Questioning Dampers

The story "Transparent Curtain Walls Perform Well in Blast Tests" attributes innovative concepts to the improved performance of structures to blast loadings (ENR 5/3/04 p. 18). Although responding (non-rigid) window frames are known to outperform non-responding (rigid) window frames, the article doesn’t demonstrate how dampers actually reduced "the effective pressure" on facade elements tenfold.

Dampers have been considered for blast-resistant design of building components in the past and have been rejected. Unlike seismic excitation, which involves multiple cycles of relatively low frequency base motion, for which dampers are very effective, blast loading is characterized by a single sudden high-intensity pulse for which dampers are spectacularly ineffective. For example, slotted connections may be used to dissipate some energy through friction (Coulomb damping) but eventually the bolt will reach the end of the slot and the system will be subjected to a secondary shock loading.

The article doesn’t indicate details of the loading, nature of the nonlinear blast analysis that was performed, nor does it adequately describe blast tests that were performed to justify the results. Because of the relatively small mass of glass and frames, it is difficult to imagine how a tenfold effect could have been achieved.

Frictional dampers would need a large stroke (travel distance) to achieve a tenfold effect, while viscous dampers would require a balance between the frequency of the system and the loading. Yielding dampers provide much less energy dissipation than the yielding permitted in the inelastic design of the frame. Larger deformation limits, permitting greater inelastic response, will have a much greater effect on improving the effectiveness of blast-resistant facade systems; however, quite often, the conventional performance criteria dictate the stiffness of the facade system.

There is no doubt that more flexible systems that allow for greater rotation and deformation are beneficial (to the extent they don’t interfere with wind and water infiltration), but this is not a revolutionary concept nor does it relate to damping.

More on Dampers, Security

Publicly releasing the details of any security system is inappropriate. However, there is some additional information that may help to understand how our system works. The device has nothing to do with seismic dampers.

By "tuning" a Coulomb damper to slip at a predetermined tension load, the system has reduced effective peak pressures from 180 psi for a conventional curtain wall to 10 psi. The approach completely avoids the problem of "bolt travel" and resulting secondary shock load.

The beauty of this system is that "conventional" performance criteria for wind and water can be easily met and still provide increased life safety under blast loading. If you take to the extreme the statement that "flexible systems that allow for greater rotation and deformation are beneficial," one can understand the secret behind this invention.