing failure1. Wood material performance2 Manufacturing errors in factory Materials & products2. Manufacturing errors in factory3. Poor manufacturing principles
4 O it lt ti
Materials & products
4. On-site alterations5. Poor principles during erection Construction work
6. Poor design / lack of design with respect to mechanical loading
7. Poor design / lack of design with respect t i t l ti
Design/planning
to environmental actions
8. Overload in relation to building l ti
Codesregulations
9. Other / unknown reasons
Multiple failure types: estimate of the weight of each failure type
Failure cause (127 cases)( )Wood material performance 1%
Manufacturing errors in factory 5%Other / unknown 5% Manufacturing errors in factory 5%
Manufacturing principles 4%Overload / codes 4%
Poor principles during erection 16%
Design (mechanical loading)
On-site alterations12%
42%
Design (environmental actions) 11%actions) 11%
Failure cause (127 cases)( )Unknown / other 5%
Materials and productsOverload / codes 4%
Materials and products 11%
Construction work27%
Design / planning53%
27%
Type of buildingsType of buildingsi t fin percentage of cases
Public 51Industrial 23Industrial 23Agricultural 7Apartment 8Apartment 8Other / unknown 11
• NOTE: Failure surveys in general can not be seen as representative for the general population of structures (cover up of mistakes is common, random sampling is impossible)impossible)
Span100
Span
80
90
16% < 10 m
60
70
80
84% > 10 m
50
60
span
[m]
30
40
s
25 m
10
20
0
Age at failureAge at failure2525
2020
15res15ures
10% o
f fai
lur
10of fa
ilu
5
%
5
%
01 2 5
00 1 2 3 4 5 6-10 11-15 16-20 21-25 26-30 31-35 36-40
years
Type of structural elements that failedType of structural elements that failed
in percentage of cases
beam 47truss 34bracing 29bracing 29joint 23
dowel-type 57punched metal plate 10glued 7
arch 8column 4
gother 27
column 4frame 2
Correlated with typical structural elements?!
Failure modesFailure modesi t fin percentage of cases
• instability 30• bending failure 15• bending failure 15• tension failure perp. to grain 11• shear failure 9shear failure 9• drying cracks 9• excessive deflection 7• tension failure 5• corrosion of fasteners / decay 4• withdrawal of fasteners 3• compression (buckling) 2• other / unknown 21
Timber, steel and concrete buildings:failure causes
F il Ti b St l C tFailure cause[in % of cases]
Timber [own survey]
Steel [Oehme, Vogt]
Concrete [Brand, Glatz ]
Design / planning 53 35 40Design / planning 53 35 40
Construction work 27 25 40
Maintenance / reuse 35
t i l 11material 11
other 9 5 20
difficult to compare – definition of categories, number of cases etc.
Q ti A i b tt t d i iQuestion: Are engineers better at designing steel- and concrete structures !?
How can we learn from previous failures?How can we learn from previous failures?
53 % d i / l i human errors53 % design / planning27 % construction work
Errors of intentErrors of knowledge Errors of performance
improved training and education
more efficient Quality Assurance (QA)
more efficient Quality Assurance (QA) ?(Q ) (Q )
Training & educationTraining & educationSh ld f t h i l t hi h t i l• Should focus on technical aspects which are typical causes for failure
• Training of engineers and control in the design / planning phase most important (most errors!)
• Training & education measurementsLectures on good and bad examples for students / practicing– Lectures on good and bad examples for students / practicing engineers
– Database on good / bad examplesC t i t h i l t– Certain technical aspects
– …
Learning from each others mistakesLearning from each others mistakes
Training & education: gexamples for issues to be emphasized
Bracing to avoid instability both during construction and• Bracing to avoid instability both during construction and in the finished structure
• Situations with risk for perpendicular to grain tensile failure (joints, double-tapered beams, curved beams,…)
• Consideration of moisture effects
D i f j i• Design of joints
• Estimation of loading conditionsEstimation of loading conditions
• Estimation of real behavior of the structure
Training & education: building site professionals
• Increasing the competence of building site professionals – Professional trainingProfessional training– Assigned training / certified personnel to perform
certain taskscertain tasks– Continuous courses and seminars
• External quality control by impartial and certified personnelp
ExamplesExamples1 Si A D k1 – Siemens Arena, Denmark2 – Jyväskylä Exhibition Hall, Finlandy y3 – Buckling of trusses
Siemens Arena DenmarkSiemens Arena, Denmark
• Inaugurated February 2002• Failed January 3 2003• Failed January 3, 2003• 8000 seats• Building costs: 6 millions Euros• Calculated cost to rebuild structure:
25% of new price25% of new price
Martin Hansson, 2004
Siemens Arena the fish shapeSiemens Arena – the fish shape
No diagonals! R=380 m
5 m
R 175about 6.5 m R=175 m
72 m(double: center to center 10 m)
FailureFailure
No load at failure2 out of 12 trusses failedout o t usses a ed
Failure initiationFailure initiation
The failure jointThe failure joint
Slotted-in steel plates
Reduced section
F il ti Tension failureFailure section - Tension failure
The failed cross sectionThe failed cross section
glued in spacerglued-in spacer block 45x160533
430
160 160 160
The failed cross sectionThe failed cross section
4 slotted-in 8 mm steel plates in 10533 steel plates in 10 mm wide slots430
160 160 160
The failed cross sectionThe failed cross section
Dowels 12 mm533
430
160 160 160
Experts explanation
Early stage estimation not corrected during detailing!A d ti t id d• Area reduction not considered- depth in bottom chord from 533 to 430 mm- slots for steel platesslots for steel plates- holes for dowels
533
430430
160 160 160
Experts explanation
Early stage estimation not corrected during d t ili !detailing!
• Moment caused by very stiff connection –y yassumed as hinged
• Angle between stresses at surface andAngle between stresses at surface and grain direction
• Used tensile strength about 50% higher• Used tensile strength about 50% higher than code value
Experts explanationTotal effect:The actual strength in joints was about 25% of theThe actual strength in joints was about 25% of the
“designed strength”The actual stresses were 40% higher than the characteristic 5% strengthcharacteristic 5% strengthAll trusses will fail sooner or later…
Why those two? the trusses had large knots and low quality glueline glulam quality determined which of the trusses failedthe trusses failed
Why just then? Duration of loady j
Restauration- cables
Design related causesX strength design
environmental actionsConstruction related causes
X
Poor principles during constructionAlterations on-site of intended design or products
Deficiency in design rules / codesDeficiency in design rules / codesprediction of capacityExtreme loading
Causes related to wood material and wood productsInadequate quality of wood materialPoor manufacturing principles for wood products
(X)g p p p
Manufacturing errors in factoryCauses related to utilisation of the structure
Misuse or lack of maintenance of the structure
(X)
Misuse or lack of maintenance of the structureOther causes
Jyväskylä Exhibition hall, Finland
• Glulam truss, about 55 meter span• Inaugurated January 17, 2003• Failed February 1 2003 (the day after an• Failed February 1, 2003 (the day after an
exhibition)• Number of people in building: 10• Nobody hurt• Nobody hurt
Martin Hansson, 2004
StructureStructure
Double trusses
4.8 m
55 m(center to center 9 m)
The failureThe failure
Failure loadFailure load
• 52 kg/m2 snow load (25 % of design value)• Failure load = 51% of expected
characteristic 5% capacitycharacteristic 5% capacity
Failure initiationFailure initiation
Failure investigationFailure investigation
Slotted-in steel plateDowels (hidden, no inspection!)Dowels (hidden, no inspection!)
Extremely poor manufacturing of dowel jointsof dowel joints• steel plate was placed incorrectly
compared to center line of beams• Positioning of dowels was poor: Some not penetrating the steel plate
• Drilled holes varied in depth andDrilled holes varied in depth and were oversized
• Partly too short embedding length f d lfor dowels
Failure investigationFailure investigation
Slotted-in steel platedoweldowel
In failure joint only 7 of 33 dowels were found !
Inadequate formula in old draftof Eurocode 5 for block shear failure Corrected years beforefailure. Corrected years before the incident.
Failure investigationFailure investigation
• Roof elements constructed as two-span, but support reactions on trusses designed for single-span (+25%)
• Lateral stability of trusses not sufficient progressive failure
Design related causes(X)strength design
environmental actionsConstruction related causes
(X)
Poor principles during constructionAlterations on-site of intended design or products
Deficiency in design rules / codesDeficiency in design rules / codesprediction of capacityExtreme loading
Causes related to wood material and wood productsInadequate quality of wood materialPoor manufacturing principles for wood productsg p p pManufacturing errors in factory
Causes related to utilisation of the structureMisuse or lack of maintenance of the structure
X
Misuse or lack of maintenance of the structureOther causes
Buckling of trussesBuckling of trusses
Buckling of trussesBuckling of trusses
Design related causesstrength design environmental actions
Construction related causes
X
Poor principles during constructionAlterations on-site of intended design or products
Deficiency in design rules / codesDeficiency in design rules / codesprediction of capacityExtreme loading
Causes related to wood material and wood productsInadequate quality of wood materialPoor manufacturing principles for wood productsg p p pManufacturing errors in factory
Causes related to utilisation of the structureMisuse or lack of maintenance of the structureMisuse or lack of maintenance of the structure
Other causes
What can we learn from failures?What can we learn from failures?
• All materials are different: steel, concrete and timber have common but also their special problems and failures types
An engineer good at designing steel and / or concrete structures is not automatically excellent at designing timber structures
What can we learn from failures?What can we learn from failures?
E t d ti / t i i h ld b id d tExtra education / training should be provided to introduce engineers to the special problems in timber engineeringtimber engineering
ImplementationImplementationdatabase / books on failure cases – learn from each others mistakesH db k/ t i l f d i f ti b t tHandbook/ material for design of timber structures including good and bad solutions, focus on typical problems Courses in design of timber structures
t l t l i t l t lexternal control – internal control
LiteratureLiterature• Kaminetzky, D.: Design and Construction failures - lessons from
forensic investigations McGraw-Hill 1991forensic investigations, McGraw-Hill, 1991• Martin Hansson: presentations on failures in timber structures, 2004• Accident Investigation Board Finland
http://www.onnettomuustutkinta.fi/2601.htmp• http://www.maintenanceresources.com/ReferenceLibrary/FailureAnalys
is/FailureModes.htm• Dröge, G., Dröge, T.: Schäden an Holztragwerken, Schadenfreies
Bauen Band 28 Fraunhofer IRB Verlag 2003Bauen, Band 28, Fraunhofer IRB Verlag, 2003• Brand, B., Glatz, G.: Schäden an Tragwerken aus Stahlbeton,
Schadenfreies Bauen, Band 14, Fraunhofer IRB Verlag, 2005 • Oehme, P., Vogt, W.: Schäden an Tragwerken aus Stahl,Oehme, P., Vogt, W.: Schäden an Tragwerken aus Stahl,
Schadenfreies Bauen, Band 30, Fraunhofer IRB Verlag, 2003• Frühwald et al, 2007: Design of safe timber structures – how can we
learn from structural failures in concrete, steel and timber? Division of Structural Engineering Lund University Report TVBK 3053Structural Engineering, Lund University, Report TVBK-3053
• STEP, Timber Engineering, 1995