As a result of the poor performance, due to a combination of material and workmanship deficiencies, noted in shear resisting elements of wood frame construction following the January 17, 1994, Northridge earthquake, various changes in the building code have been implemented. Los Angeles City, Los Angeles County, and Ventura County have all made changes to their adopted building codes which significantly impact all new wood frame construction.

WALL BRACING FAILURES

Damage to single and multi-story wood frame dwellings was largely due to inadequate connection of the finish material serving as lateral bracing for the exterior shear walls. Significant damage was commonplace in homes where stucco, without plywood backing, was used as the only shear resisting element. Failure in the connection between the stucco lath and the foundation sill plates was commonly observed in slab-on-grade and raised floor construction. In many cases where the embedment of the wire lath backing in the stucco coating was insufficient, large curtains of stucco became detached from the wall frame spanning from the bottom sill plate to the double top plates of the ground level stories. For this reason recent code changes have disallowed the use of staples or self-furring lath. All lath is now required to be attached with 11 gage, 7/16" diameter head, 1-1/2" long, galvanized nails furred at least 1/4 inch.
A major problem with shear walls braced with plywood and/or gypboard was the extreme deflection experienced in the shear panels when subjected to the dynamic cyclic loading associated with a high magnitude earthquake. In tests conducted at University of California at Irvine, the actual drift at the top of an eight foot high 28 inch wide wall panel with plywood sheathing was measured in excess of 2 inches when subjected to simulated earthquake loading. The allowable story drift by 1994 U.B.C. standards was 0.005 times the story height. In the above case (0.005) x (8 ft) x (12"/ft) = 0.48 or approximately 1/2", which is only 25% of that which actually occurred.

Accordingly, post-earthquake code changes have limited the height to width ratio for stucco panels and gypboard to 1:1. Further, allowable shear values have been reduced from 180 lbs/ft to 90 lbs/ft for stucco, and from 100-120 lbs/ft down to 30 lbs/ft for gypboard. The values for plywood shear panels have been reduced by 25% from the prior listed table values, and the height to width ratio has been reduced to 2:1 in an attempt to reduce high story drift in tall narrow panels.

Other problems with plywood panel failures related to edge nail pullout and overdriven nails, have been addressed in the revised code by provisions requiring special inspection of plywood shear walls where the shear loads are greater than 300 lbs/ft. It is hoped that failures from nails too close to the panel edges and overdriven nails from pneumatic guns which break the outer veneer and compromise the panel strength, may be avoided through more careful inspection.

SILL PLATE SLIDING AND SPLITTING

Raised wood floor structures without sill bolts experienced sliding of the foundation sill plates along the top of the stemwalls. Where sill bolts were present but the bracing for the exterior shear walls failed, the sill plates were observed to split longitudinally (with the grain) in both directions from the bolt holes. A contributing cause of the sill plate splitting is the common practice of oversizing the bolt holes to facilitate the pre-framed stud walls being lifted into place. Revised code changes require the use of square plate washers (in lieu of round cut washers) under the nut at sill bolts and at holddown post connections at the ends of shear walls. The plate washers provide a greater bearing area for the bolts. Also, a decrease in the allowable maximum spacing for sill bolts (2 and 3-story structures) and an increase in the bolt diameter (3-story structures) have been written into the post earthquake code changes. Since sill plates are subjected to a combination of axial forces, shear loads, and bending both with the grain and across the grain (see diagram) during seismic ground acceleration, field inspection should be more sensitive in this area.

Other possible upgrades which would aid in the prevention of sill plate failure would be the use of 3x sill members and the application of a quick-set, shrinkage resistant drypack to all bolt holes with a diameter greater than 1/16" or more than that of the anchor bolt itself.

For further information about PACE Investigative Services, please contact Sherrie Henderson at 818-251-1200 or e-mail her at shenderson@hfinc.com.