By Andrew P.
DiGiammo
As a design-build contractor specializing in custom homes near
thewater, I have to take wind loads into account on every
project. The New England coastline where I work has been hit by
hurricanes before and no doubt will be again; homes here are
supposed to be ready to handle a storm.
I've been using wind-resistant design and construction details
for as long as I've been a builder, but this year I've been
making some adjustments. That's because I face the same
situation as many builders in other coastal states: Rhode
Island and Massachusetts, where I work, are moving to the
2000 International Residential Code (IRC) and the
International Building Code (IBC). The International
Codes contain provisions for wind-resistant construction that
are quite a bit tougher and more detailed than the rules we
used to have.
Even for builders like myself who have long experience with
wind-resistant details, now is a good time to review our
approach to wind-resistant construction. If you don't know much
about wind issues, and you plan to build near the coast, doing
some homework will help you stay out of trouble.
The new rules cover a whole range of complicated issues,
including the way houses are anchored to foundations and the
way components and claddings are attached to the structure.
There's also a new set of requirements for windows. That's too
much to cover all at once, so in this article I'll focus on one
aspect of a wind-resistant building: shearwalls.
Understanding Wind Loads
Every framer has an instinctive awareness of gravity loads:
Common sense says that your walls have to hold up the floors
and the roof. Lateral loads aren't so obvious, but near the
coast they are important: Wind applies a sideways force to your
building that can be stronger than the gravity loads on the
floors and roofs. The Rhode Island waterfront house shown in
this article, for example, had design wind pressures of 35 psf
on the building. That house needed several beefy interior
shearwalls to pick up the load.
Wind acts on a
building's structure in several primary ways. Uplift, the
suction force that could tear the roof off or lift the building
off its foundation, is important to consider, but it's not
central to this discussion. What matters more here are the
racking, sliding, and overturning effects: When winds try to
roll the house over or slide it horizontally, floor diaphragms
and shearwalls come into play (see illustration, below).
Conventional structurally sheathed exterior walls withstand
some of the lateral force exerted by high winds but are not
strong enough in 110- and 120-mph zones. By contrast, shear
panels, which can be incorporated into standard frame walls,
are engineered specifically to handle all sliding and
overturning forces without relying on other elements of the
frame.
In reality, the force of wind is variable and unpredictable,
and there is no way to define it exactly. To get numbers we can
use for design loads, we have to oversimplify. In the code, it
comes down to wind speed zones: The higher the wind speed zone,
the greater the pressure. There are also exposure categories:
If you're building right on the water where wind has a clear
shot, you use a higher-design wind load than if your site is
protected by woods, rough terrain, or other buildings.
Shearwalls at Work
How do shearwalls handle wind? The easiest way to understand
wind is to think of it as a constant load pushing against a
wall face. It loads the wall the way furniture and people load
a floor, only horizontally rather than up and down. Wall
sheathing collects the wind pressure and delivers it to the
studs, which in turn carry the load to the top and bottom of
the wall and apply a force to the floor systems. Next, the
plywood-sheathed floor picks up the force, acting like a deep,
thin, sideways beam, and carries the load over to the
shearwalls. The shearwalls, in their turn, restrain the floor
diaphragm and carry the load to the foundation and into the
earth. On larger buildings, end shearwalls typically need help
from interior shearwalls.
To do their part, the shearwalls have to be stiff enough to
resist racking, and they must also be anchored against sliding
and overturning. The stiffness comes from plywood or OSB
sheathing (I use plywood). To pin the wall in place, the easy
choice is a manufactured hold-down like Simpson Strong-Tie's
HD5 series, which I used on my current Rhode Island project.
You could design your own connector if you could find an
engineer to okay it, but in my experience it's so convenient to
work with Simpson products that I don't bother looking for an
alternative.
Stud and plywood framing isn't the only way to build a
shearwall. In commercial construction, engineers might call for
a steel moment frame or a reinforced masonry shearwall to pick
up lateral loads. But steel and masonry aren't easy to mix with
wood framing; they interfere with wiring, insulation, flashing,
and everything else. In houses, wood-framed shearwalls make
more sense.
Even without special detailing, a stud wall with plywood or
OSB sheathing has a lot of shear capacity. A "braced wall
section" that will satisfy the IRC prescriptive path is just
like a normal sheathed wall. It's permitted in lower-wind-speed
zones. But it's not an engineered solution — code
acceptance of braced wall sections is based on tradition and
experience.
To get an actual shearwall with predictable strength, you
don't change much: It's still 16-inch on-center stud framing
with a single shoe and double top plate, typically sheathed
with a 1/2-inch panel (or thereabouts), using 8d nails. But you
need hold-downs of some sort at both ends of the wall
— concrete anchors, or a floor-to-floor connector in
the case of a second-floor shearwall. The hold-downs need to
connect to a double stud, or sometimes even a single 4x4
member.
To strengthen the basic shearwall, you can increase the
perimeter nailing, use thicker plywood, or go up to 10d nails
— or some combination of all three. If the wall has to
handle a bigger load, it may also need beefier hold-downs.
Specifications that match nail size, spacing, and panel
thickness to shearwall allowable capacity are available from
APAThe Engineered Wood Association at
www.apawood.org.
New Codes, New Loads
While design and construction of shearwalls have not changed,
the codes that tell you where and when you need a shearwall
have. Based on the lessons of hurricane damage in the 1990s,
the IRC and IBC have incorporated a number of important
changes. One is that the map of wind-speed zones has been
modified to reflect new data collected by modern instruments.
The new map is published in ASCE-7, the American Society of
Civil Engineers handbook that governs wind and seismic building
design.
The IBC now requires houses in wind speed zones greater than
110 mph to be designed according to ASCE-7, or according to one
of the documents based on it (see "
For More Information," below). If your house is exposed to
a wind speed of less than 110 mph, you can use the prescriptive
methods given in the IRC, and for speeds of less than 100 mph,
the prescriptive methods give you more leeway. Below 90 mph,
conventional construction is allowed.
Of course, states that adopt the IRC and IBC can (and do)
amend them, adding their own special rules and compliance
paths. In Rhode Island, for example, the state has tried to
simplify even the prescriptive wind provisions in the IRC. In
place of the IRC's extensive section on "braced wall" options,
Rhode Island's "Appendix L" just calls for a 4-foot-wide
shearwall at every building corner and at least every 25 feet
— blocked at panel edges, nailed at 6 inches
on-center, and with an anchor and doubled studs at each end.
(Alternatively, a window opening may occur as close as 2 feet
from the corner, but only if the shearwall section is made 8
feet wide instead of 4.) It's simple, but if you deviate from
it, you need an engineer to review and approve the
change.
I don't particularly like the limits on window location or the
prescriptive tables they replaced. I never use the prescriptive
methods, even if I'm in a zone where they apply, because I know
that at some point I'm going to want a window or something
where the rules don't allow it. So I just get an engineer on
board from the beginning and assume that he'll be reviewing the
plans. That way I can control both the appearance and the
structure of the house.
Designing Around Shearwalls
Near the water, I like to use the shingle style, a tradition
that includes a lot of jogs and bays. The corners and short
wall sections add rigidity to the structure.
I always identify the shearwall requirements at the beginning,
before I start to make decisions about the floor plan. When I
see that I'm going to need interior shearwalls, I can place
walls that define the shapes of rooms in the house in a way
that lets me use those same walls for shearwalls and also as
bearing walls for the roof and floor gravity loads. It
simplifies both the engineering analysis and the construction
of the house, eliminating a lot of aggravation and
expense.
To simplify the structural analysis, it's helpful to locate a
few wall areas in each structure that can be left without
windows. That way I don't have to analyze walls with openings
("perforated" shearwalls). Instead, I have the freedom to slide
windows around any way I please in all the other
walls.