Award of Excellence: Strengthening Category
Ashford Perimeter Parking Garage Strengthening
Submitted by Structural Preservation Systems, Inc.
Glenborough Realty Trust
Simpson Gumpertz & Heger
Structural Preservation Systems, Inc.
Master Builders, Inc.
The Ashford Perimeter Parking Garage supplies approximately 1000 parking spaces for a commercial office. The lowest level of the four-level garage is on-grade with three above in a terraced configuration. No internal structural ramps exist as each level is accessed on grade. The deck is 8 in. thick, 2-way, post-tensioned, flat slab.
The column-strip post-tensioning in the E-W direction was inadequate. A repair was developed using a heavily-reinforced Gunite beam added to the underside of the slab column strip. Soon after, delaminations between some beams and slab were noticed. An NDT investigation (pulse velocity test), concluded that approximately 30 percent may have more than 40% delamination. The engineering firm then recommended more NDT inspection utilizing impact echo. Of the 133 beams tested, 105 showed some delamination, and 19 had 33% or more delamination.
The first phase of the repair included an in-depth in-situ load testing of the existing slab under various conditions that existed as well as strengthened with FRP sheets. A load testing procedure was proposed to provide a non-destructive demonstration of the performance of structural components, utilizing hydraulic jacks to induce force equivalent to those resulting from distributed loads
Test results indicated that the beam in the E-W direction with a minimum of 70% bond performed well. A span with no beam demonstrated poor performance. Results also showed CFRP strengthened slabs (after removing the Gunite beams) were satisfactory. Accordingly, 19 locations E-W beams would remove and the slab would be strengthened with CFRP sheet. Typical span in the N-S direction were found to be acceptable without additional upgrade.
Large overstress in the E-W end spans and excessive deflection to the 2nd level cantilever required another strengthening approach. External steel-framing was installed reducing the clear span, and additional steel supports were installed under the cantilever slabs. At steel support points, "pancake" jacks were permanently installed to engage the steel frame.