Shopping Mall Prestressed Concrete Parking Structure Load Test
A newly constructed parking garage for a prominent anchor store in an upscale mall exhibited distress in the form, visible cracking, and limited but substantial concrete spalling prior to county approval for public access. Typical bays in the garage were constructed from precast, prestressed double-T beams supported by precast concrete columns and pilasters. This structure provided support for the two elevated levels of the garage, with a parking area of approximately 175,000 square feet.
Discovery of the distress motivated concern for whether additional, undiscovered problems might exist. Preliminary visual site inspection, nondestructive testing and general design review conducted by others indicated the distress was caused by the precast concrete design and fabrication. These investigations identified numerous potential problems with the garage. However, confirmation or denial of potential problems by either calculation or observation proved prohibitively expensive and time consuming. This prompted the county building department and the structural engineer-of-record to require load-testing of 100% of the structure.
The property owner engaged CTLGroup to address the concerns of the project stakeholders and resolve questions regarding the condition of the structure. Schedule constraints required resolution of the structural issues within a period of approximately five months, including condition assessment and design and installation of any structural modifications.
- Feasibility Study: Typical in-situ load tests focus on a limited number of critical structural elements and are conducted over an area of hundreds or several thousands of square feet. Load testing approximately 175,000 square feet of structure is unprecedented. A preliminary feasibility study identified that load testing the entire parking structure would potentially be more expensive and time consuming than any other method of assessing the garage.
- Consensus Building: The owner and CTLGroup worked with the project stakeholders to develop an achievable assessment program without 100% load testing. The result of this effort was a four-phase program combining inspection, analysis, load test and strengthening.
- Inspection: CTLGroup conducted extensive visual, non-destructive and destructive exploratory testing of a statistically significant sample of structural components. This effort served to identify potential deficiencies and variability in as-built condition of the structure.
Analysis: CTLGroup and precast designer analyzed the majority of the structural elements and connections to determine the sensitivity to construction tolerances as determined by the inspection.
- Load Test: CTLGroup conducted a load test on a representative region of the structure. The load test included more surface area and structural elements than almost any reported to date, but was still significantly reduced scale than the original 100% requirement. Load test development and execution included: preparation of a detailed protocol and acceptance criteria based on American Concrete Institute provisions in ACI 318, Chapter 20; installation of extensive instrumentation of to measure response of members and connections; test load calculation and application; data collection and analysis. Load was applied using an innovative application of water-filled bladders with a custom designed water metering and distribution system. This approach provided rapid, controlled load application and removal.
- Strengthening: A significant number of components were strengthened in lieu of inspection, analysis or testing. Strengthening focused on the beam supports at the columns, was cost-competitive with the alternative assessment actions, and the strengthening could be conducted on a parallel track with other activities to improve completion time.
Site inspection and analysis served to resolve concerns over a large percentage of the parking structure. The targeted strengthening was installed without difficulty and addressed concerns at connection locations that would have been challenging to assess by testing or inspection. The parking structure satisfied the load test, providing a demonstrated performance level that, in conjunction with the other efforts, served to restore the confidence of the project stakeholders in the suitability of the parking structure. All four assessment phases were conducted in a parallel schedule and were completed on time, allowing the parking structure to open to the public by the target date.