Fig. 255.

The way to put in such rods is to leave the lower end free to move vertically, that is up and down, but secure it against lateral movement and then to attach to the lower end a heavy weight, proportioned according to circumstances.

In Figuring the allowance for wind it is customary to take only one-half the usual allowance for circular or surfaces slanting to the direction of wind. This is done because they offer less resistance than a flatly opposed surface, allowing the wind to slip by more readily. In a circular steeple we should take as our area the half circumference of base multiplied by the length on the rafter line and

Vertical tie-rods in steeples.

Wind allowances for different surfaces multiply this by only one-half the pressure for wind as given in Table XLIV. This usually would be 20 pounds per square foot.

For an octagonal roof we should assume full pressure on the central surface and only half pressure on the two side surfaces. Where the octagon is a regular one, this would amount practically to assuming double pressure on one surface only.

There is no danger of a steeple blowing over diagonally, as the sides would in such a case present slanting surfaces to the wind, allowing it to slide off readily; and besides the base line, being the diagonal of the square, would be so much longer.

On vertical (wall surfaces) the writer usually allows only for a maximum wind pressure of 30 pounds per square foot, for, as a rule, they (or part of them) are low down near the ground and therefore not exposed to the full force of the wind.