Download PDF version (769.2k) Log In or Register to view the full article as a PDF document.

Framing Around Hold-Downs The 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 trimmers, and king stud assemblies, so that they match the location of the hold-down hardware. In some cases — at inside corners 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. At 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. All 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 (Figure 4).

0997sc7.jpg (10336 bytes)

Figure 4. Some hold-downs, 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 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. Hold-downs can also be used to connect two floors together (Figure 5).

0997sc10.jpg (7570 bytes)
Figure 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 (right).

Tying to the Roof

To 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 A35F clips are installed on 16-inch centers to tie the blocking to the top plate (Figure 6).

0997sc8.jpg (9751 bytes)

Figure 6. Here, Simpson A35F clips tie eaves blocking to the ceiling diaphragm below.

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.

Roof 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.

Shear Basics

The plywood sheathing on the walls of a house does double duty. First, it provides resistance against racking — the tendency for a row of studs to collapse like dominoes along the length of the wall. Second, when lapped across the wood members and nailed properly, plywood ties the pieces of the framing together. A good example of this is the joint between a first and second story. If the plywood sheathing spans from the lower to the upper floor and is properly nailed into the first-floor studs and top plates, the second-floor rim joist (or blocking), and the second-story studs and plate, the plywood ties the two floors together. Typically in residential construction, 1/2-inch Structural #1 (called "struc one") plywood is specified for shear paneling. Plywood ratings call out the veneer grades, adhesives, and the structural span of the plywood. Do not substitute a different grade of plywood for the specified grade without the engineer’s approval. Plywood shear panels should be placed and nailed so that they connect the sole plate or bottom plate of the wall to the vertical framing members. Shear walls should also be blocked between studs at the top of each sheet of plywood to accept edge nailing. The plywood sheet must be nailed so that all four edges have framing members behind them to allow for full perimeter nailing. All our plans have a shear nailing schedule that specifies the spacing and size of nails to be used (See sample schedule, below).

Shear Wall Schedule

Symbol

Wall Sheathing

Sill Connection

3/8" ply. str II,

blocked w/8d

@ 6" o.c. E.N. & 12" o.c. F.N.

A34 @ 24" o.c. 16d @ 6" o.c.

3/8" Ply str II,

blocked w/8d

@ 4" o.c. E. N. & 12" o.c. F.N.

A34 @ 24" o.c. 16d @ 6" o.c.

3/8" Ply str II,

blocked w/8d

@ 3" o.c. E.N. & 12" o.c. F.N.

A34 @ 16" o.c. 16d @ 3" o.c.

1/2" Ply str I,

blocked w/10d

@ 3" o.c. E.N. & 12" o.c. F.N.

A34 @ 8" o.c. 16d @ 3" o.c.

5/8" G.W.B

blocked w/6d cooler nails

@ 4" o.c. E.N. & F.N.

A34 @ 48" o.c. 16d @ 8" o.c.

7/8" stucco over paper backed lath

w/16 GA. staples

@ 6" o.c. top & bottom & E.N. & F.N.

A34 @ 32" o.c. 16d @ 8" o.c.

This typical shear wall schedule specifies plywood size and type as well as edge-nailing (EN) and field nailing (FN) size and spacing

For instance, a schedule that calls for a 4- and 12-inch nailing pattern refers to a 4-inch nail spacing along the plywood perimeter and a 12-inch spacing in the rest of the sheet. Shear paneling should be installed so that the joints are staggered. The shear plywood should continue to the top plate of the wall and in two-story construction should tie the first and second floors together. It is critically important that sheets are gapped 1/8 inch at the joints (we use 16d nails as spacers.) This allows the sheets to expand and contract with moisture and temperature changes without buckling.