Session 25 ic2011 vatovec

DESIGN

ENGINEERS BENEFIT FROM WOOD
REHABILITATE INVESTIGATE

SCIENCE AND TECHNOLOGY
KNOWLEDGE

Milan Vatovec

www.sgh.com

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.

© 2007 Simpson Gumpertz & Heger Inc. Proprietary and Confidential

1

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

2

Fungal Decay


Damage Is Often Buried or Hidden


3

Could Also Be Hard to Reach


It Could Also Be Visible and Causation Known


4

Insect Attack

© 2011Simpson Gumpertz & Heger Inc. Proprietary and Confidential

Marine Borers


5

Deterioration of Architectural Members
 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.


Rotting of Wood Members Due to Exposure to Water


6

Understanding the Type and Extent of Damage


Remedial Solutions Need to Consider the
Underlying Cause of Problem


7

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.


8

Splits, Shakes, Slope of Grain


Knots, Reaction Wood


9

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. N e, C
am ode D ensions (in.)
im Classification MC R / in.
ings SlpName, Code
No.
of G rn Knots onN Classification
arrowFace
Dimensions (in.) MC
EdgeKnots onw Knots on NarrowM Edge Knots onW Face Shakes Shakes Checks
Rings / in. Slp of Grn
ideFace Face Knots on wide Face Middle Knots on Wide Face C
iddle ide hecks Splits
Splits
Width Depth
Width Depth Length(in.) (D , B S P T, S )
IM & , & R (%) 1in: S Length (in.) Location
ize (DIM, B&S, P&T, SR) (%)
Size 1 in: Size
Location Location
S Size
ize Location Size
Location Location
(in.) (in.)
(in.)(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

1 par 6 12 96 B&S 11 8 3
12par 6
2
12 96 B&S 11
1.75
6 15 2
3 0.5 2.5
1 1
1.51.5 1.5
2
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

ALLOWABLE PROPERTIES
No. Species Fb Fcpar No. FcperpFb
Species Fcpar Fv
Fcperp Fv Ft
Ft E E
(psi) (psi) (psi) (psi) (psi) (1000 psi)
(psi) (psi) 1 SYP(psi) 1678 1172 (psi)
602 116 (psi)
1126 1566 (1000 psi)
2 SYP 1849 1295 602 129 1241 1566
1 SYP 1678 1172 3 SYP 602 1678 1234 116
602 129 1126
1126 1566 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

10

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


11

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 finishes
most 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.

12

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 Wood and Timber Structures

 Design of new, large residential homes: many new
McMansions require complex engineering – detailing
important
 Can we really neglect seismic forces?


13

Design of New Wood and Timber Structures

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


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.


14

Troubleshooting
 Imminent failures must be recognized and remediated.


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


15


 “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


16

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?


17

Session 25 ic2011 vatovec

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Session 25 ic2011 vatovec