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, using the bracket to locate the hole and a torpedo level to stay plumb.
He hammers the rod through and sets the brackets.
Then he drills holes into the studs for lag screws.
In the basement, he hammer-drills holes in the concrete.
Then he anchors the threaded rod into the hole using high-strength epoxy.
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.
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.
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.
For More InformationStandards 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.