session 25 ic2011 vatovec

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DES

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BILI

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www.sgh.com

ENGINEERS BENEFIT FROM WOOD SCIENCE AND TECHNOLOGY KNOWLEDGE

Milan Vatovec

© 2007 Simpson Gumpertz & Heger Inc. Proprietary and Confidential

Introduction

Objective to show importance of understanding wood as a material in engineering projects and applications

Real projects used as case-study examples:

– Evaluation of wood biodeterioration

– Structural assessment, in-situ stress grading

– Dimensional stability (moisture movement) investigations

– Analysis, design, repair and rehabilitation of wood structures.

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Wood Science, Technology and Engineering

Applying knowledge pertaining to physical, mechanical, and chemical properties of wood and wood products, as well as historic and current construction practices, to engineering applications.

Organic material: behavior is influenced by physical and mechanical properties, the natural growth characteristics, and effects of biological and other deterioration agents.

Because of its complexity, its material composition, orthotropic nature, and variable response to environmental conditions, optimal use of wood often benefits from special knowledge requiring integration of material science, structural analysis and design, and construction practices.

Rare amongst most structural engineers


Biodeterioration of Wood - General

Microbiological agents: fungi (rot), insects, bacteria, marine borers.

Risk of attack varies depending on application (end use) and on present conditions.

Special conditions needed for fungal deterioration: temperature, oxygen, right amount of moisture, food source.

Often, insect, fungal, or marine-borer attack can be hidden (is not visible from the outside of the member), and the structural integrity can already be lost - can result in sudden, catastrophic failures.

Incipient decay is a significant factor – Material properties can be significantly affected without change in appearance

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Fungal Decay


Damage Is Often Buried or Hidden

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Could Also Be Hard to Reach


It Could Also Be Visible and Causation Known

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© 2011Simpson Gumpertz & Heger Inc. Proprietary and Confidential

Insect Attack


Marine Borers

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Damage could be economically staggering even if not structurally significant

Problems frequently due to poor waterproofing design or construction detailing

Contractor or designer at risk of being blamed for a deficient product that resulted in a multi-million dispute over decay damages (e.g. window and framing problems with condominium structures)

Engineer must not only be able to recognize and correct the damage, but also to evaluate the cause, extent, and the ensuing cost to repair the damage –understanding wood as a material is essential.

Deterioration of Architectural Members


Rotting of Wood Members Due to Exposure to Water

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Understanding the Type and Extent of Damage


Remedial Solutions Need to Consider the Underlying Cause of Problem

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Wood Biodeterioration - Takeaways Usually caused by water infiltration through isolated

leaks, condensation, systemic breaches, etc.

Important to understand the exposure, risks, causation, extent, and degree of problem before offering design, detailing, or remedial solutions.

Water path is not always obvious: wood-educated inspector must be able to recognize all signs of distress, should understand current and historic methods of construction, and be able to recognize potential locations and conditions for attack.

Understanding biodeterioration mechanisms helps prevent wood loss in service, allows effective evaluation and remediation, and enables prediction of remaining useful life: crucial for wood applications.


Natural Growth Characteristics of Wood

Reclamation, renovation, and redevelopment of old timber buildings is resulting in the need for evaluation and reuse of old wood and timber structural members.

Drawings are seldom available – engineer must assess the condition and evaluate wood members for the new role (e.g. higher loads due to change of use).

Conventionally educated engineers lack understanding of particularities associated with wood species, natural growth characteristics, their effect on member strength, etc.

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Splits, Shakes, Slope of Grain


Knots, Reaction Wood

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In-Situ Stress Grading

Ability to accurately assess the existing member strength can be a powerful tool, often resulting in significant savings due to less required retrofit.

ASTM D 245 – Standard Practice for Establishing Structural Grades and Related Allowable Properties for Visually Graded Lumber

Knot size, species, wood defects and their location, slope of grain, moisture content are considered to arrive at allowable strength for individual members –typically higher than based on conventional grading.

Extremely useful in certain situations (small areas seeing large loads, condition assessments, use of reclaimed timber, etc.)


In Situ Wood Stress Grading (ASTM D 245)No. Name, Code Classification MC Rings / in. Slp of Grn Shakes Checks Splits

Width Depth Length (in.) (DIM, B&S, P&T, SR) (%) 1 in: Size Location Size Location Size Location (in.) (in.) (in.)

1 par 6 12 96 B&S 11 8 12 2 1.75 3 1 1.5 2

2 par 6 12 96 B&S 11 7 15 1.5 1 2 1 1.5 1

3 par 6 12 96 B&S 11 6 15 2 0.5 2.5 1 1.5 1.5

4 par 6 12 96 B&S 11 8 9 0.5 2 1 1 1.5 2

5 par 6 12 96 B&S 11 9 14 0.25 1.75 3 1 1.5 2

6 par 6 12 96 B&S 11 11 12 1 1.25 2 1 1.5 1

7 par 6 12 96 B&S 11 12 11 1 1.75 2.5 1 1.5 1.5

8 par 6 12 96 B&S 11 8 15 2 2 1 1 1.5 2

9 par 6 12 96 B&S 11 7 9 1 1 3 1 1.5 2

10 par 6 12 96 B&S 11 8 12 1.5 1.5 2 1 1.5 2

11 par 6 12 96 B&S 11 9 8 2 0.5 2.5 1 1.5 2

12 par 6 12 96 B&S 11 8 18 3 0.25 3 1 1.5 1

13 par 6 12 96 B&S 11 8 19 2 1.75 1 1 1.5 1.5

14 par 6 12 96 B&S 11 6 12 1 1.75 3 1 1.5 2

15 par 6 12 96 B&S 11 8 8 2 0.5 2 1 1.5 2

16 par 6 12 96 B&S 11 9 10 1 1.25 2.5 1 1.5 2

17 par 6 12 96 B&S 11 9 12 1 1.75 3 1 1.5 1

18 par 6 12 96 B&S 11 8 12 2 1.75 1 1 1.5 1.5

19 par 6 12 96 B&S 11 8 11 1 0.75 2.5 1 1.5 2

20 par 6 12 96 B&S 11 8 12 0.5 1.75 3 1 1.5 2

Dimensions (in.) Knots on Narrow Face Edge Knots on wide Face Middle Knots on Wide Face

ALLOWABLE PROPERTIESNo. Species Fb Fcpar Fcperp Fv Ft E

(psi) (psi) (psi) (psi) (psi) (1000 psi)

1 SYP 1678 1172 602 116 1126 1566

2 SYP 1849 1295 602 129 1241 1566

3 SYP 1678 1234 602 129 1126 1566

4 SYP 1289 1018 602 116 865 1409

5 SYP 1800 1172 602 116 1208 1566

6 SYP 1678 1265 602 129 1126 1566

7 SYP 1484 1141 602 129 996 1566

8 SYP 1678 1295 602 116 1126 1566

9 SYP 1289 1018 602 116 865 1409

10 SYP 1678 1265 602 116 1126 1566

11 SYP 1289 1018 602 116 865 1409

12 SYP 1289 1172 602 129 865 1409

13 SYP 1678 1295 602 129 1126 1566

14 SYP 1678 1172 602 116 1126 1566

15 SYP 1289 1018 602 116 865 1409

16 SYP 1484 1141 602 116 996 1566

17 SYP 1678 1172 602 129 1126 1566

18 SYP 1678 1265 602 129 1126 1566

19 SYP 1484 1141 602 116 996 1566

20 SYP 1678 1172 602 116 1126 1566

No. Species Fb Fcpar Fcperp Fv Ft E

(psi) (psi) (psi) (psi) (psi) (1000 psi)

1 SYP 1678 1172 602 116 1126 1566

No. Name, Code Classification MC Rings / in. Slp of Grn Shakes Checks Splits

Width Depth Length (in.) (DIM, B&S, P&T, SR) (%) 1 in: Size Location Size Location Size Location (in.) (in.) (in.)

1 par 6 12 96 B&S 11 8 12 2 1.75 3 1 1.5 2

Dimensions (in.) Knots on Narrow Face Edge Knots on wide Face Middle Knots on Wide Face

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Hygroscopic Nature of Wood

Wood, when under FSP, undergoes dimensional changes in service due to fluctuations in temperature and humidity of the environment.

Wood floor and finish performance very sensitive to system design intricacies and installation procedures

Environmental control during installation and in service critical.

Compatibility between materials and components is key


Floors Buckle, Delaminate, Move

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Compatibility, Restraint, Movement


Hygroscopic Nature of Wood - Takeaways

Designer (or the installer) must be aware of the sensitivity to moisture fluctuations and should anticipate wood movement in service – cause of a lot of investigations involving responsibility allocation.

Problems with wood flooring and woodworking finishesmost common troubleshooting projects, often caused by incompatible materials.

Development of new systems: lack of behavioral and compatibility consideration in design can be disastrous.

Shrinkage or swelling can cause structural distress as well

Special engineering techniques: dimensional back-calculations, FEM, etc.

Wood-behavior knowledge and familiarity with standard design and construction practices are essential.

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Design of New Wood Structures, Repair and Rehabilitation of Existing Wood Structures

Wood design offered maybe as a one-semester course at many accredited engineering schools in the US – covers codes, analysis and design methodology

Little emphasis placed on the orthotropic nature of the wood, detailing, old construction practices, analysis of existing structures, heavy timber structures and connections, etc.

Literature is available, but few use it

Several examples discussed here:


Design of new, large residential homes: many new McMansions require complex engineering – detailing important

Can we really neglect seismic forces?

Design of New Wood and Timber Structures

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Wood structures often exposed to view and may require unorthodox structural solutions – understanding of available options important.

Design of New Wood and Timber Structures


Troubleshooting Engineer must be able to recognize weak points in the

existing timber structures (notched members, tension members, serious natural defects, unorthodox connections) both as a designer and as an investigator, even though failure may not be imminent.

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Imminent failures must be recognized and remediated.

Troubleshooting


Repair and Rehabilitation

Important to understand the existing structure to be able to determine the underlying cause of problem and arrive at an adequate and cost-effective repair solution

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“Repairs” sometimes require repairs;

Shoring may be needed until permanent repairs are done



Strengthening of existing structures: count on load sharing, consider creep, jack load into new elements, understand the interaction between the structure and the strengthening elements – must understand wood


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Conclusions In-service wood performance problems can have significant

public safety, loss of utilization, and economic consequences.

Specific wood-material and wood-engineering knowledge allows engineers to extend the useful service life of wood structures and systems:– Biodeterioration mechanisms– Moisture-driven compatibility of displacement issues – Microscopic wood species and problem identification – In-situ grading– Special construction and detailing knowledge, etc.

This special skill will be more significant and needed in the future, with technological advancements allowing utilization of engineered-wood products and multi-material systems in a wide range of previously unattainable applications.


QUESTIONS?