Design Solutions for Wood-Frame Multi-Story Buildings Resisting Uplift and Lateral Forces

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Jul 18, 2012 (5 years and 1 month ago)

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Design Solutions for Wood-Frame Multi-Story Buildings –
Resisting Uplift and Lateral Forces
By Randy Shackelford, P.E. and Steve Pryor, P.E., S.E., Simpson Strong-Tie
Published in the August 2007 issue of Construction Specifier magazine

Due in part to the more liberal height and area restrictions of the International Building
Code (IBC), the use of wood-framed Type V construction for mid-rise multi-family and
mixed-use buildings has greatly increased in recent years.

Building with wood on multi-story projects has several advantages. Wood framing and
sheathing are readily available and economical. There are also countless construction
crews with the necessary skills to construct wood-frame buildings. Additionally, the use
of wood framing results in a clear load path easily understood by both designers and
contractors.

With the prevalence of wood-frame multi-story buildings, however, it is important
designers understand how these structures must resist high wind forces. Some of the
fastest growing markets in the United States are areas along the Gulf Coast and
Eastern Seaboard, which are more susceptible to hurricanes and high wind events.

Understanding high wind forces
Regardless of the type of construction, buildings are subjected to two basic types of
loads under high winds. Uplift loads result from air flowing over the roof, causing a
suction force. Lateral loads result from wind blowing on the windward wall, as well as
wind blowing past the leeward wall. These two lateral forces act in the same direction
and combine to create a force that tries to push the building over or slide it in the
direction of the wind. Lateral loads can also result from wind blowing on a steep pitched
roof.

These loads and the requirements for their application are described in the IBC, or in
the American Society of Civil Engineers (ASCE) ASCE 7, Minimum Design Loads for
Buildings and Other Structures. In fact, with the removal of the simplified wind loading
requirements from the 2006 IBC, ASCE 7 is now the single source for designers in
calculating wind loads on a structure. (The wind loading section of the 2006 IBC only
includes such basic information as wind speed maps, exposure classification, opening
protection requirements and some roofing requirements.)

When designing based on the 2006 IBC, wind loads must be established using ASCE 7-
05. Within this reference document, there are different options for determining wind
loads.

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The simplified procedure
Method 1 is very similar to the simplified procedure contained in the 2003 IBC. The
simplified procedure can be used on simple diaphragm buildings that have a mean roof
height less than both 18.2 m (60 ft) and the least horizontal dimension of the structure.
The structures must be ‘regular-shaped,’ having an approximately symmetrical cross-
section with either a flat roof or a gable or hip roof with a roof slope ≤ 45 degrees, and
be enclosed as it relates to the wind-borne debris requirements.

The analytical procedure
Within Method 2, there are two sets of requirements—the low-rise procedure, and the
procedure for buildings of all heights. Building size and construction type will determine
which of these methods is applicable.

The wind tunnel procedure
The final method for determining wind loads in ASCE 7 is the use of a wind tunnel. This
is typically not cost-effective for all but the largest of buildings.

Design guidance
Although wind loading information has been removed from IBC, there is still design
guidance given in Chapter 16. Section 1604.4 in the code describes the extent of
analysis required, by stating:

Any system or method of construction to be used shall be based on a rational analysis in
accordance with well-established principles of mechanics. Such analysis shall result in a
system that provides a complete load path capable of transferring loads from their point
of origin to the load resisting elements … Every structure shall be designed to resist the
overturning effects caused by the lateral forces specified in this chapter.

In large, multi-story wood-frame construction, the shear forces can become quite high,
especially on the lower stories. This is intensified by the large number of openings for
windows and doors that building owners often demand. When wood shear walls are
loaded, they must resist both shear and overturning forces.

Shear force
Shear force must be transferred from the top of the wall to the bottom. It is resisted by
nailing panels at their edges to the wall framing (reduced nailing at the panel interior is
also provided to resist panel buckling and wind suction loads). By varying the nailing
and panel thickness, different levels of shear resistance can be achieved.


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The amount of shear in each panel depends on the wind speed, exposure, building
height (and size in the perpendicular direction), and the amount of sheathing on the
shear wall. The more sheathing on the wall, the less each panel has to work. Since
current plans often require many openings in the walls, there is less and less sheathing
to act as shear walls, so each panel tends to have to resist more force.

Tips for shear wall specification

1. The designer should take advantage of shear wall capacity increases. IBC
permits the shear wall shear capacities listed in Table 2306.4.1 to be increased
by 40 percent for wind design.

2. More nails can be used to achieve higher capacities. In other words, the fastener
spacing should be decreased at panel edges before one opts to increase the
panel thickness. This is a more economical option, and thinner panels tend to be
more readily available.

3. If possible, sheathing grade should be specified. Sheathing-grade wood
structural panels are more plentiful than Structural I grade. If higher capacity is
needed, nail spacing should be first decreased, followed by an increase in
thickness. (Structural I sheathing should be seen as a last resort.)

4. Framing species must be specified for highest shear values. Allowable shear
capacities listed in Table 2306.4.1 are for wood species having a specific gravity
of 0.50 and higher (Douglas Fir-Larch and Southern Pine). Since most other
common framing species have a lower specific gravity, the capacity must be
reduced. Southern Pine or Douglas Fir-Larch framing (plates and studs) should
be specified if the tabulated shear capacities are required.

5. If necessary, wood structural panel capacity can be combined with gypsum
capacity. For wind design, IBC permits the capacity of the wood structural panels
on one side of a shear wall to be added to the capacity of gypsum board on the
opposite face of the shear wall. This option should only be used when the
designer is confident the gypsum board will not get wet or is water-resistant.
Additionally, the fastener type and spacing for the gypsum shear wall must be
specified. One should be wary of using this combination when shear wall aspect
ratios exceed 1:1. As the aspect ratio gets higher, the deflection of the shear wall
under the increased allowable loads for wind can render the wall ineffective for
load sharing with gypsum interior sheathing.


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6. Staples should be avoided in high wind areas. Staples are harder to place
correctly so that both legs penetrate into framing. They tend to also have a less
corrosion-resistant finish since they are thinner than nails.

7. Larger anchor bolts should be employed at the sill plate. If high shear forces
result in close spacing of 12.7-mm (1/2-in.) anchor bolts, anchor bolt size should
be increased to 15.9 mm (5/8 in.), rather than spacing smaller anchor bolts closer
together. The 5/8-in. anchor bolts have almost 50 percent higher shear capacity
than their 1/2-in. counterparts.

8. On multi-story buildings, a continuous tie-down system, tied off at every floor,
should be used to achieve economies in rod sizing, as well as decreased
deflections.

9. Compression, as well as tension forces, must be considered at edges of shear
walls. Use of a holdown system is understood to resist overturning of shear walls.
In a shear wall, the edge opposite the holdown will be under compression, and
this force can exceed the overturning force due to dead load in the wall.


Overturning force
Overturning force is the tendency for a wood panel to tip over (overturn) when shear
force is applied. Overturning is resisted by anchoring the panel’s end to the foundation.
These forces depend on the amount of shear in the panel, the shear wall aspect ratio,
and any uplift from shear walls above. Again, as the shear forces increase due to
modern wall configurations, the overturning force also increases.


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Figure 1

Figure 1 shows a simplified example of how the shear and overturning forces increase
from the upper stories to the lower stories. Each floor must carry not only the shear and
overturning from that particular level, but also the shear and overturning from all the
walls above. This particular example shows extreme differences, since the shear from
wind on the roof was neglected, but it does illustrate the general principles.

In this four-story application, the shear on the lowest story is seven times the shear at
the roof level, and the overturning force at the lowest floor is 16 times that at the top
floor level. It is important to keep in mind this is a greatly simplified example, since it
does not consider the reduction in overturning from dead load, or the increase in
overturning due to the holdown anchor not being located at the actual edge of the shear
wall.


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Traditional holdown methods
In wood frame construction, shear wall overturning is resisted by hardware known as
holdowns. The goal is to anchor the edge of the shear wall so it doesn’t lift up, limiting
the horizontal wall movement, or deflection. The allowable wall deflection is specified in
the building code for seismic resistant construction, and is typically not permitted to be
more than 0.025 times the wall height for residential structures. The code requires
structural systems to be designed to limit deflections and lateral drift, but there are
currently no deflection limits for wind-induced in-plane shear deformations.

With the building owner, it is prudent for designers to evaluate acceptable levels of
architectural damage and user discomfort, and to limit the lateral drifts of shear walls
when loaded during a wind event. Designing shear walls with limited anchorage
deflection is one way to reduce wall lateral drift.

In one-story buildings, the holdown is often attached directly to a post, which then
connects to the foundation with an anchor bolt. Straps embedded into the foundation
and then attached to the post above are another popular solution (Figure 2).



Figure 2

For multi-story buildings, the traditional way to anchor an upper story shear wall against
overturning has been to install two holdown anchors (one on the upper story studs and
the other inverted on the lower story studs), which are then connected with a threaded
rod through the floor. This ‘floor-to-floor’ holdown system effectively transfers the
overturning force from the upper story to the one below. For upper-story shear walls

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where the overturning force does not exceed 17,793 to 22,241 N (4000 to 5000 lb), a
strap from the upper story studs to the lower story studs can be a cost-effective option
(Figure 3).



Figure 3

For typical multi-story applications as described above, the holdown at each floor level
must resist the overturning at that level, plus the overturning from the shear walls
above. Floor-to-floor holdown systems generally work well for structures three stories
and under. For higher buildings, it is possible for holdown forces at the lower floors to
exceed the capacity of readily available holdown anchors.

Continuous tie-down systems
Continuous tie-down systems have evolved as a cost-effective solution for buildings that
have these very high overturning forces. In this type of system, the overturning force is
collected in a single continuous rod. Continuous tie-down systems use steel tension
elements such as threaded rods up to 28.6 mm (1 ⅛ in.) in diameter or more,
sometimes from high strength steel, to resist overturning forces in excess of 177,929 N
(40,000 lb). This rod is then anchored to the foundation.

One type of continuous tie-down system is a simple and cost-reducing variation of the
holdown system. It uses a continuous rod from the foundation to the upper most

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holdown. Every holdown is tied into the rod. At each level, the anchor rod can be sized
for overturning force above that level, but the holdown can be designed for the
overturning force at that level only. As the force in the rod is reduced in upper stories, a
reducing coupler nut may be used to facilitate smaller rods in these locations (Figure 4).



Figure 4

In a more common type of continuous tie-down system, overturning forces are resisted
by holding down the end post top(s) in the shear wall through bearing from above. (This
is instead of holding down the bottom of the post with a directly attached holdown.) The
overturning uplift from each floor is then continuously and cumulatively collected in the
continuous tie-down element, usually a steel rod or cable, by bolting it to a large plate
washer that then restrains the surface above the wall (Figure 5).


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Figure 5

Skipping floors
As previously described, each story of a multi-story shear wall has had direct
engagement with the overturning restraint system and has thus been ‘tied-off’ at each
story. With continuous tie-down systems, there is the temptation to not tie-off each
story. Instead, some may skip stories or even just run a rod or cable from the foundation
all the way to the roof, employing a single point of bearing to resist all the overturning in
the stacked multi-story shear wall. However, skipping stories can have very negative
consequences on structural performance (Figure 6).



Figure 6

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When stories are skipped, the force in the continuous tie-down element is constant over
the entire height of the skipped area—it does not reduce in the upper stories as
described earlier. Since the continuous tie-down element will stretch under load, having
a higher load over a longer element length leads to increased deflection both vertically
in the tie-down element/posts/floors and laterally in the shear wall system as a whole.

Each shear wall looking for overturning resistance pushes up until it finds the
continuous tie-down system. Consequently, all the vertical movement due to rod stretch
and wood compression accumulates at the bottom of the posts in the lowest story of the
group tied together through skipping. This can lead to early failure of the nailing at the
lowest level shear wall at the bottom of the shear panel where it is nailed to the sill
plate, thus compromising the entire assembly’s structural integrity. While this effect is
easy to rationalize through calculations and engineering mechanics, this behavior has
also been observed in full-scale tests.



Figure 7

Figure 7 shows testing performed at a manufacturer’s large-scale structural engineering
research facility. In this particular test, a three-story tall shear wall assembly was not
tied off at each floor level. The picture shows the broken studs in the first story. When
the sheathing-to-sill plate nailing fails due to too much post uplift, the only load path left

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to transfer the shear in the sheathing to the foundation is through weak axis bending of
the studs. The failure of the sheathing nailing rapidly progresses up the studs through
all field and vertical edge nailing until the distance through which the shear must travel
down the stud to the sill plate becomes too great for the stud to handle.

Another issue to consider with continuous tie-down systems is wood shrinkage, which
can lead to gaps forming between the structure and the restraint points of the
continuous tie-down system. To compensate for this, manufacturers offer some form of
shrinkage compensation device (Figure 8).



Figure 8

Continuous load path
As illustrated in this article, a complete load path for resisting horizontal wind loads is
made up of a system of sheathing, nailing, framing, holdown anchors, and shear
anchors at the bottom of the wall. This system acts together to transfer the shear forces
to the foundation.

In the same way, a complete system of framing and connectors is used to transfer the
uplift loads from the top of the wall to the foundation. Just as the overturning resisting

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system must be designed from the top of the building working down toward the
foundation, so is the uplift resisting system.

First, the uplift loads on the roof-to-wall connection must be determined. This will involve
using either the simplified or analytical procedure of ASCE 7. These loads are then
used in an analysis of the roof framing system to determine the magnitude of the uplift
force at the framing member support. In wood framing, this force is typically resisted
using a hurricane tie, although toenails may be used for very low uplift resistance. The
traditional method then uses connectors at each joint in the wall along with the vertical
wall framing members to transfer the force from the roof to the foundation.

A connector would be specified to attach the top plate to the studs at the top level. Next,
straps would be used to connect the studs above each floor level with the studs in the
story below. At the lowest level, connectors attach the lowest stud to the sill plate.
Finally, anchor bolts with large plate washers transfer the uplift force to the foundation.
Tips for connector specifications are discussed below.

Hurricane tie location
Contractors generally prefer to install the roof-to-top plate connector on the inside of the
wall, especially on a multi-story building where the outside is hard to access. When the
roof-to-top plate connection is on the inside, it is important to ensure the top plate-to-
stud connection is also installed on the inside of the wall. Full-scale testing at Clemson
University (South Carolina) have shown a significant reduction in connection capacity if
these connections are made on opposite sides of the wall.

Once the connection has been made into the stud, it is acceptable for the rest of the
connections to be on the outside of the wall. For example, the floor-to-floor strapping
can be on the outside of the wall, even if the stud-to-plate connectors at the top of the
wall are on the inside (Figure 9).


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Figure 9

Avoiding connector overlap
Some high-capacity connectors are designed to be very wide so they can spread out
the load into the top plates. However, when using one of these from the roof framing to
the top plate and another type from the top plate to the studs, they can overlap when
studs and roof framing line up. Consequently, the specification should call for
connectors that wrap around the plate, instead of nailing to the side to avoid nail overlap
(Figure 10). Alternately, a single connector can be specified to connect the roof framing
directly to the stud below.


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Figure 10

Using straps for the floor-to-floor connection
It is acceptable, and often preferable, to install floor-to-floor strapping over the wood
structural panel sheathing that makes up the shear walls. When this occurs, full-length
nails can be specified through the straps and sheathing and into the studs. On another
point, straps are rated by their manufacturer based on not having a gap between the
two connected members. For floor-to-floor connections, the designer should refer to the
manufacturer’s clear span table, which will list allowable loads with a gap in the middle
of the strap for the floor system.

Know your nails
It is important to keep in mind the fasteners required to install the specified connectors.
Typically, specific hurricane ties, stud-plate ties, and straps are designed to use either a
short 10d nail or a short 8d nail, but not both. These days, contractors in coastal areas
have special pneumatic tools that install nails into connectors.

These special tools do two very important things—they install the correct size nail, and
they place the nail into the existing hole in the connector. Since contractors are likely to
be using one of these tools, it will be inconvenient to have to switch out nails when
installing different connectors. It is best to specify a series of connectors that require the
same type of nail.


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Know your wood species
The allowable load of a connector (or any fastener, for that matter) varies depending on
the wood type to which it is fastened. Connectors achieve their highest allowable loads
when nailed to Southern Pine or Douglas Fir-Larch lumber. However, other species
(e.g. Spruce-Pine-Fir) have become very common in recent years. Connector
manufacturers have worked hard to retest and publish connector capacities for Spruce-
Pine-Fir, but they are typically lower than values for Southern Pine or Douglas Fir.

Both wood members to which the connector is nailed must be Southern Pine to use that
species’ values. If these higher connector capacities are required, it is crucial to specify
the type of wood for framing (e.g. top plates and studs). As an added benefit, shear
walls will have higher allowable shear loads when constructed of Southern Pine or
Douglas Fir framing.

Include dead load
Allowable stress load combinations in IBC and ASCE 7 permit 60 percent of the dead
load likely to remain in place during a design event to resist wind loads. In a multi-story
building, dead loads increase as the load path moves from the top of the building toward
the bottom. In this way, with discrete connectors, the connector size can be reduced as
the wind load decreases due to increasing dead load.

Avoid cross-grain bending in sill plates
At the bottom of the wall, connectors may be nailed to the outside or inside face of the
sill plate, while the sill plate is held down in the middle by the anchor bolt. This causes a
phenomenon known as ‘cross-grain bending’ in the bottom plates.

Design standards require this be avoided if possible. It can be minimized by one of
three methods:
⋅ installing connectors on both sides of the plate;
⋅ installing a U-shaped stud-to-plate connector that wraps under the sill plate and up
both sides of the stud; or
⋅ specifying a large plate washer beneath the nut on the anchor bolt that will help hold
down the sill plate’s edge.

Alternate methods
The continuous load path using connectors at each joint in framing has been
successfully used for years for wind-resistant wood construction. In recent years,
alternate methods have been proposed to achieve the same result.

Continuous restraint system

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One of these alternate methods is similar to the continuous tie-down system in that it
uses a single all-thread rod or cable from the top plate to the foundation. The rod tie-
down’s capacity can be calculated fairly easily as the lesser of the tensile capacity of the
rod, the bearing capacity of the washer at the top, and the capacity of the anchorage to
the foundation at the bottom. However, design of the entire anchorage system requires
extensive attention to detail.

At first glance, the rod spacing would seem to be easily determined by dividing the
linear uplift force at the top plate by the rod tie-down capacity. One important item is the
design of the top plate to resist bending from the uplift loads in between rods. At the
normally suggested 1.8 to 2.4 m (6 to 8 ft) spacing, the ability of the top plate to carry
uplift loads in bending between anchors is severely limited. This is especially true when
there is a splice in the double top plate. If the wood structural panel sheathing is to help
resist bending in the top plates, additional fastening of the panel must be designed,
since the existing fasteners are being used to resist shear.

Another critical issue when using a continuous system in multi-story applications is
wood shrinkage. As wood further dries after installation, it shrinks in the two directions
perpendicular to the grain. Consequently, horizontal members (e.g. plates and rim
joists) become shorter, causing the overall wall height to become less. If the continuous
tie-down system is not able to take up this shrinkage, it will not restrain the top plate
until the entire wall has lifted up an amount equal to the shrinkage.

In addition to wood shrinkage, wall uplift can occur when the tension members
themselves stretch under load. The stretch of a tension member can be calculated by
the following equation:

A
E
PL



Where:
P = total force in member
L = total length of member
A = cross-sectional area of member
E = modulus of elasticity of member (constant 29,000,000 for steel)

It can be seen that deflection will increase as the force in the rod or cable increases, the
length of the rod or cable increases, or the diameter of the rod or cable decreases.
Building codes limit the allowable load for a single uplift connector to 3.2 mm (1/8 in.) of
movement. Use of this limit is currently not codified for these types of systems, but
should be considered to maintain traditional performance.

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Use of a continuous tie-down system for shear wall overturning restraint has already
been discussed in this article. If the same rod is to be used to resist uplift and
overturning, its capacity must be increased. If separate rods are used to resist uplift and
overturning, their anchorage must be analyzed to ensure their capacity is not reduced
due to close spacing of anchors.

Use of cables for a continuous tie-down system presents many of the same challenges.
An additional concern with the use of cables is their reduced actual cross-sectional
area, which results in much more deflection than solid rod systems. Pre-tensioning is
ineffective, since the cable relaxes over time due to cable relaxation, wood shrinkage,
and compression settling until much of the pre-tensioning is no longer present.

Sheathing system
Another method of providing an uplift load path is to use the wood structural panel to
resist both the shear and uplift at the same time. The most common way to do this is to
install additional fasteners from the panel to the top and bottom plates to resist the uplift,
in addition to the ones used for resisting shear. One challenge in using this method is
the ability to install additional nails to achieve a significant uplift capacity without causing
splitting in the top and bottom plates. In multi-story wood frame construction, shear
walls may have very close fastener spacing, especially at lower levels.

While the shear capacity of nails in the plates can be calculated using the National
Design Specification for Wood Construction (NDS), the capacity of plates to resist
splitting or cross grain tension from this type of design cannot be calculated. This author
has participated in testing to explore the performance of walls with sheathing systems
resisting combined shear and uplift.

The only current code-referenced design guidance for this type of system is contained
in the Southern Building Code Congress International (SBCCI) SSTD 10-99, Standard
for Hurricane Resistant Residential Construction. This standard was written in the 1990s
to comply with older Standard Building Code requirements. This author was part of a
team that constructed a wall using the guidelines from this document, testing it for
resistance to combined uplift and shear loads. Unfortunately, the wall was unable to
achieve the design uplift and shear loads, even with a minimal safety factor of two. The
failure was splitting of the bottom plate as shown in Figure 11.


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Figure 11

In addition, when this standard was written, the perforated shear wall method was just
becoming popular, and the use of sheathing to resist uplift and shear was permitted for
all types of shear walls. However, requirements added to the 2003 IBC state that for
perforated shear walls:

perforated shear wall bottom plates at full-height sheathing shall be anchored for a
uniform uplift force, t, equal to the unit shear force, v, determined in Section 2305.3.8.2.5.

This is due to an increased uplift load applied to the sill plate from the lack of
overturning restraints at openings within the shear wall. A dearth of testing has
prompted recent discussions in the design industry on whether perforated shear walls
can resist any additional uplift, since the nails at the bottom plate are already resisting
uplift and shear. Testing is continuing to be performed on sheathed walls to determine
the conditions under which this type of system can be effective.

Mid-rise wood-framed buildings can be economically constructed to withstand high wind
events. All it takes is solid design, attention to detail, and good construction to create
the continuous load path needed for both uplift and lateral loads. The string of natural
disasters in recent years has shown well-designed and well-built buildings with a good
load path can endure these types of forces. They also have shown even a few key
connections lacking the strength to create a sufficient load path can have devastating
results for the entire structure. Through ongoing testing and research in both the public
and private sectors, new solutions are being developed that are helping to overcome
these structural challenges and ensure we have strong, safe structures now and in the
future.


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About the Authors
Randy Shackelford, PE, is a research engineer/building code specialist with Simpson
Strong-Tie. He has focused his career on wind-resistant construction for the last 20
years, including 13 with Simpson Strong-Tie and seven with the Texas Department of
Insurance. He currently serves on the American Forest and Paper Association Wood
Design Standards Committee and the ICC Ad-Hoc Committee on Wall Bracing.

Steve Pryor, PE, SE is the building systems research and development manager for
Simpson Strong-Tie. Joining the company in 1997, he now manages the company’s
Tyrell Gilb Research Laboratory in Stockton, California, which features state-of-the-art
seismic and high wind testing equipment. Steve is considered a subject matter expert in
structural building design and is a member of several national building code
committees.