by Doris Eichburg, Beth Anne Feero, EIT, and David Nicastro, PE
Bearing pads are used widely in precast concrete parking garages. They function as buffers between the separate concrete members to prevent damage and facilitate movement (much like cartilage between bones at joints). Bearing pads should last for the design life of the structure, but need to be replaced when they are not sufficiently durable. The ‘surgery’ to replace the pads properly is a complex operation, with few published guidelines.
In typical precast concrete garage construction, a bearing pad is installed beneath the end of each double-tee beam stem and under other beams and panels. The pads accommodate expansion, contraction, and rotation at the bearing area, preventing spalling and cracking of the supporting concrete members. Traffic in garages also induces movement, noise, and vibration which need to be absorbed or transferred by the bearing pads. (Similar bearing pads are used in bridges and other types of structures, but they are beyond the scope of this article.)
When correctly designed and installed, good-quality bearing pads are expected to be permanent components—that is, their component design life should be equal to that of the structure. Unfortunately, the actual service life of bearing pads may be shorter than their design life, requiring replacement when they prematurely fail.
The authors have investigated the following failure mechanisms of bearing pads that can necessitate early replacement.
Slippage or ‘walking’ is a phenomenon where a bearing pad moves out from its original location. When a bearing pad is loose-laid and not secured to the support below (e.g. with epoxy), it is held in place only by the concrete member’s weight. Bearing pads may walk due to incremental one-way movement because expansion and contraction forces are not exactly equal, particularly when the bearing substrate is not parallel to the double-tee stem ends. Bearing pads may also walk out due to sun camber on the top level, which reduces loading on them (Figure 1).
Other factors contributing to slippage include excessive shear load due to expansion or contraction of the concrete members and rotation of the double-tee beams from cars driving in a relatively empty (and unloaded) garage. The bearing pad’s elastic memory, an inherent tendency to recover to its uncompressed state, may also cause the pad to walk out.
Crushing and tearing
Bearing pads may crush or tear over time due to excessive loading or inadequate pad size/material. Bearing pads compress slightly under load, which is to be expected. However, if the compressive load significantly exceeds the pad’s capacity, the pad will first spread (increase surface area) and then tear. Non-uniform loading of the bearing pad due to camber, misalignment of the concrete members, or improper installation of the bearing pad can cause a concentrated point load that may also cause crushing of the pad (Figure 2).
Cracking or crazing of the bearing pads may be caused by extreme heat, ultraviolet (UV) light, and ozone. Heat and UV light are not typically significant issues, since most bearing pads are not directly exposed to them. However, ozone is a powerful oxidant that occurs naturally in the atmosphere, and in greater concentrations in polluted areas (where parking garages are also found).
Natural rubber and most synthetic bearing pads are degraded by ozone as it breaks the carbon-carbon double bonds on their surface. Degradation due to environmental attack can be mitigated by including protective additives during the manufacturing process of the bearing pads.
Bearing pad types
American Association of State Highway and Transportation Officials (AASHTO) has standards for bearing pads in bridge applications, but there are no industry standards for the common types of bearing pads used in the construction industry.
Natural rubber bearing pads were the first type used. As the name implies, the main ingredient is latex extracted from rubber trees. The rubber used in today’s bearing pads is typically vulcanized (a chemical process forming crosslinks in the polymer chains of the rubber) to improve tensile strength, elasticity, and durability. One of the material’s biggest attributes is its ability to remain flexible and less stiff in colder temperatures.
Neoprene (or polychloroprene) bearing pads are synthetic rubber. They resist ozone attack better than natural rubber. Other benefits of neoprene bearing pads include high tensile strength, good weatherability, and resistance to heat and flame. However, neoprene bearing pads do not have the ability to remain as flexible as natural rubber in colder climates.
Random-oriented fiber (ROF) products are the most widely used bearing pads in parking garages. They are engineered to be strong, with a high compressive strength. ROF products are made of rubber elastomer with synthetic fibers added for strength, and vulcanized to form the final composition.
Bearing pad replacement procedure
Although it is commonly implemented, there is little published guidance and no industry standards regarding how to replace bearing pads in precast concrete parking garages. The general process requires the double-tee beam stems to be lifted to replace the bearing pads, and then be lowered back down. For efficiency, it is recommended the bearing pads under both stems of a double-tee beam end are replaced, even if one of the bearing pads is still in good condition, since both stems have to be lifted together.
The authors’ firm has designed and monitored numerous bearing pad replacement projects using the general procedures outlined in this article. As every garage is unique, the actual scope of work should be designed for each project by a licensed engineer. Caution is paramount because structural damage can occur when the lifting operation is done incorrectly. All personnel should be trained to stop immediately when structural damage to any concrete member occurs, and to secure the garage until the engineer inspects the conditions and provides further guidance.
Pre- and post-lifting inspections
The topping slab, the double-tee beam stems to be lifted, and the adjacent double-tee beam stems must be closely inspected for concrete cracking and spalling before commencing lifting operations and again after completion. This helps identify new distress caused by lifting operations.
The documentation should include dimensions and locations of distress on drawings, as well as markings on the concrete. If all bearing pads are not scheduled to be replaced at the same time, then the pads should also be inspected to document crushed, torn, or ‘walked’ pads. It is important to note sometimes the bearing pads appear to be in serviceable condition, but they are actually torn or crushed under the stems. The authors also recommend measuring the bearing length and comparing to the design capacity (Figure 3).
Cracks identified during the initial inspection requiring structural repairs (such as epoxy injection) should be addressed before lifting operations.
Where necessary to lift double-tee stems, welded shear connections should be temporarily saw-cut at the ends of the double-tee flanges from the abutting components. After lifting, the connections can be re-welded. Any connections that are cut should be reviewed prior in order to ensure the stability of the tee and attached components are not compromised.
Shoring should be installed for the lifting device to distribute the double-tee weight and lifting equipment loads down to the slab-on-grade or to multiple lower floors in tall garages. If the garage has any single-?ledger beams supporting double-tee beams, one must verify they are secured against rotation. For safety, it is important to ensure the shoring is secured and does not move during lifting, especially on ramps (Figure 4).
It is important to ensure the double-tee beam being lifted carries no load other than self-weight. Using a hydraulic jack, both stems of a single double-tee unit should be slowly, uniformly, and simultaneously lifted, with a differential deflection maintained at no greater than 3.2 mm (1?8 in.). The lifting should be just enough to remove the bearing pads—any more could lead to cracking due to torsion.
The jacking pressure is not usually measured due to the difficulty in accounting for friction, resistance, and dead loads during the process. Sometimes it is necessary to lift adjacent double-tee beams at the same time to ensure the structural stability or integrity of adjacent members (e.g. stairs tied in to the sides of the double-tee stems) are not compromised (Figure 5).
Bearing pad replacement
After removing the existing bearing pad, all debris should be cleaned from the surfaces. The new bearing pad should be installed as far back as possible under the stem, with the long dimension centered on the tee stem. The bearing pad should not be extended beyond the edge of the supporting member. The pad can be adhered with epoxy if slipping is a concern. When the substrate is not level, shims can be used beneath the bearing pad to reduce excessive non-uniform loading that can cause premature failure—thereby defeating the project’s entire purpose (Figure 6).
The double-tee beam stems should be lowered in the reverse order of lifting to ensure the differential deflection between any two stems does not exceed 3.2 mm (1?8 in).
The engineer should be informed of any new cracks or ones that have increased in length or width. Any spalling or cracking resulting from the lifting operations must be repaired, and any shear connector plates that were cut or broken during the work should be re-welded. After the shoring and lifting devices are removed, the garage can be returned to service.
Test results from several manufacturers indicate bearing pads should last the structure’s lifetime when correctly designed and installed. However, several factors contribute to their premature degradation (including inferior products, environmental attack, movement from traffic, non-uniform loading, incorrect placement, and incorrect size for application), which necessitates early replacement. Industry standards should be developed for the manufacturing of bearing pads so specifiers would have confidence in selecting durable products.
As there is a significant chance of damage from the lifting operations, bearing pad replacement should only be undertaken after evaluating the relative risk of leaving the failed pads unrepaired. For those cases where an engineer recommends proceeding, standardization of the procedures (along the lines presented in this article) would ensure durability of the replaced bearing pads and minimize the risk of structural damage to the garage.
Note: The authors gratefully acknowledge the continuing support and leadership of David W. Fowler, PhD, PE—the faculty advisor for the research being performed at The Durability Lab, a testing center at The University of Texas at Austin.
Doris Eichburg is a principal with Building Diagnostics, specializing in the investigation of problems with existing buildings, designing remedies for those problems, and resolving disputes arising from them. She participates in the research being performed at The Durability Lab—a testing center established by Building Diagnostics at The University of Texas at Austin (UT). Eichburg can be reached by e-mail at firstname.lastname@example.org.
Beth Anne Feero, EIT, is a graduate student studying architectural engineering at UT. She serves as the graduate research assistant for The Durability Lab, which researches and tests the durability of building components, identifying factors causing premature failure. She can be contacted at email@example.com.
David H. Nicastro, PE, is the founder of Building Diagnostics. He is a licensed professional engineer, and leads the research being performed at The Durability Lab. He can be reached by e-mail at firstname.lastname@example.org.