Shearwalls for Coastal Homes, continued
Nuts and Bolts of Hold-Downs
For shearwalls to work, it's critical to block and nail the
plywood panels to the framing according to the engineer's
nailing schedule. It is also vital to properly install all
hold-downs in the specified locations. The well-fastened
plywood gives the wall enough strength to resist racking forces
from the applied lateral loads, while the hold-downs keep the
wall from lifting, rotating, or sliding.
To link stacked shearwalls through the
wood floor, a carpenter drills a tight hole from upstairs
(above left), using the bracket to locate the hole and a
torpedo level to stay plumb. He hammers the rod through (above
right) and sets the brackets, then drills holes into the studs
(bottom left) for lag screws. In the basement, he hammer-drills
holes in the concrete, then anchors the threaded rod into the
hole using high-strength epoxy (bottom, center and
right).
For the house shown, my engineer specified 1/2-inch plywood on
one face of each shearwall, nailed at 4 inches on-center on the
perimeter with 8d common nails. He called for Simpson HD-type
hold-downs at both ends of each wall (there are several
varieties of the HD and similar connectors, suited for use in
different situations).
To secure the basement shearwalls to the slabs and footings
they rest on, we first drilled a hole through the wall plate
and into the concrete, using a masonry drill and bit. Then we
placed threaded rod in the hole using an epoxy formula supplied
by Simpson Strong-Tie.
Neither my engineer nor I recommend placing anchors in the
concrete during the pour. The anchors have to be placed
accurately, and it is much easier to drill a hole in the plate
and footing than it is to locate a rod in the right place
during concrete work. And epoxy is actually stronger than a
concrete bond anyway, as long as you are sure to blow out all
the dust with compressed air before you put the adhesive in the
hole.
Epoxy used to be a hassle, back in the old days, with two
tubes to mix by hand. But the new systems use a mixing nozzle
attached to the tube — you squeeze the epoxy out just as
if you were applying caulking or subfloor adhesive. Nothing
could be easier — and now my local lumberyard even stocks
the adhesive, so I can pick it up whenever I need some.
The epoxy has to set up before the nuts and bolts can be
torqued down. You can put the nuts on finger tight about a half
hour after setting the rod into the epoxy, but you should wait
a day before tightening down the fasteners to the specified
torque.
To tie together wood-framed shearwalls on different stories,
we install one bracket above and one below, linked by a length
of threaded rod long enough to get through the floor system and
both wall plates. You set the bracket in place, mark the spot,
then take the bracket away for a moment to drill the hole.
Holes should be snug enough that it takes some force to send
the rod through — there's no wiggle room allowed when
you're fastening down a shearwall. As with the concrete-set
hold-downs, the nuts need to be tightened to the supplier's
specifications, but since there's no epoxy involved, you don't
have to wait.
Hold-downs are not cheap, and they take time to install, so I
like to use them efficiently. With the walls stacked one above
the other as they are in this design, I needed just six of
these standard fasteners for each foundation-to-attic shearwall
stack. The system is easy to inspect and understand as well as
to build — a plus when I get inspected by the building
official.
Finding the Right
Engineer
Many of the projects I build are large and
elaborate, and I know from the outset that I'll
need a lot of engineering help. But even for
smaller jobs, I like to get an engineer involved.
Near the coast, I consider an engineer to be an
indispensable part of the team.
One big reason is to handle surprises. There's
always a chance that you'll hit a snag with the
building department about some structural element.
The inspector might come to check the framing and
say, "That beam wasn't called out in the plan. It's
fine that it's there, but prove to me that it
works." If you call an engineer cold at that point,
he might get back to you in six weeks — or
never. For $150, why should he involve his
professional license in some stranger's project
that he hasn't followed from the outset? If
something else goes wrong, he's at risk, though he
took no part. The relationship is important: I
bring the engineer on board from the beginning so
there will be someone I can call up on a Thursday
morning and say, "I need help figuring something
out" and get an answer that week.
For coastal houses, selecting an appropriate
person is important. You're looking for an engineer
who specializes in this field, or is at least
experienced with wood frame platform construction.
I've had disappointing results with some engineers,
who were very skilled but didn't know this kind of
framing — they specified connectors that were
not available, for instance. You need your engineer
to come up with a solution that works well with
both the design and the process of your specific
building.
And he can do that only if he wants to. In my
experience, some of the larger firms don't want to
bother with homes — they won't give you their
real attention. If they take the work, they'll
think of it as something they can do between big
jobs when they have time on their hands. They may
give you a solution that covers their rear end but
leave you with a logistical problem — you end
up with something that will work the way they drew
it but will be a hassle to get in place. You need
someone who knows how to specify the wood members
and connectors you typically work with.
Good engineers will do as much or as little for
you as you need them to. I like to bring my plans
to the engineer as if I were ready to go to the
building inspector for a permit. I'll have the
footprint, all the elevations, and the floor plan
laid out, and I'll know how I intend to frame. If
I've already planned for the lateral loads, at
least in principle, everything is easier: He will
calculate what I need for sheathing, nailing, and
hold-downs, but he'll be working with my framing
concept, not telling me how to lay out the
building.
The engineer then supplies me with a plan overlay
showing where all the shearwalls are. He calls out
all the connections I need between those shearwalls
and the foundation, the floor system, or the walls
below, and he specifies the plywood and the
nailing. He can also provide a letter that you can
take to the building department with wording like,
"At your request I have reviewed the construction
documents for your project. My review includes an
analysis and design of the main structural support
beams, a general overview of the typical framing
systems, and a wind analysis." Most of the time,
such a letter with the engineer's stamp and
signature is enough to satisfy building inspectors
— it saves your time and theirs, because you
don't have to go over your plans with them page by
page.
— A.D.
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The frame was simple to analyze, efficient to build, and
economical, to boot. There was no need to build and set a
center girder down the long axis of the basement, and using the
full-length I-joists saved labor, even though we had to block
between the joists above the shearwalls.
Because they run the long way, I couldn't use the attic floor
joists to tie the rafters together. Instead, I used a supported
ridge, which I was able to post straight down through my
shearwall assemblies to the basement footing — a strong,
direct, efficient load path (see photos below).
This setup gives me several long rooms that span the width of
the house from ocean side to landward side. The bays function
to open up those spaces and make them broad as well as
long.
As a result, the structurally critical shearwalls support,
rather than interfere with, the other functions of the
building.
We've ended up with a structure that meets or exceeds the
capacity required by code. The engineer's analysis includes
just the walls we designated as shearwalls. Of course, all the
other elements in the house contribute some degree of strength.
So although we have designed for a wind between 110 and 120
mph, in a severe exposure category, I'm comfortable that this
house could handle an even rougher storm. That's okay —
the extra capacity will add stiffness and solidity to the
structure and security to the owner's life and well-being.
Andrew P. DiGiammois a design-build contractor and a
partner in an architectural firm in Assonet, Mass.Standards referenced in the
International Codes (available from the
International Code Council at 888/699-0541,
www.iccsafe.org/e/catalog.html): Minimum Design Loads for Buildings and
Other Structures (ASCE Standard No.
7-02), $98.00. The Wood Frame Construction Manual for
One- and Two-Family Dwellings, 1995 SBC High-Wind
Edition, $30.00. Referenced by the
2000 International Residential Code. The Wood Frame Construction Manual for
One- and Two-Family Dwellings, 2001
Edition.Referenced by the 2003
International Residential Code. SSTD 10-99 Standard for Hurricane
Resistant Construction, $33.00.
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