this page updated 8/1/07
On July 10, 2006, part of the ceiling collapsed in the D Street portal of the I-90 connector tunnel in Boston, Massachusetts (also known as the Big Dig tunnel accident), causing one fatality. The National Transportation Safety Board (NTSB) released a synopsis of its Highway Accident Report on July 10, 2007. The NTSB found that the probable cause of the collapse was "the use of an epoxy anchoring adhesive with poor creep resistance, that is an epoxy formulation that was not capable of sustaining long-term loads." Over time, the epoxy deformed and fractured until several ceiling support anchors pulled free and caused part of the ceiling to collapse.
The adhesive used in this project was not a Simpson Strong-Tie product. However, the NTSB report also concluded: "There is a general lack of understanding and knowledge in the construction community about creep in adhesive anchoring systems."
As a manufacturer of anchoring adhesives, Simpson Strong-Tie is knowledgeable about anchor performance and creep resistance. Most of our products, including ET, SET, Acrylic-Tie and VGC, have passed the creep test defined in ICC Evaluation Service AC58 and are qualified for resisting long-term tensile loads. To help educate our customers about our creep testing and the appropriate use of our products in construction applications, including those for long-term sustained loads, we have created the following FAQ.
Frequently Asked Questions
What is creep?
Creep is the slow continuous deformation of a material under constant stress. Creep occurs in many construction materials, including concrete and steel when the stress is great enough. The creep characteristics of adhesives are product dependent. Adhesive anchors that are not creep resistant can pull out slowly over time when sustained tensile loads are applied. The ability of an adhesive to resist a load over the short term does not indicate the adhesive's ability to support lesser loads over the long term. This behavior is well-known to anchor manufacturers and is considered in the formulation of structural adhesive anchoring products.
Because of the creep phenomenon, it is important for designers to consider the nature of applied loads and to determine if loads will be continuously applied to the anchor over the long term. If this is the case, a product that is suitable for resisting sustained loads over the long term must be selected.
How are adhesive products tested for creep resistance?
Manufacturers conduct a "creep test" in which an anchor is loaded and monitored for movement over time. Presently, the most widely used creep test is specified in ICC Evaluation Service AC58. According to the NTSB, the adhesive product that failed in the Big Dig accident did not pass this test. According to AC58, anchors that pass the creep test are determined to be suitable for resisting long-term tensile loads. Anchors that do not pass the creep test are to be limited to short-term loads, such as wind and seismic forces only.
Which Simpson adhesive anchoring products are creep resistant?
ET, SET, Acrylic-Tie and VGC. These products passed the creep test defined in ICC Evaluation Service AC58 and are qualified for resisting long-term tensile loads. Simpson's published literature and ICC-ES evaluation reports reflect this information.
Which Simpson adhesive anchoring products are NOT creep resistant?
ETF - this product has not passed the ICC Evaluation Service AC58 creep test and is only recommended to resist short-term loads such as wind or seismic forces. Simpson's published load tables and ICC-ES evaluation report for ETF indicate this limitation.
What are Simpson's recommendations regarding the use of adhesive anchors in overhead applications?
Overhead installations with Simpson's adhesives are safe when properly designed and installed in accordance with Simpson's printed literature.
If the overhead attachment will resist long-term tensile loads, a creep-resistant adhesive that is qualified for long-term tensile loads should be selected. If the overhead attachment will only resist short-term tensile loads, either a non-creep resistant adhesive that is restricted to resisting short term tensile loads may be selected, or a creep-resistant adhesive that is qualified for long-term tensile loads may be selected (see table below).
In any application, especially overheard applications, the designer must give proper consideration to fire exposure conditions. Adhesives lose load carrying capacity at elevated temperatures (see the "In-Service Temperature Sensitivity tables in Simpson literature) and the designer should consider the likelihood and consequences of anchorage experiencing elevated temperatures.
Overhead installations should utilize a Simpson adhesive retaining cap (ARC) to retain the adhesive in the hole.
Note: All other design and installation recommendations in Simpson literature must also be met.
Overhead Application Suitability Table for Anchoring Adhesives
Long-term and |
Short-term loads |
ET |
ET |
Where can I find more information about the "Big Dig" accident?
The NTSB has published a press release and a synopsis of their report on their website, www.ntsb.gov.