Monday, June 22, 2015

What Do Structural Engineers Have to Do with Disaster Resilience and Climate Change Adaptation?

Structural Engineers have the most direct impact on the disaster resilience of new and existing buildings and their ability to adapt with ever-changing climate. Although we typically cannot impact the location of a new structure, we can often impact its orientation, shape and exposure to various risks. The orientation and shape of the structure and the location of movement joints may significantly impact how much force is transferred to the building cladding, framing and lateral system during a severe wind event, earthquake, extreme temperature event, tidal event or terrorist attack. The first step in the process is to identify which risk factors pose a significant risk to the structure.

Structural Engineers must determine the potential risks for a given location over the service life of the structure even if this risk is not addressed by the governing code. For instance, current FEMA flood elevations do not directly factor in the impacts of future sea-level rise; depending on the service life of the structure, this may have a significant impact on the building’s exposure risk. We must first determine what code-prescribed requirements apply for addressing these various disaster risks, based on the occupancy type and importance of the structure. Some questions that need to be addressed are as follows:
  • What design codes are currently available for addressing that specific risk?
  • Do these codes specify loading and serviceability requirements?
  • Where codes or design aides are not yet available, what design practices can be implemented to help address this risk?
  • Building codes represent minimum/baseline requirements. Are these sufficient for the structure given its location?

After the various risk factors have been identified and the magnitudes of forces to be applied to the structure are understood, the Structural Engineer must then select the proper structural system and determine how the exterior cladding is anchored and transfers forces back to the structure. Below are some items to consider in this exercise:
  • Building Material Type: Structural steel, reinforced concrete, wood, masonry, light gage, etc.
  • Lateral System Type: Load-bearing or non-load bearing shear walls, moment frames, braced frames, dual systems, etc.
  • Level of Ductility: Response Modification Coefficient (R-value) of lateral system, provision of alternative load paths (to address progressive collapse), decoupling of lateral system from gravity system, system redundancies (e.g. use of full depth shear connections that can transmit diaphragm shear in addition to vertical shear), etc.
  • Structural Movement Joints: Are structural movement joints required? Where and how often are structural movement joints to be placed? Where are the lateral system frames located relative to these joints? For instance, locating lateral frames further away from the center of the diaphragm will increase member forces during an extreme temperature event.
  • Cladding Anchorage: How and where is the cladding anchored to structure? It’s generally best to apply any lateral forces directly to floor diaphragms. Does the superstructure support the weight of the cladding? How can cladding spans be configured to absorb the most energy and to minimize the forces transferred to the building frame during a disaster?

Once the risks have been determined and the building frame system has been chosen, the Structural Engineer must determine the required level of performance during a design event based on the client’s objectives, the acceptable amount of damage after one of these events, and the client’s budget. For example, ASCE 41 categorizes the performance of a structure during a seismic event in three levels: Immediate Occupancy, Life Safety and Collapse Prevention. Similar performance levels can be established and specifically tailored for different types of disasters. The goal is to determine how quickly after an event the building must be operational. In addition, the Structural Engineer must determine how, where and what level of ductility and redundancy must be provided to avoid a disproportionate level of damage or progressive collapse of the structure.

There are many ways Structural Engineers can be proactive about disaster preparedness, providing the best possible service to their client, serving their local community as well the overall engineering community. A Structural Engineer must effectively communicate with the design team and owner on what potential disasters must be considered, identifying what the code requirements are, and what the most efficient tools and methods for addressing those risks are. Structural Engineers can serve on code committees to advance current literature on addressing the risks posed by various disasters. It’s also important to be prepared for disasters by undergoing training on performing structural assessments of existing buildings post-disaster (e.g. ATC-20-1: Postearthquake Safety Evaluation of Buildings, ATC-45: Safety Evaluation of Buildings After Wind-Storms and Floods). Our ability to quickly assess the condition and level of damage of a structure after such an event is critical to the local government’s ability to assess the overall impact to a geographic area and the resources that will be required to reconstruct or remediate the area, as appropriate.

Structural Engineers play a critical role in impacting the disaster resilience of new and existing buildings and it is incumbent on all of us to be mindful of these risk factors in our day to day work. As climate change, sea level rise, and the frequency of extreme weather events continues to escalate, our ability to address these risks on a variety of fronts will be critical to the long term sustainability of our communities.

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