04 - Building Design
Two aspects need to be considered with regards to seismic resistance of a structure; the load bearing system and the non-structural elements. The former refers to those elements, e.g. columns, beams, foundations, slabs, that contribute to resisting the inertial forces generated by an earthquake. The latter includes contents, equipment, machinery, etc. that do not contribute to the load resisting system. Damage to non-structural elements occurs at lower seismic shaking levels and can disrupt business operations, even if the structure sustains no damage.
Local design codes provide guidance on force levels and detailing requirements for new buildings and extensions to existing ones. Some issues are listed below:
- Age of building compared with latest version of structural design code. Older buildings may not meet state-of-the-art requirements in terms of force levels, detailing, dimensioning of structural elements, material properties, etc.
- Buildings with unique architectural configurations, eg. irregular plan shapes (footprint), changes in vertical profile (setbacks, overhangs). Such features require special design methodologies and detailing requirements as their performance during an earthquake is difficult to predict.
- Discontinuation of force-resisting elements, ie. walls and columns terminate at some level and are not continuous through the height of the building. Such elements must provide a continuous load path for the forces from all storeys to the foundations otherwise “soft storey” effects may result in collapse of weak storeys (pancaking effect).
- Changes in the structure due to upgrades, changes in occupancy, installation of new equipment, modifications to internal layout through partitioning, removal of structural elements, etc. Sometimes such “simple” modifications can significantly impact the seismic performance of the building if not planned properly with the support of a structural engineer.
- Roof-mounted equipment can change the dynamic characteristics of the structure with undesirable consequences on seismic performance.
The FEMA document “FEMA E-74: Reducing the Risks of Nonstructural Earthquake Damage – A Practical Guide” is publically available online and provides valuable guidance on the reduction of seismic damage to such elements. Some issues are highlighted below:
- Unreinforced masonry or concrete blocks. Such elements are commonly used as partitions or façade elements and have demonstrated poor seismic performance. If not detailed properly, eg. through provision of separation gaps between these walls and the adjoining force-resisting system, providing “frames” or internal reinforcement, then damage and even collapse of these walls is highly likely. This poses a hazard not only to equipment and machinery but also to life safety.
- Precast or glass façade elements require special seismic considerations with regards to detailing and connection to the structural elements.
- Bracing of piping. All critical piping, whether for transport of production-critical material, eg. gas, or of the fire-fighting system, must be braced for seismic forces. Damage to piping can result in leakage of hazardous material and/or fire following earthquake, through disruption of fire-fighting water supply and/or release of combustible material.
- Ensure that all piping penetrating walls are provided with a sufficient gap to prevent shearing of the pipes during an earthquake. These gaps are to be filled with fire-resistant foam or suitable material that can also deform under seismic loading.
- Before undertaking any modifications or changes to the structures, or any other capital expenditure project to existing buildings a detailed structural analysis considering the actual building configuration and condition, in terms of material properties, etc. should be undertaken in accordance with the requirements of the most recent edition of the structural design code. Material properties of existing structures can be determined using tools such as a Schmid Hammer (effective concrete compressive strength), Profimeter (to establish the reinforcement in the concrete), etc.
- Connections and material properties of façade elements, eg. precast or glass, are to be designed to withstand the expected seismic lateral deformations in accordance with the national structural design codes.
- Affix all loose furniture, eg. book shelves, suspended ceilings, partitions, which could collapse under seismic load and cause injury.
- Follow best practice guidelines or, if available, national seismic design requirements for steel storage racks. These can collapse and result in loss of stored material as well as injury to staff. Publically-available documents exist, e.g. FEMA 460) describing maintenance of such elements in seismic regions.
- Machinery and equipment are to be anchored with structural anchors to the foundations. The configuration and types of anchors as well as foundation capacity are all to be checked by a qualified structural engineer. This issue is especially critical for tall equipment and rotating machinery.
- Ensure that roof-mounted equipment have been considered in the seismic analysis during building design. If such equipment is subsequently added to the building, the potential influence on the structure’s dynamic characteristics should be controlled by a qualified structural engineer.