Project Profile: Reinforcing Concrete Joists to Increase Load Rating

We’re excited to share another fiber-reinforced polymer (FRP) project that required both flexural and shear strengthening (photo below) of reinforced concrete joists to enable the slab floors to carry more live load. The structure is in Southern California, and appears to have been built in the 1950s or 1960s when pan joist construction was common. The EOR for this project, Structural Focus, is an experienced structural engineering firm known for seismic retrofit solutions. The FRP applicator was FD Thomas Structural Specialties, a contractor with decades of FRP installation experience.

Applying the FRP to the bottom surface of the joist was the simplest solution for flexural strengthening and worked in most cases. However, in some locations the required strengthening was significant enough to require FRP on the joist sidewall as well. Adding to the challenge was concrete blocking, located at the joist mid-span, making it difficult to maintain the structural continuity of the flexural FRP strips. Using FRP through-anchors on one side of the joist solved this problem. The anchors are very hard to see in the photo below, but the elevation drawing that follows illustrates more clearly how this problem was resolved.

 

Because of its role in resisting gravity loading, it is common to require that the FRP be coated in these applications to help protect the resin against fire, but this is mostly for the purpose of limiting smoke development and fire spread. The video below shows how this coating, in this case Simpson Strong-Tie® FX-207 slurry seal, can be sprayed using a hopper gun to efficiently cover large areas.

As shown in the photo below, this building retrofit also required post-installed rebar dowels, anchored with Simpson Strong-Tie SET-XP®, to strengthen the structure’s lateral system through the addition of ductile-behaving shearwalls and boundary elements.

At Simpson Strong-Tie, we are always thrilled to investigate new solutions for engineering challenges on the jobsite; it’s what keeps our job exciting. If you have a challenge or simply want to learn more about our solutions, please contact our Engineering Technical Support team at (800) 999- 5099.

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Jason Oakley

Author: Jason Oakley

Jason is a California registered professional engineer who graduated from UCSD in 1997 with a degree in structural engineering and earned his MBA from Cal. State Fullerton in 2013. He is a field engineer who teaches specifiers about concrete anchorage and fiber reinforced polymers (FRP) in the S. California and Pacific region. Before joining Simpson Strong-Tie in 2002, he was a design engineer for 5 years working on subterranean parking lots, movie sets, offshore drilling platforms, nuclear power plants, oil refineries, blast-resistant structures, fall protection, dry-dock ship supports and vibration.

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