A common concern for many of our clients is to improve the schedule of I like to call the “ABC’sa job in order to increase revenue and profit. One of the most common ways for a project to gain time in a schedule is to install temporary wall bracing, typically using tilt-up style metal braces. This alleviates the need to put the lid on a foundation prior to backfilling. When trying to design the most efficient temporary wall bracing plan, one might want to consider what I like to call the “ABC’s”:
A. Angle: brace capacities are given as an axial load. After calculating the required horizontal bracing force, the designer must consider how the angle of the brace is going to transfer that horizontal load into an axial load. This can drastically affect your brace spacing if your brace angle is 60 degrees versus 45 degrees.
B. Bottom: this is typically the main complication of a bracing plan. The temporary brace resists the overturning of a wall near the top, but there is still the total horizontal load that needs to be resolved at the bottom. For example, assume that the average load against a 12’ high wall is 5,000 lbs, and it is applied at 1/3 the height (this scenario is similar to backfilling a wall). The overturning of that backfill is (5,000 lbs) X (12 ft) x (1/3) = 20,000 ft*lbs. If the brace is installed at 10’, then the required horizontal capacity is (20,000 ft*lbs) / (10 ft) = 2,000 lbs. However, if the original load against the wall is 5,000 lbs and the brace is only resisting 2,000 lbs, then the bottom of the wall still needs to resist 3,000 lbs. Typically this is accomplished by installing the slab on grade. If the slab on grade is not installed, then the designer must analyze the wall itself to resist the load or specify an additional permanent support. If the wall itself is not sufficient, then it is typically in the best interest of the contractor to install the slab on grade.
C. Connection: connections will need to support shear loads vertically on the wall, horizontally on the slab, and vertically on the slab. There may be limitations in the existing structure due to substrate thickness, edge/spacing distances, and ground bearing capacity.
By keeping these guidelines in mind, designers maximize the efficiency of bracing for the contractor and the project. Need help with a wall bracing design? You can contact me or another engineer to discuss your next project.