In some cases, it may not be possible to retrofit an existing building to limit the extent of collapse to one floor on either side of a failed column. If the members are retrofitted to develop catenary—the natural curve created by a flexible cord freely suspended between two fixed points—behavior, the adjoining bays must be upgraded to resist the large lateral forces associated with this mode of response. This may require more extensive retrofit than is either feasible or desirable. In such a situation, it may be desirable to isolate the collapsed region rather than risk propagating the collapse to adjoining bays.
The retrofit of existing structures to protect against a potential progressive collapse resulting from the detonation of a terrorist explosive threat may therefore best be achieved through the localized hardening of vulnerable columns. These columns need only be upgraded to a level of resistance that balances the capacities of all adjacent structural elements. At greater blast intensities, the resulting damage would be extensive and termed global collapse rather than progressive collapse. Attempts to upgrade the structure to conform to the alternate path method will be invasive and potentially counterproductive. Care must be taken not to weaken a structure in the attempt to make it more robust.
Conventionally designed columns may be vulnerable to the effects of explosives, particularly when placed in contact with their surface. Standoff elements such as partitions and enclosures may be designed to guarantee a minimum standoff distance; however, this alone may not be sufficient. A steel jacket or a carbon fiber wrap may be used to provide additional resistance to reinforced concrete structures. These systems effectively confine the concrete core, increase the confined strength and shear capacity of the column, and hold the rubble together to permit it to continue carrying the axial loads. The capacity of steel flanged columns may be increased with a reinforced concrete encasement that adds mass to the steel section and protects the relatively thin flange sections. The details for these retrofits must be deigned to resist the specific weight of the explosives and the standoff distance. See also WBDG Designing Buildings to Resist Explosive Threats, section on Column Reinforcements.
Floor slabs are typically designed to resist downward gravity loading and have limited capacity to resist uplift pressures or the upward deformations experienced during a load reversal. Therefore, floor slabs that may be subjected to significant uplift pressures, which may overcome the gravityloads and subject the slabs to reversals in curvature, require tension reinforcement at the top fiber of the mid-span locations and bottom tension reinforcement at the underside near the supports. If the slab does not contain this tension reinforcement, it must be supplemented with a lightweight carbon fiber application that may be bonded to the surface at critical locations. Carbon fiber reinforcing mats bonded to the top surface of slabs would strengthen the floors for upward loading and reduce the likelihood of slab collapse from blast infill uplift pressures as well as internal explosions in mailrooms or other susceptible spaces.
This lightweight high tensile strength material will supplement the limited capacity of the concrete to resist these unnatural loading conditions. These retrofit options are currently the subject of a Technical Support Working Group (TSWG) research project, being performed by Weidlinger Associates (WAI) at the Energetic Materials Research and Test Center (EMRTC), and the initial results show the retrofits to be effective. An alternative approach is to notch grooves into the top of the concrete slabs, and then to epoxy carbon fiber rods into those grooves. Although this approach may offer greater capacity, it is much more invasive and has not been evaluated with explosive testing. See also Designing Buildings to Resist Explosive Threats, section on Floor Slab Reinforcements.(ArticlesBase SC #3983699)