Creating a Seismic ShelterSep 21 2017 · 0 comments · NISHKIAN DEAN, Seismic ·0
By Edwin T. Dean, PE, SE
Having completed the initial designs for the innovative CoreFirst system, we at Nishkian Dean believe that it is a viable alternative to doing nothing and accepting fate when it comes to the next earthquake that may devastate Oregon buildings and put occupants in harm’s way. There is little argument that a full seismic strengthening of a building is the best solution, but for many building owners, it is simply not an expense that they can afford. If a full seismic upgrade is not a financially viable option, an alternative that would potentially provide a robust sanctuary to shelter occupants as the building around them shakes apart during a seismic event is a good one.
The CoreFirst system functions as a seismic shelter erected within an existing building, providing improved life safety during a seismic event without the need to retrofit the building to current seismic standards. Coupled with an earthquake early-warning system that can provide more than 60 seconds to evacuate, CoreFirst both alerts building occupants and provides a safe place to congregate during an earthquake.
Composed of a one or two-story steel special moment frames oriented in both principal directions, the CoreFirst system includes a steel grating plank platform at each level to provide protection from debris and existing building failure. The moment frames are isolated from the existing structure, ensuring that they only resist load generated by the seismic weight of the CoreFirst shelters. While the moment frames are not tied to the building’s existing seismic-force-resisting system, they are designed with a large reserve capacity for additional lateral load, with the added benefit of potential use as a component of a future comprehensive seismic upgrade of the building.
The shelters are designed so that their floor levels are located below the floor structure of the existing building. These floor levels are framed out with infill gravity framing supporting steel planks or channels that form a debris shield, preventing debris from falling through the floor of the existing structure into the shelter’s protective zone. The platforms are designed for a floor live load of 100 psf, a roof live load of 20 psf, and a vertical seismic load of 50 psf (representing both the dynamic load of debris falling on the platform and the static load of accumulated debris). Ultimately, all the gravity load is supported by the moment frames.
The moment frames are designed to the requirements of a Risk Category IV structure, which primarily impacts the drift limit, or the typical governing limit state for steel moment frames. Based on the seismic weight of the frames and grating platforms, seismic loads are generated for the frame per the equivalent lateral force procedure of ASCE 7. To enable the potential use of the frames as a component of a future full seismic upgrade of the building, additional seismic load is assigned at each level of the moment frames. It is not always possible to predict what shape a future seismic upgrade would take, and what loads the moment frames might need to carry, but a conservative load is estimated by assuming that a certain tributary area is assigned to the moment frames based on their location in the building. The total lateral load the frames are designed for is indicated on the construction drawings for future reference.
Footings are also designed for the additional seismic load described above. Because the frames are isolated from the main structure, they have very little dead load to resist overturning. As a result, there are three footing options:
1) Very large isolated footings with enough weight to prevent overturning
2) A mat footing designed to resist overturning
3) Small isolated footings/pile caps utilizing helical piles to resist uplift and downforce
The seismic gap required around all interfaces between the shelters and the existing structure is determined by estimating the maximum seismic drift of the building (based on the drift limits for the building’s structural system at the time it was constructed), determining the maximum seismic drift of the moment frames, and calculating the resulting maximum drift in any direction for both cases by combining the maximum drift in one direction with 30% of that drift in the orthogonal direction. The sum of the two maximum drifts is the minimum required seismic gap.
We believe that building owners could benefit from this affordable system. If you have any questions about CoreFirst, please contact us at the Nishkian Dean office or visit the CoreFirst website. We are happy to discuss this innovative system!
Edwin T. Dean, PE, SE is Vice President and Managing Principal of Nishkian Dean a structural engineering consulting firm in Portland, Oregon.