Our company typically braces masonry walls using 2 x 10 planks that are 16 feet long and installed at an angle with a vertical 2 x 10 against the wall. Stakes are driven into the ground at the opposite end. These braces failed and the wall blew over during a heavy windstorm.
Fortunately, this incident occurred on a Sunday when nobody was at the site and there was only minimal damage to the other partially completed portions of the wall. The braces were positioned on both faces every 15 or 20 feet along the length of the wall. The wall was 24 feet tall, constructed out of 12 inch CMU, and was completed about one week before the incident.
How can this problem be avoided in the future?
There are several reasons why bracing systems such as the one that you described fail. A 16-foot long brace positioned at a 45 degree angle only extends to a height of about 11 feet 4 inches, assuming that the end of the brace was level with the bottom of the wall. This figure is less than half of the wall’s height. The system should brace the wall so that there is no unsupported portion higher than 10 times the wall thickness.
A 16-feet long 2 x 10 member without an intermediate brace buckles when subjected to wind because the wood plank is very long and slender. An intermediate brace or diagonal attached at mid-length of the 16 foot brace is needed to prevent buckling under a compression load. This intermediate brace must be designed and detailed to function in both tension and compression.
Another possible reason for the failure pertains to the anchorage of the base’s braces. Depending on the condition of the soil and the type and length of the stakes installed in the ground, the stakes may have failed during the windstorm causing the brace to slip.
Studies have shown that reinforcing rods driven into the ground do not provide sufficient anchorage. Two 2 x 4 members driven at least 1 foot into the ground provide better support; however, even these often fail, depending on the soil conditions. This problem is especially acute if the soil becomes saturated following the installation of the stakes. Reinforcing rods are occasionally driven into frozen ground during cold weather. These stakes have very little strength when the soil thaws.