Earthquake Risk and Engineering towards a Resilient World

When a structure is subjected to a very short lived impact or blast at one location, a pulse of energy is transmitted through the structure. This energy pulse manifests itself as displacement, velocity and acceleration within the structure. The overall effect is known as In Structure Shock (ISS). Although this shock has a very short duration and is attenuated as it passes through the structure, it theoretically has the potential to disrupt equipment resting on or anchored to the building structure. The paper will examine how to deal with such shock transmission, using the example of a facility built from multi-cellular concrete shear walls. The findings are applicable to all high hazard facilities where such shock transmission can occur.

The paper will examine how the effects of shock loading may be modelled within the building. It will compare and contrast the techniques available for shock loading with those commonly used for seismic loading, and discusses the differences associated with the extremely short duration of loading.

The use of spectra for shock typically shows significant accelerations for frequencies greater than 20Hz; with very high accelerations (many multiples of g) at the peak, which is in the region of 100 to 300 Hz. The dynamic deflections associated with these accelerations are small (typically less than 1mm) at frequencies of 15 to 25 Hz, with even smaller deflections at higher frequencies. The paper will discuss how these effects can be allowed for. The consequence of these high strain rates on material properties will also be considered.

The duration of the pulse is of the order of 10 ms to 100 ms. In this timescale the energy absorbed is small, and as such, if the structure system or component (SSC) was to yield and a mechanism form, then only a very limited amount of plastic deformation would occur. Collapse would only occur if a brittle fracture occurred in the few milliseconds of the pulse. In this time scale, even failure from conventional brittle failure modes such as buckling and shear will not occur in steel. Failure modes are limited to components shaking loose; failure of brittle materials; and other similar modes.

Based upon the above, the paper will examine for the structure in question, a number of different techniques that were adopted to deal with the various SSCs. These were:

• Comparison with seismic spectra

• Adequate Margin

• Ductility

• Use of shock fragility data in accordance with UFC 3-340-01, 2002

• Provision of vibration isolation