Prior to joining Simpson Strong-Tie, my career involved the design of projects in California’s San Francisco Bay Area. When designing the primary lateral force resisting system, I would have several pages of seismic base shear calculations and, oh yeah, a one- or two-line calculation of the wind forces – just to show that seismic governed. There was no need for complete wind analysis, since the seismic design and detailing requirements were more restrictive. Of course, building components such as parapets, cladding or roof screens needed a wind design. Unfortunately, when wind appears to control, meeting the seismic requirements is not so simple.
The building code requires that every structure be designed and constructed to resist the effects of earthquake motions. The 2006 & 2009 International Building Code, Section 1604.10 states that:
Lateral-force-resisting systems shall meet seismic detailing requirements and limitations prescribed in this code and ASCE 7, excluding Chapter 14 and Appendix 11A, even when the wind load effects are greater than seismic load effects.
Often, these seismic detailing requirements and limitations get lost in the shuffle of building design when it appears that wind controls.
Wind vs. Seismic
Calculated wind pressures on a structure produce actual loads the building is expected to experience during a wind event. A good structural system for wind design is typically a strong, heavy system with robust connections to help resist loads as the wind blows across and over the structure.
In seismic conditions, however, it’s expected that buildings will undergo cyclic loading as the ground moves back and forth and the building’s inertia catches up with the ground movement. So the building is designed to absorb and then dissipate seismic energy through damping and yielding of the lateral systems.
This concept requires a good understanding of the ductility in the lateral system and controlling the amount of deflection the system will undergo during this loading condition. Going a step further, the seismic loads on a building are not the actual loads a building will experience during a seismic event. The ability of the system to undergo inelastic displacements and retain structural integrity – or ductility – is utilized more than the strength characteristics.
Currently, the code-calculated seismic forces on a structure are based on 2/3 of the actual expected ground accelerations. The seismic load is further reduced based on the ductility of the lateral-resisting system chosen. Many lateral-resisting systems, such as light-frame wood shear walls, have a ductility variable assigned and tabulated in the ASCE7 reference standard.
When comparing wind and seismic lateral loads on a structure, it may appear that the wind load will control over the seismic load. However, if the ductility of the lateral-resisting system is less than the initial value used to calculate the seismic force (perhaps due to poor detailing or limitations), the seismic load may actually control. This may not be obvious unless a full wind and seismic analysis is performed.
Assume that the simple structure below has a calculated lateral load (in-plane) on the wall of 1500 lbs. due to wind, and 1000 lbs. due to seismic. You may quickly dismiss the seismic load and base the rest of your design on the wind load. Makes sense because it’s 50% higher, right?
But when you analyze and apply the detailing and seismic limitations to the design, you reach a much different conclusion. The height-to-width ratio of wood light-frame shear walls shown in this structure is 3.5:1 (7ft/2ft), which is acceptable for wind conditions. For seismic loads, a ratio of 2:1 is required for full shear wall capacity per the Special Design Provisions for Wind and Seismic. The SDPWS does allow the height-to-width ratio of the shear walls to be increased to 3.5:1 for seismic conditions provided shear capacity of the wall is multiplied by 2W/H. Applying this limitation to demand load yields an adjustment to the seismic force of 1.75 [shear wall height / 2x shear wall width = 7 ft / (2×2 ft) = (7ft /4ft) = 1.75], and the adjusted seismic force is now 1750 lbs. compared to the 1500 lbs. force due to wind.
There are many seismic details and limitations just like this one that may affect the outcome of the structural design, including connections, drag struts, anchorage to concrete and framing designs. We can’t simply compare a wind and seismic base shear to make a decision about which load controls. Depending on the shape of the building, wind may control in one direction while seismic controls another.
It is essential to be compliant in all seismic detailing and limitations to ensure the safety and welfare of the building occupants. Often, the only way to do this is to design the structure completely for each load effect and apply the appropriate design for each application.
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