20 - Friday, August 2 26, 1994 - North Shore News

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IN PREVIOUS Quake Tips, we have discussed factors such as the type of earthquakes, soils, slopes, and building codes and the roles that they all play in our understand- ing of earthquake effects on North Shore buildings and structures,

QUAKE TIPS

By Ross Peterson

But what about buildings them- selves? What makes them safe or resistant? Are some types of build- ings more resistant than others?

In this two-part article, we will look first at how buildings are affected by earthquakes and what has been done to make them resis- tant; and then consider the types of buildings we have on the North Shore and how they might'react differently to earthquake motion. Following this, future Quake Tips columns will deal with the various things we can do as building and home owners to ensure these struc- tures are safe places to work and live.

The structural elements of buiidings are designed to distribute and carry the weight of the build- ing and its contents to the support- ing foundation in the ground. Some of the weight is distributed along horizontal planes (beams, roofs, and floors) but the heaviest weight will be borne by vertical support- ing members such as walls .and

columns leading to the foundation. -

Earthquake shock waves gener- ate opposing horizontal and verti- cal vibrations in the ground which push and pull upon a building's foundation. This in tum causes the building's walls to actually expand and compress in response to verti- cal shock waves; and to bend or sway from side to side from the horizontal motion.

The building structure tends to resist these ground motions and this natural inertia snaps the build- ing up and down and back and forth. This inertia response is anal- ogous to a person in a sharply braking car or one’s sensation in a rapidly rising or descending eleva- tor.

Buildings are obviously designed to carry their heavy verti- cal loads effectively, so vertical motion from moderate earthquakes may be adequately resisted by most

building design. A very strong ver-

Initial

Location

Ground Reversal Ground Movement. Ground ,Movement Movemen

Final Ground Location

Ground

WHEN THE ground moves, wave force is transmitted to the structures through the foundations.

tical motion however may exceed building strengths with resulting damage.

The Jan. 1!7 Northridge California earthquake had a strong vertical motion which pulled build- ings off their foundations and roofs away from walls.

Horizontal carthquake motions where they exceed the building’ lateral strength and flexibility, cause the more conventional cracked and broken studs and columns, and separation of walls from roofs.

Strong horizontal motions can also burst the mortar seams of bricks or concrete block walls if they are not reinforced or inade- quately connected to the building frame. There are many exaraples of this effect in the January Northridge earthquake.

In order that buildings can absorb and distribute the earth- quake forces without collapse or severe damage, the building code requires several techniques of rein- forcement including lateral bracing and sound connections between all of the structural components.

In addition, the foundation, buildings materials, structural design and quality of workmanship are also important to the resistance of the building to earthquake motion,

Lateral Bracing

Lateral bracing of the walls is the best way to ensure that the sideways motion of an earthquake can be carried from one component . of a building to another without damage. Lateral bracing strength- ens the connection between the horizontal and vertical parts of a structure and stiffens the walls,

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columns and studs against failure. Many building failures in the January Northridge earthquake were the result of inadequate later- al bracing of wood frame build- ings, Lateral bracing also reduces the damage to exterior and interior finishings (such as sidings and wallboard), windows and doors, and other elements.

Latera) bracing is a strengthen- ing component in all modern build- ings. Latge buildings may have a series of connected box shaped frames of steel or reinforced poured concrete as bracing. Wood frame buildings such as most hous- es generally have plywood sheath-

BRACING FOUNDATION

SEVERAL FACTORS come into play in determining a

buliding’s earthquake readi- ness,

ing nailed to the studs and support- ing columns for lateral bracing. The plywood sheathing allows the naturally flexible wood supports to bend or deform just enough to

See Structures page 21

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