By Erin Kelly, P.E.
As you may be aware, there is a document used in assessing the seismic performance of existing structures entitled, ASCE 41 - Seismic Evaluation and Retrofit of Existing Buildings. Among other topics, this guide offers instruction for basic assessment of seismic vulnerabilities through “checklists” on general building configurations as well as one specific checklist for each building type. The checklist on configurations is used for all building types and is referred to as the “Basic Checklist.”
A quick scan through the Basic Checklist will highlight several of the concepts that you’ll need to be comfortable with as you prepare for the CA Seismic P.E. exam, so I thought it would be helpful to run through some of them here.
Consider a building you know well as you go through this list. Maybe it’s a building you designed, or maybe it’s the one you’re sitting in right now.
Here are some of the criteria:
Load Path: The structure shall contain a complete, well-defined load path, including structural elements and connections, that serves to transfer the inertial forces associated with the mass of all elements of the building to the foundation.
· As we covered previously, lateral loads are applied to the exterior walls of the building, then transferred to the diaphragm, then to the vertical elements of the LFRS, then to the foundations. Each of these elements needs to be sufficiently connected (i.e. dowels in concrete construction, bolts/welds in steel construction). Most buildings will pass this one.
Adjacent Buildings: The clear distance between the building being evaluated and any adjacent building is greater than 4% of the height of the shorter building.
· We cover this concept specifically in the course as we discuss both drift and separation. Pounding can cause considerable damage, particularly if the adjacent buildings are not the same height or do not have the same floor-to-floor heights. If the shorter building is displaced toward the taller building, and the contact point is between floor heights, the contact could occur at the midpoint of a column and cause catastrophic damage. If the buildings are the same height and experience contact during a seismic event, the damage will be less significant but could cause damage at the roof/wall connection.
Weak Story, Soft Story, Vertical Irregularities, Geometry, Mass, Torsion:
· You should recognize each of these as some of the Horizontal and Vertical irregularities from ASCE 7. These irregularities were only codified in the 1994 Uniform Building Code (UBC), so buildings designed and constructed prior to the adaptation of the 1994 UBC are more likely to have these irregularities. Also notable is that weak story and soft story are listed here separately. While they are the result of similar configurations, “weak story” relates to strength and “soft story” relates to stiffness.
Liquefaction, Slope Failure, Surface Fault Rupture:
· These all relate to the soil below the structure. However, we now know that the soil conditions can be a large factor in how the buildings will behave during a seismic event. Liquefaction relates to a type of soil in which the cohesion between the soil particles is likely to decrease to a point of instability when saturated. This can occur during an earthquake, as was seen specifically in the 2011 Christchurch Earthquake. Slope Failure relates to landslide hazard, which is relatively common after an earthquake. Surface Fault Rupture relates to the proximity to the closest known fault. If a structure is located very close to a fault, the building could be damaged by the surface rupture during an earthquake. Based on recent legislation, structures shall not be built immediately on top of or within 50 feet of these fault lines.
If you have time to review the building-type-specific checklists, you’ll notice some of the improvements that we discussed in the Building Code Blog Post. These checklists are designed to show how much of the modern seismic detailing can be found in these existing structures, and I think they double as a great study tool for this exam.