Strengthening a House in Earthquake Country

Many Americans believe that earth quakes strike California and only California. The fact is that earthquakes occur in many areas of North America with enough force to shake an ordinary building to pieces. If you plan to build a vacation retreat in any of these areas, you may wish to reinforce the structure to help it survive a major tremor.

Begin with a strong foundation. One of the strongest is a masonry-block foundation wall, stiffened with a skeleton of steel reinforcing bars (rebars) and filled with concrete grout. In earth quake country, masons build the wall with masonry blocks called speed blocks, especially designed to accommodate vertical and horizontal rebars. Other specially designed blocks permit horizontal rebars to round the corners of the wall without interruption. These lateral stiffeners, em bedded in grout in every third or fourth course and in the top course of the foundation wall, are called bond beams.

Strong materials are as important as sound design. Mortar for laying the blocks should be a mixture of 1 part port land cement to 2 1/4 parts damp, loose sand. In the concrete grout, use sand as the aggregate in a ratio of 3 parts sand to 1 part portland cement. One cubic yard of concrete will make enough grout to fill about 70 blocks.

To reinforce the wall, use No. 5 steel rebar. Vertical bars should be cut long enough to extend 23” beyond the top of the footing, then bent to a right angle 6” from one end; the corner bars should have a fairly tight bend to make them fit easily into the corner blocks. Have all of these pieces cut and bent beforehand by a steel fabricator.

A strong foundation alone is not enough to protect a cabin in an earth quake area. Stud walls, for example, should be braced with structural ply wood sheathing, fastened to the studs with nails spaced at 6” rather than the usual 12” intervals.

As a final earthquake-proofing touch, if you have a gas line, have a qualified gas serviceman install an earthquake valve in it. The valve is designed to shut automatically when shaken by an earth quake, reducing the danger of fire from a ruptured line.

90 Earthquake areas of North America. In this map of the United States and the most densely populated parts of Canada, dark-red sections represent areas where the risk of severe earth quake is greatest; if you are building in these areas, the special precautions described on the following pages are advisable. In light-red areas, major earthquakes are less likely but still possible at some locations; in these areas, consult local building officials before you begin construction to find out whether reinforcement is necessary at your cabin site.

In uncolored areas, earthquakes are rare and few builders construct reinforced dwellings.

Special Blocks for a Strong Foundation

Speed block. These masonry blocks have beveled, open ends that slip sideways around vertical rebars rather than down over them, as conventional blocks would; the single web of each speed block is recessed to accommodate horizontal rebar. Each course of speed block is mortared to the course below and vertically mortared to corner blocks; other vertical joints are not mortared. Concrete grout is added after each course is laid to fill the triangular spaces between the beveled ends of the blocks.

Preparing a bond-beam corner block. A pre scored block is designed for easy removal of webs and sides to make a path for a horizontal rebar. With light taps of a hammer, knock out the scored section on one side of the block—the side you choose will depend on the direction the block will face when built into the foundation—then knock out both webs.

If pre-scored blocks are unavailable, make your own bond-beam corner block out of an ordinary two-core corner block. With a brickset and a ball-peen hammer, chisel notches 4” wide and 3” deep as needed into the webs and side of the block.

1. Supporting the vertical rebars. Wire pre-bent vertical reinforcing bars to one line of the horizontal bars laid out for the foundation footing and to additional stubs driven 2’ into the ground. Drive two bar stubs at each corner, using the core of a corner block set temporarily into the trench as a guide. Set the remaining stubs 16” apart and directly below a string tied between batter boards to mark the center of the block wall. When all the bars are wired in place, pour the footing.

Reinforcing a Foundation Wall

91b 2. Grouting the wall. Build corner leads, using bond-beam corner blocks in the course that will be reinforced with horizontal reinforcing bars, lay the first course of speed blocks on all four sides of the foundation, and pour concrete grout into the hollows of the first course and down into the first course of the leads. As you pour the grout, have a helper stir it with a stick so that it fills the holes completely, leaving no air pockets.

Trowel the grout flush with the tops of the blocks. Lay and grout the second course in the same way. Do not grout the third course (or, if the wall consists of four courses, the fourth) until you have completed Step 3.

3. Making a bond beam. In the channel within the top course of blocks, lay two reinforcing bars, parallel to each other and 2” apart, completely around the foundation. At joints between bars, overlap the bars 20” and wire them together. At corners, use bars pre-bent by the supplier to pass through the notches in the webs and side of each bond-beam corner block. Grout the bond beam and embed anchor bolts in the grout (Step 5).

Stiffening a Frame Wall

93a Nailing plywood. Before applying whatever exterior covering you plan, sheathe the wall with full and partial panels of %” Structural 1 C-D plywood, covering the joists at the top and bottom. Wherever two panels meet without a stud behind the joint, nail 2-by-4 horizontal blocking between studs to support the edges of the panels. Space eightpenny nails 6” apart along the edges of all panels; within a panel, space the nails 12” apart.

An Array of Earthquake Valves

Types of earthquake valves. Designed for professional installation, earthquake valves come in a wide range of designs and prices. Simplest and least expensive is the valve at left, which consists of a metal ball perched on a ramp. When the valve is jolted by a tremor, the ball rolls off the ramp and into the gas line, stopping the flow of gas. In the more sensitive valve at center, a metal ball triggers a latch mechanism that clamps against the outlet end of the valve like a closing door. Most sophisticated is the valve at right, which bears the seal of approval of Underwriters’ Laboratories, the fire-protection organization whose recommendation is most widely accepted by fire-fighting agencies and insurance companies. In this valve, a pendulum weighted lower pin supports an upper pin set into a disk. When an earthquake strikes, the pendulum holds the lower pin stationary; the up per pin, dislodged by the tremor, falls downward and the disk plugs the valve seat.