Avoiding the Sag: The Importance of Erecting Trusses at Zero Camber

A specialized element of structural design is the use of trusses and plate girders, which are often used as transfer members to eliminate columns when a space needs to be opened up to be more attractive or increase its usefulness. Transfers must be erected with zero camber—a lack of downward or upward curvature—to ensure that the transferred column will be starting at the correct elevation and not affect the elevation of the rest of the building.

Zero camber can be difficult to achieve when stick-building trusses. The top or bottom chord of the truss is erected first and will deflect significantly under its own weight. If not corrected, the transfer columns will start off too low, causing the entire building to be too low at those column locations. The result is sagging floors, and no one wants that.

Our recent project, The Harper, located at 19th and Chestnut streets, used five trusses and two plate girders to transfer out nine columns. The Harper is a mixed-use residential project featuring ground and second floor retail stacked below a 21-story residential tower. The owner wanted to reduce the number of columns to allow for longer spans in the retail spaces to make them more versatile and rent at a higher rate, so we elected to use trusses at the base of the tower to take on the load from the removed columns.

The trusses were stick built and the deflection in the top chords, just from self-weight, was between 5/8 of an inch and 1 ¼ inches. First, the trusses were erected, all of the bolt holes were stuffed, but not tightened. To resolve the deflection issue, the steel erector jacked up the trusses to set zero camber. Shoring and spreader beams were added to distribute the load from the jacking operations. Then, the tower crane was used to pull up on the truss until the crane hit its load limit, and then jack pressure was applied. As the jacks took load off the crane, the crane applied more force, and so on until the process was complete.

After the surveyor called the truss at the proper elevation, all of the bolts were torqued to spec (1 1/8” A490-SC TC bolts). When the bolts were tightened, the jacks and the crane were released and the surveyor did a final check of elevation. After torqueing all the bolts, deflection due to self-weight of the truss was minimal, 1/16 of an inch or less (almost exactly zero camber).

To learn about transferring columns in existing buildings, read “Transferring the Load: Historic Columns Removed from Renovation Projects.”

By Chris Gottschall