## 193. Flitch-Plate Girders

In the floor framing of heavy buildings not fire-proof and where wooden girders occur resting on columns as shown in plate, the columns are some-times placed farther apart than would be safe if wooden girders alone were used. In such cases the flitch-plate girder is sometimes adopted (see plan and section). It is composed of two wooden beams connected by an iron plate with 3/4-inch bolts placed about two feet apart.

It has been found in practice that the thickness of the plate should be about one twelfth of the whole thickness of the beams. The elasticity of iron being so much greater than that of wood, the beams should be proportioned so that they will bend at the same time as the iron, otherwise the whole strain might be thrown on the plate. The modulus of elasticity of wrought iron is about thirteen times that of hard pine; or a beam of hard pine one inch wide would bend thirteen times as much as a plate of iron of the same size under the same load.

Let D = depth of beam; B = thickness of wood; L = clear span in feet; t = thickness of iron plate; f = 100 pounds for pine; W= total load on girder.

Then for a distributed load, girder supported both ends, D= û[WL/(2fB+1500t)].

For a load in centre take one half, or D = û[WL/(fB+1500t)].

For example, what should be the depth of a girder composed of two 6" timbers supporting 26.16 tons or 52,320 pounds, span 20 feet, load equally distributed, the thickness of the iron plate being 1 1/8 inches, or one twelfth the thickness of the entire girder?

D =û[(52,320x20)/(2x100x12+1500x1 1/8)]= û256, or 16 inches.

For a safe load in pounds equally distributed =(2D2/L)(fB+750t), or Safe load =(2x16x16)/20(100x12+750x1 1/8) = 52,320 lbs.

## 194. Sidewalk Elevator

These hand elevators are used to hoist from 500 to 3000 pounds, and extend from lower floors under store floor to sidewalk. The platform is of wood, with an iron- or wooden-frame hoisting apparatus, securely and firmly braced. The plan is shown with square iron guides at N and platform M. Extending from the pillow block P and under the platform is a 2 1/2-inch-diameter shaft connected with the iron-frame hoisting apparatus. This is composed of two 30-inch-diameter wheels H and C arranged on the cast-iron frame D, and worked by the crank I and 6-inch-diameter pinion wheel E, as shown in plan and elevation..

A heavy chain is used for raising the platform. It is connected to the four corners, and extends over the three wheels in each cross-head 0 and down each side of the platform to the drum wheels A.

To lower a weight from the sidewalk, the pinion wheel E is thrown off the small shaft by the arm G; then a brake on wheel K worked by the lever arm L prevents any sudden falling of the platform. The framework is 4 feet high by 3 feet long by 22 inches wide. The crank arms extend 14 inches on each side, beyond the width of frame.

In setting the framework care should be taken that the platform lowers to the level of the floor; this is accomplished by building a ditch a trifle larger than the car, and placing the shaft B below floor level.

## 195. Wrought-Iron Gratings

Wrought-Iron Gratings are generally used for basement areas, and are made of 2 1/2 X 1/2-inch frames, with 1/2 X 3/8-inch filling bars placed 1 1/2 inches apart between centres, and secured to the frame by a tenon cut on each end, mortised into the frame and riveted cold.

Thimbles are placed on a 3/8-inch-diameter rod which passes through and stiffens the tars. The outside frame is set flush with top of coping, and dowelled with 1 1/2 X 1/2-inch flat dowels as at A.

The gratings are also made to open in sections as a door, working on butt or pin hinges as shown at B and C.

 Spans. Filing Bars. Frames. 3 ft. and less 1 1/4"x 3/8" 1 3/4" X 1/2"; one row of thimbles. 3 ft. to 5 ft. 1 1/2" x 3/8" 2 "X 1/2" " " " " 5 ft. to 8 ft. 1 3/4" x 3/8" 2" X 1/2"; two rows of thimbles. 8 ft. to 10 ft. 2" X 3/8" 2 1/4" X 1/2" " " "

## 196. Cast-Iron Perforated Plates

Cast-Iron Perforated Plates are used in front of doorways in connection with the above gratings cast from 1/2 to 3/4 of an inch thick, and of various designs.

## 197. Knee Gratings

Knee Gratings are frequently employed for forming platforms, treads and risers in connection with the cast perforated plates, and made of the same iron as that used for plain gratings.