Technical Papers

Earthquake Retrofitting

Wednesday, June 4th, 2014 by Michelle Dalinghaus

With the recent earthquakes around the Southern California area, seismic retro fitting, is something many are or should be considering, especially in older homes and buildings that may not be up to the new building codes.

Wikipedia describes seismic retrofitting as follows:

Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures and with our recent experiences with large earthquakes near urban centers, the need of seismic retrofitting is well acknowledged. Prior to the introduction of modern seismic codes in the late 1960's for developed countries (US, Japan etc.) and late 1970's for many other parts of the world (Turkey, China etc.),[1] many structures were designed without adequate detailing and reinforcement for seismic protection. In view of the imminent problem, various research work has been carried out. State-of-the-art technical guidelines for seismic assessment, retrofit and rehabilitation have been published around the world - such as the ASCE-SEI 41[2] and the New Zealand Society for Earthquake Engineering (NZSEE)'s guidelines.[3] These codes must be regularly updated; the 1994 Northridge Earthquake brought to light the brittleness of welded steel frames, for example.[4] 

So what is Seismic retrofitting, according to Saber Getechnical Construction Solutions?

The seismic retrofitting process involves the installation of engineered metal connectors and other reinforcement materials at key stress points throughout your home’s structure. The specially designed hardware includes hold downs, anchors & anchor bolts, strap ties and framing angles. Wood blocking and structural sheathing may also be incorporated into many retrofitting packages.

The goal of the retrofitting process is to counteract three main earthquake-induced forces that can cause major damage to a building: racking, sliding and overturning. In engineer-speak, the retrofitting process transforms a building with loosely connected parts into a unified structure with a “continuous load path” that extends from the building foundation up through the walls and all the way to the roof framing. Our experienced crew will utilize an impressive arsenal of seismic connectors to keep your house structure together in spite of seismic forces that can easily tear an un-reinforced house apart.

To safe guard your home and your family against an earthquake its important to understand how forces from an earthquake can affect your home.

The ground forces of an earthquake can try to tear apart your home in three ways, however; there are two types of forces, these two types are lateral force and uplift force.  

Lateral forces can cause your home to move or tilt from side to side - this is called racking, or the lateral earthquake force can shake your house and weaken the frame, causing it to slide off the foundation - this is called sliding.

Uplift forces can cause the walls of your home to lift or rotate off the foundation - this is called overturning.

So in order to protect your investment from the damages of an earthquake, you must strengthen the frame of your home.

Strengthening the structural frame of your home includes creating a "continuous load path" within your home.  So what is a continuous load path?  It is a method of construction that uses a system of wood, metal connectors, fasteners (such as; nails and screws) and shearwalls to connect the structural frame of the house together.  A continuous load path is like a chain that ties the house together from the roof to foundation.  

A continuous load path is critical during an earthquake because it helps hold the house together when ground forces try to pull it apart.  When all parts of a home are connected together; roof, walls, floors and foundation, it is more likely to stay intact and withstand an earthquake.

Although building codes require home to be built with a continuous load path, not all parts of the country follow these national building standards.  The age of your home can also help determine whether it has a continuous load path.  Home built before, 1985, in most cases do not have a continuous load path.

With the continuous load path there are five different types of connection, according to the Simpson strong tie website:

     ~Rafter to top plate connection

          A home roof must be secured to the walls of the home,   Metal connectors called hurricane ties are             often used to fasten the roof rafters to the top plate, which runs horizontally along the top of the                 homes walls.  

     ~Top plate to stud connection

          The top plate should be attached to the wall studs with metal connections such as stud plate ties and           joist hangers.

     ~Floor to Floor connection

          If the home has two-stories or more, each floor must be connected to the one below it.  For                       example, in a two-story home the second floor is connected to the first floor with connector straps.

     ~Stud to mudsill connection

          A homes walls must be secured at the top as well as at the bottom.  The wall studs are connected to           the "mudsill", which runs horizontally along the bottom of the homes walls.  Metal connectors, such             as hold-downs and stud plate ties, are typically used to make this connection.

     ~Mudsill to foundation connection

          Which runs horizontally along the bottom of the homes walls, needs to be anchored to the home's               foundation.  Anchor bolts or mudsill anchors are used to fasten the mudsill to the concrete                         foundation.

So, therefore; building your home with a continuous load path is the best way to protect your home and your family from earthquakes and even high winds.

Seismic retrofitting is smart because…

  • Your home and your family will have a better chance of surviving an earthquake.
  • Proper installation of seismic hardware will help minimize earthquake damage, reducing home repair costs following a seismic event.
  • Professional retrofitting has the potential to reduce home insurance premiums.
  • A comprehensive home seismic retrofit “package” can typically be installed in just 1-2 days.

 

Don’t wait until it’s too late! Contact Saber Foundation Repair today for a free seismic inspection and seismic retrofitting estimate. We provide our professional seismic retrofitting and foundation repair services to residents all throughout Greater Los Angeles, including: Los Angeles, San Diego, Long Beach, Bakersfield, Santa Ana, Riverside, Anaheim, San Bernardino, Oceanside, Huntington Beach and many cities nearby in San Diego, San Bernardino, Riverside, Ventura, Los Angeles and Orange Counties!

 

Information for this paper, was found on the Simpson Strong Tie website, Saber is a proud installer of Simpson Strong Tie products. 

 

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