expansion joints, snare drum, slot test

One of the most disturbing revelations about the Silver Spring Transit Center is that the 580 ft. x 315 ft. SSTC doesn’t have expansion joints. The SSTC was supposed to have been designed according to WMATA design criteria, which require that expansion joints in structures be spaced no farther than 100 ft. apart. (Providing expansion joints in a building or bridge isn't a special, unique requirement of WMATA; it's Building 101.)

Without expansion joints, expansion and contraction of the building from temperature changes create residual stresses that become “trapped” in the structure. Further, without expansion joints, the large structure is rigid; accumulated residual stresses are spread over the entire structure, rather than being disbursed in smaller pieces of the structure. These accumulated stresses can be quite large, and can result in concrete failure (cracking), especially when combined with stresses from external loads (buses, cars, etc.).

The problem of no expansion joints and resulting concrete failure is further compounded by post-tensioning. In a post-tensioned building, steel tendons, or cables, are embedded in slabs, beams and girders, and the tendons are “stressed” after concrete is poured. Since the large structure is rigid (slabs, beams, girders and walls acting together without expansion joints to “intercept” residual stresses created by stressing post-tensioning tendons), cumulative residual stresses from post-tensioning have the same effect (except to a greater degree, depending upon the loads applied to the tendons) as residual stresses from temperature changes. Strength and quality of the concrete, which are affected by, among other factors, cement and water content, temperature during curing, etc. also influence concrete strength and failure (cracking).

 

Media reported that WMATA called for “slot stress tests” at the SSTC. A Los Alamos National Laboratory paper entitled “Residual stress measurement by successive extension of a slot: The crack compliance method” (which can be found and downloaded on the internet by searching the title) describes experience with these tests as limited and applying mostly to manufactured machine parts, i.e., experience with large buildings is very limited. The paper also describes results of these tests as “approximations” of residual stresses, as opposed to quantifiable measurements. This can be attributed to the highly indeterminate nature of a large structure with integral slabs, beams, girders and walls (as opposed to a large structure with regularly spaced expansion joints).

 

A representative for the general contractor for the SSTC, Foulger-Pratt, was described in media reports as saying that the SSTC was “like a tightly wound snare drum”. The problem with the “snare drum” analogy is that a tightly wound snare drum can be loosened, while a large concrete building without expansion joints cannot.

 



 

Note: This exhibit was provided by Montgomery County to the Washington Post. The first line reads: "Cracks do not impact load support..." I take exception with this statement. While residual stresses are the result of "internal" loading caused by temperature change or post-tensioning, as opposed to external loads (buses, cars, etc.), residual stresses are cumulative to stresses caused by external loads, and can result in concrete failure (cracking).