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

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Jul 18, 2012 (4 years and 9 months 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, the
use of wood-framed Type V construction for mid-rise multifamily and mixed-use buildings has
greatly increased in recent years. Constructing multi-story buildings with wood has several
advantages. Wood framing and sheathing are readily available and economical. There are
countless construction crews that have the necessary skills to construct wood-frame buildings.
The use of wood framing also results in a clear load path that is easily understood by both
designers and contractors.

However, with the prevalence of wood-frame multi-story buildings, it is important that designers
understand how to design these structures to resist high wind forces. Some of the fastest growing
markets in the U.S. 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 together 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 International Building
Code (IBC), or in the reference document 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 ASCE 7-05, there are four options for determining wind loads. Method 1 is the simplified
procedure. This method is very similar to the simplified procedure that was contained in the 2003
IBC. The simplified procedure can be used on simple diaphragm buildings that have a mean roof

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height that is 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.

Method 2 in ASCE 7 is called the analytical procedure. Within this procedure, 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 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.

Even though wind loading information has been removed from the IBC, there is still design
guidance given in Chapter 16. Section 1604.4 in the IBC 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.” The section goes on to say that “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 two
separate forces: shear and overturning.

Shear Force
Shear force must be transferred from the top of the wall to the bottom of the wall. It’s 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 the
thickness of the panel, different levels of shear resistance can be achieved. The amount of shear
in each panel depends on the wind speed, exposure, size of the building in the perpendicular
direction, height of the building, and the amount of sheathing on the shear wall. The more
sheathing on the wall, the less each panel has to work. Since plans today often require many
openings in the walls, there is less and less sheathing on the walls to act as shear walls, so each
panel tends to have to resist more shear.


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Tips for Shear Wall Specification:
1. Take advantage of shear wall capacity increases. The IBC permits the shear wall shear
capacities listed in Table 2306.4.1 to be increased by 40% for wind design.
2. Use more nails to achieve higher capacities. To achieve higher shear capacities, try
decreasing the fastener spacing at panel edges before increasing the panel thickness. This
is a more economical option, and thinner panels tend to be more readily available.

3. Specify sheathing grade, if possible. Sheathing grade wood structural panels are more
readily available than Structural I grade. If higher capacity is needed, decrease nail
spacing first, then increase thickness. Use Structural I sheathing as a last resort.

4. Specify framing species 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. Specify Southern Pine or Douglas
Fir-Larch framing (plates and studs) if the tabulated shear capacities are required.

5. If necessary, combine wood structural panel capacity with gypsum capacity. For wind
design, the 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.
Use this option only if you are confident that the gypsum board will not get wet or is
water resistant. Also, be sure to specify the fastener type and spacing for the gypsum
shear wall. Be careful 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.

6. Stay away from staples in high wind areas. Staples are harder to place correctly so that
both legs penetrate into framing. Also, they tend to have a less corrosion-resistant finish
since they are thinner than nails.

7. Use larger anchor bolts at the sill plate. If high shear forces result in close spacing of 12.7
mm (1/2”) anchor bolts, increase anchor bolt size to 15.88 mm (5/8”) rather than spacing
smaller anchor bolts closer together. The 15.88 mm (5/8”) anchor bolts have almost 50%
higher shear capacity than 12.7 mm (1/2”) bolts.

8. On multi-story buildings, use a continuous tiedown system tied off at every floor to
achieve economies in rod sizing, as well as decreased deflections. (see discussion below).

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9. Consider compression as well as tension forces at edges of shear walls. Use of a holdown
system is well understood to resist overturning of shear walls. The opposite edge of the
shear wall 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 to it. Overturning is resisted by anchoring the end of the panel 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.

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. Keep in mind that 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.

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 that it stays upright, 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 does require 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.

It is prudent for designers to evaluate, with the building owner, 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.


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

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 story below. For upper-story shear walls where the overturning force does not
exceed 4,000 to 5,000 pounds, a strap from the upper story studs to the lower story studs can be a
cost-effective option.

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. Generally
speaking, floor-to-floor holdown systems work well for structures three stories and under. For
buildings higher than that, it’s possible for holdown forces at the lower floors to exceed the
capacity of readily available holdown anchors.

Continuous Tiedown Systems
Continuous tiedown 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 tiedown systems use steel tension elements such as threaded
rods up to 28.6 mm (1⅛”) in diameter or more, sometimes from high strength steel, to resist
overturning forces in excess of 40,000 pounds. This rod is then anchored to the foundation.

One type of continuous tiedown system is a simple and cost-reducing variation of the holdown
system. This system uses a continuous rod from the foundation to the upper most holdown. Each
holdown is tied into the rod. At each level, the anchor rod can be sized for the amount of
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 the upper stories.

In a more common type of continuous tiedown system, overturning forces are resisted by holding
down the top of the end post(s) in the shear wall through bearing from above (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 tiedown element, usually
a steel rod or cable, by bolting it to a large plate washer which then restrains the surface above
the wall.

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Skipping Floors
As described above, 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 tiedown
systems there is the temptation to not tie-off each story, instead skipping stories or even just
running a rod or cable from the foundation all the way to the roof, employing a single point of
bearing to resist all of the overturning in the stacked multi-story shear wall. However, skipping
stories can have very negative consequences on structural performance.

When stories are skipped, the force in the continuous tiedown 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 tiedown element will stretch under load, having a higher load over a longer element
length leads to increased deflection both vertically in the tiedown element/posts/floors and
laterally in the shear wall system as a whole. Since each shear wall that is looking for
overturning resistance pushes ‘up’ until it finds the resistance offered by the continuous tiedown
system, all of the vertical movement due to rod stretch and wood compression will accumulate at
the bottom of the posts in the lowest story of a group of stories that are 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 structural integrity of
the entire assembly. While this effect is easy to rationalize through calculations and engineering
mechanics, this behavior has also been observed in full-scale tests.

The picture above shows testing performed at the Tyrell Gilb Research Laboratory, Simpson
Strong-Tie’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 to transfer the shear in the sheathing to the foundation is through weak
axis bending of the studs. The failure of the sheathing nailing progresses rapidly 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 tiedown systems is wood shrinkage. Wood shrinkage
in multi-story structures can lead to gaps forming between the structure and the restraint points
of the continuous tiedown system. To compensate for this, manufacturers of these systems offer
some form of shrinkage compensation device. One such device is shown below.


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Continuous Load Path
So it can be seen that a complete load path for resisting the 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 system must be
designed from the top of the building working down toward the foundation, the uplift resisting
system is designed the same way.

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 in order to determine the magnitude of the uplift force at the framing
member support. In wood framing, this force is typically resisted through the use of 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 Specification
1. 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
of the wall is hard to access. When the roof-to-top plate connection is on the inside of the
wall, ensure that the top plate to stud connection is also installed on the inside of the wall.
Full-scale testing at Clemson University and laboratory testing at Simpson Strong-Tie
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, then it is
acceptable for the remainder 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 of the wall.

2. Avoid 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

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overlap when studs and roof framing line up. Specify connectors that wrap around the
plate instead of nailing to the side to avoid nail overlap.

3. Use 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, simply specify full length nails through the straps and
sheathing and into the studs. Straps are rated by their manufacturer based on no gap
between the two connected members. For floor-to-floor connections, refer to
manufacturer’s Clear Span Table, which will list allowable loads with a gap in the middle
of the strap for the floor system.

4. Know your nails. Keep in mind the fasteners that will be required to install the connectors
that you specify. 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 will 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’s best to specify a series of connectors that require the
same type of nail.

5. Know your wood species. The allowable load of a connector, or any fastener for that
matter, will vary 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, such as 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 in order to use the Southern Pine values. If these higher connector capacities are
required, be sure to specify the type of wood for framing such as 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.

6. Include dead load. Allowable stress load combinations in the IBC and ASCE 7 permit
60% 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.

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7. 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 that this be avoided if at all possible. It can be
minimized by either 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 by
specifying a large plate washer under the anchor bolt that will help hold down the edge of
the sill plate.

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

Continuous Restraint System
One of these alternate methods is similar to the continuous tiedown system in that it uses a single
all thread rod or cable from the top plate to the foundation. The capacity of the rod tiedown itself
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 does require 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 tiedown 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 m (6
ft.) to 2.4 m (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 be used 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. But if the designer must use the sheathing to resist uplift at the top plates, this begs
the question of why not just use sheathing itself to resist uplift? This will be discussed later.

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. So horizontal members, such as plates and rim joists will become shorter, causing
the overall wall height to become less. If the continuous tiedown 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.

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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 equation
AE
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)

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

Use of a continuous tiedown system for shear wall overturning restraint was discussed
previously. 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 its capacity is not reduced due to close spacing of anchors.

Use of cables for a continuous tiedown 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 not effective, since the
cable will relax 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
fasteners that must be used to resist shear. One challenge in using this method is the ability to
install enough 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.


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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. Simpson Strong-Tie has performed
testing in our Tyrell Gilb Research Laboratory 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
SBCCI SSTD 10-99, Standard for Hurricane Resistant Residential Construction. This standard
was written in the 1990’s to comply with older Standard Building Code requirements. Simpson
Strong-Tie constructed a wall using the guidelines from this document, and tested it for
resistance to combined uplift and shear loads. Unfortunately, the wall was unable to achieve the
design uplift and shear loads with even a minimal safety factor of two. The failure was splitting
of the bottom plate as shown in Figure XB357Y.

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 lack 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. Simpson Strong-Tie is continuing to perform
testing 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 that well-designed and built buildings with a good load path can endure these types of
forces. They also have shown that even a few key connections that lack 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.


Simpson Strong-Tie Company Inc. 5956 W. Las Positas Boulevard Pleasanton, CA 94588 Phone: 925.560.9000 Fax: 925.847.1605 www.strongtie.com