keep houses from shaking apart in earthquakes or being blown away
in hurricanes, engineers and architects specify hold-downs, metal
framing connectors, and plywood shear paneling. The idea is to use
metal connectors and plywood to create a continuous "hold-down
path" that ties the structural wooden frame together from foundation
to roof peak. In California, where I live and work, every set of
house plans addresses the issue of tying the structure to the foundation.
piece of hold-down hardware, whether it's a simple 1/2-inch J-bolt
or a custom metal clip, will be marked on the foundation plan and
in the structural details. When reviewing a set of plans, I use
a colored marker to highlight every hold-down and shear panel anchor
bolt. As I mark each one, I keep a running count of each type and
with their locations. Hold-downs are typically called for in pairs,
located, for instance, at each end of a shear wall or on both sides
of a window or door where the shear wall is interrupted by the opening
(see Figure 1). With experience, it's pretty easy to find (and even
memorize) the location of the hold-downs.
1. Hold-down installation begins with a careful review of
the plans. An overlooked hold-down bolt or embedded strap can
turn into a costly retrofit once the slab is poured. The author
uses colored highlighters to locate every piece of hardware
specified by the engineer.
we usually keep an extra supply of the most popular clips in the
storage shed, it's expensive to go back and do a special installation
of an omitted hold-down, especially for the larger, more costly
anchors. So if you are new to these kinds of anchor systems, be
pre- pared to do a close examination of the plans to avoid a costly
retrofit installation. Nevertheless, this is a common problem, especially
on a complicated set of plans. If you do omit a hold-down, ask your
structural engineer for a solution. For example, a common fix for
a missing foundation anchor is to drill into the foundation or footing
and install a retrofit bolt or a length of threaded rod, secured
plans I get from architects are very specific, and it's my job to
locate the foundation hardware and to see how it will connect to
the shear walls, and to the door and window openings. I look for
places where some other building component, such as waterlines or
power conduits, might interfere with the installation of the hardware.
If the house has a second story, I'm careful to note any connection
called out between floors so that my framer can adjust the stud
and post layout to accommodate the connecting hardware.
2. Bolts for hold-downs like the type in the top photo, have
to be precisely located with respect to the framing when the
foundation is poured, as do post anchors (middle photo). Embedded
straps (bottom photo) are somewhat more forgiving.
turn to the roof framing plan and note how the engineer has created
a hold-down path up to the roof diaphragm. These systems can be
complicated, so if anything is unclear I notify the engineer and
ask for guidance. To avoid confusion between subs, I meet with our
lead framer and foundation foreman to discuss the plan and to note
our area it is usually the contractual responsibility of the framer
to supply and install the hold-down hardware. The hardware is often
a mix of inexpensive off-the-shelf sizes and expensive custom-order
styles. The custom pieces sometimes take weeks to deliver and can't
be sent back, so it is important not to miss any on the plans.
at the Foundation
the foundation forms are in place, I layout the centerline of every
door and window opening directly on the formwork. Next, I figure
backwards and mark the exact locations of rough openings, trimmers,
and posts. We work off the actual, not nominal, post sizes to determine
the setback (offset distance from the edge of the actual post) for
the centerline of the hold- down bolt. For example, the hold-down
shown in Figure 2, uses a 7/8-inch anchor bolt (the hole is drilled
1/16 inch larger), and has a centerline 21/16 inches from the face
of the post. It's possible to simply measure the distances, but
often the bolts are on site before the rest ofthe hardware is delivered,
so to be accurate we look up the centerline dimension of each style
and size of hold-down in the hardware catalog.
the same time, I also mark the location of all the shear wall anchors
and other hold-downs, such as strap ties and post anchors. I do
this layout jointly with the lead framer so that we are both in
bolts and hold-down straps must be securely fixed in place both
laterally and vertically (Figure 3). If they move sideways during
the pour, they'll miss the structural member that they are supposed
to anchor. If a bolt moves up, it won't embed into the concrete
enough. If it moves down, it gets embedded too deep into the concrete
and there won't be enough exposed thread to make a connection. (If
a bolt happens to sink into the concrete during the pour, a threaded
union and a piece of all-thread the same diameter as the bolt can
be used to lengthen the exposed shank.)
inspectors in our area require that all hold-down bolts be in place
before the pour. Using the color highlights on the plans for reference,
I spray paint the matching locations on the formwork to make the
job easier for the inspector. Then we install plywood templates
and jigs to lock the bolts in their proper locations during the
concrete pour. When we require extreme accuracy, we use the Simpson
MKP (monkey paw anchor bolt holders) to help secure bolts prior
to concrete placement.
3. The author uses plywood templates (above) or Simpson
"Monkey Paws" (drawing) to secure hold-down bolts
to foundation form boards. Embedded strap anchors are tacked
into place, while post anchors (left) are temporarily secured
to a scrap of framing lumber of the correct width.
a hold-down inspection of both numbers and locations by the job
superintendent and approval by the inspector, the concrete can be
placed. Make sure you comply with the engineered design mix and
that the concrete is mechanically vibrated into place to eliminate
concrete honeycombing or voids that could compromise its strength,
especially around the hold-downs. At the same time, make sure the
vibrator does not come in contact with hold-downs and preset anchors
or it may move them. After placing the concrete and while it is
still wet, check the hold-down locations a final time, and make
any necessary adjustments.
hold-down hardware in the walls will have to line up with the anchor
bolts in the concrete when the walls are tilted up. So before any
framing begins, the layout man needs to accurately note the location
of key structural members such as bearing posts, window and door
and king stud assemblies, so that they match the location of the
hold-down hardware. In some cases - at inside comers or near a window
or door, for instance -it may be necessary to predrill a structural
member or leave out the adjacent stud in order to have access to
the nuts and bolts.
the foundation, a typical hold- down consists of an embedded anchor
bolt connected to a metal bracket with a nut and washer. The metal
bracket is then attached to a structural member such as a trimmer,
with a set of horizontal bolts. Tolerances of plus or minus 1/8
inch must be met to make installation go smoothly. Otherwise, it
takes a pretty big hammer to bend the steel into submission.
4. Some holdowns, depending on the manufacturer, have
to be held up a specified "standoff" distance from
the plate, to ensure that the horizontal bolts don't tear
out the end grain of the post under load.
hold-downs must be installed with the bottom hole at least seven
bolt diameters from the end of the post. This prevents the horizontal
bolt from passing through the post too near the post end, where
it could tear out under load. It used to be that some of the smaller
hold-downs had to be held up a certain distance -the " standoff"
distance -when installed. But now Simpson Strong- Tie, the hardware
manufacturer I use, has built this standoff distance into all of
its hold- downs. The HDA series installs flat against the sill and
the bottom hole is automatically seven bolt diameters up. The heavier-duty
HD series has a folded down "ear" that presets the distance
are installed with commonly used tools, such as a 1/2-inch or 3/4-inch
drill and auger bit, open-end wrenches, a deep-socket set, and,
occasionally, a pneumatic impact wrench. Once the plywood shear
paneling has been installed, access to the hold-down bolts is restricted.
So we usually connect and tighten the hardware after the walls have
been plumbed and lined, but before the plywood shear paneling is
installed. Sometimes the plywood has to be nailed before the hold-downs
are connected. In this case, we use ratchets and pneumatic impact
wrenches to get into tight spots. It's occasionally necessary to
preinstall hold-downs to king post assemblies before framing the
king post into the wall.
can also be used to connect two floors together (Figure 5). In this
application, one hold-down is installed upside down at the top of
a structural framing member. We drill a hole in the top plate with
the correct off- set, then install another hold-down at the base
of the second floor framing directly over the upside-down one below
it. The holes can't be more than 1/2 inch out of alignment or the
threaded rod will bind. In the real world, of course, we often have
to "correct" an offset with a hammer to make the rods
line up. We then cut a piece of threaded rod to slide through one
hold-down, the floor framing, and into the second hold-down. Finally
we install and tighten the nuts at each hold-down, making the first
to second floor connection.
5. To provide a secure load path between stories, engineers
specify bolted hold-downs, with the lower one turned upside
down (left), or straps like the Simpson FTA (middle) or MST
are not the only way to tie together the framing between floors.
In fact, engineers are beginning to specify Simpson MST straps or
FTA floor ties because they are cheaper and .easier to install (Figure
5). These are flat steel straps that span between the two floors
with either nails or bolts to attach them. With strap connections
between floors, the framing members such as studs or posts need
to be exactly in line with each other vertically.
to the Roof
6. Here, Simpson A35F clips tie eaves blocking to the
ceiling diaphragm below.
ensure that the plywood extends (and ties) all the way up to the
top plate, we install the rafters after nailing the shear panels
flush with the top of the double top plate. Blocking is nailed into
place between rafters and A3SF clips are installed on 16-inch centers
to tie the blocking to the top plate (Figure 6). In cases where
roof overhangs are large and wind uplift is an issue, we use H-series
or hurricane offset straps to tie the rafter to the top plate.
sheathing. The building code requires us to stagger the
joints of the roof plywood. For added strength, we are often required
to block between rafters along plywood edges or to use tongue-and-groove
plywood. We also nail the sheathing to the perimeter blocking installed
between rafters. This allows the transfer of shear stress from the
roof to the walls and down the hold-down path to the foundation.
John Scoggins is
a field representative for Allen Assocites.