Quake Resistant Design

By David Jacknin

New map from the US Geological Survey showing probable areas of seismic activity.

[The recent earthquakes in Japan and Ecuador are yet another warning that it's time to prepare for the inevitable. It turns out that seismic activity in the US isn't just a California phenomenon, but also occurs in the Northwest, Midwest, and even Washington, D. C. -- remember the magnitude 5.8 quake that damaged the Washington Monument on August 23, 2011! Find preparedness plans, like the 

seven step approach shown here, and other comprehensive earthquake info at the US Geological Survey where you can download their very informative brochure Living On Shaky Ground. In this post, contractor David Jacknin offers some basic advice for building a home that is better prepared to withstand a major seismic event. -- Editor Dan Gregory]

New homes built in earthquake prone areas are subject to rigorous building codes. These codes are designed to protect the occupants from loss of life in a seismic event. It is advisable to exceed these minimum standards and take precautionary measures to minimize earthquake damage. Here are general guidelines to consider and specific actions you can take to get your house ready for a seismic event.

Site Selection – Know your soil type.
Houses built on bedrock will generally minimize the damage from an earthquake while houses built on sedimentary soils, fill or clay will encounter the maximum earthquake displacements. When building in an earthquake zone, consult a soils engineer before proceeding.

Connect your house to the mudsill. 
Your house needs to be connected to the mudsill. This is important to prevent sliding and uplift that occurs with both vertical and lateral forces. Holddowns are also very important.  In California, these are installed every four feet and connect framing members to the foundation.  The idea is to have a continuous connections from the foundation all the way to 

the roof rafters (hold-down image courtesy Mcvicker.com).

Install shear walls. These are your principal means of offsetting  horizontal/lateral forces. They are most important in the cripple wall area (below the first floor). Shear walls are created by covering your exterior walls in plywood and nailing the bottom, top plates and every stud 6” on center (see your local building code). This creates a plywood gusset that resists lateral forces. The thickness of the plywood, nailing pattern and size of the nails are specified by your local building codes.

Shear walls should be placed on all four corners of the house for equal strength in each direction. They should be added around all openings such as doors and windows. Your engineer or architect can help you with details (image courtesy Association of Bay Area Governments).

Retrofit or remove chimneys.
Masonry chimneys are very unstable and can crash through your roof and ceiling. Consider bracing or removing the masonry chimney and replacing with a metal flue. Though expensive, this measure can save lives as well as save considerably on repair costs.

Connect floors.
Split level and houses built on steep slopes face risks from weak connections between floors and between sections of the house. The house may be well connected at the foundation level while an upper floor or section of the house may move independently from the rest of the house. Connections can be improved by using mechanical fasteners and plywood gussets to unite overlapping floors and wings of the house.

Roof Connections. In keeping with the principal of connecting all framing members from foundation to roof,  it is important to connect rafters to wall top plates. Hurricane ties and other mechanical connectors are available for this purpose (example shown below courtesy Simpson Strong-Tie).

 
Reinforce Large Openings -  Finally, if your house includes large openings such as openings around garage doors, your engineer may recommend such measures as a moment frame, which is a steel structure that offsets lateral forces. All such measures will help offset the lateral and uplift forces that houses encounter in a seismic eventYour architect, engineer and local building department have the tools to help you plan and design for such an event.

Mon Apr 25 00:00:00 PDT 2016