Bracing Failures

Gravity loading--the downward force due to the weight of things--is the primary concern in most structures. Most builders have a good feel for how gravity loads work and how to keep their structures from being broken or pushed down by these loads. But when downward forces cause sideways motion (as with roof thrust), or when the forces themselves are sideways, builders' instincts are not as well developed, and small but important structural component are sometimes left out. One small but important element that is often missing from failed structures is bracing.

An everyday stud bearing wall looks something like the drawing below. The top plate distributes the load to the studs, and the studs carry the load down to the bottom plate and the floor. The vertical alignment of the studs is structurally advantageous because neither the stud itself nor the nailed joints at its top and bottom are being bent or twisted.

But if the wall is loaded from the side by wind, then the advantages mentioned above are lost and the joints get bent. Since typical nailed joints have little bending strength, the wall will then rack to one side and may collapse. Things aren't much better when the wind stops blowing. Racking deformation tends to be permanent, and now that the gravity loads are offset, they too tend to bend the joints.

The solution to the racking problem is bracing. In older structures, this was provided by one or more diagonal members running from the top plate to the bottom plate and spanning two or three studs; today it's usually provided by a sheet of plywood nailed to several adjacent studs. Diagonal members turn the wall into a truss of sorts, taking advantage of the inherent stiffness of the triangular form. Plywood bracing uses what is known as diaphragm action to achieve the same goal. The bracing is usually put at the ends of the walls. You've probably seen such bracing at the corners of houses under construction. But sometimes it gets left out.

Rack 'em up

This little garage is leaning a bit to the back. It may not be obvious at first sight, but a quick check with a plumb bob shows the studs to be over 2 inches out of plumb. The problem is lack of bracing; there were no diagonals or plywood sheets in any of the walls.

The builder apparently believed that the horizontal lap siding would brace the walls. And it probably did for several years. But because the siding boards can slide against one another, true diaphragm action cannot be attained. Over time, the nailed joints between the siding boards and the studs loosen and allow the walls to rack.

Interestingly, the owners had recently replaced this garage's roof structure because it had been sagging. The original builder had managed to make two significant errors in one small structure.

Lean and mean

Although it may look it, this picture has not been altered with Photoshop--the walls really are about a foot out of plumb. The reasons for the tilt are what you would expect: no diagonal or panel bracing; the builder mistakenly relying on the lap siding to keep the structure from racking.

This case is so extreme, we can easily see what happens to the nailed joints between the siding and the studs. This picture shows a section of siding from the front of the garage. The forces imposed on the nail holes have caused them to elongate more than half an inch in some places.

These investigations were performed by Mark Lawrence.