T" 



INTERNAL FORCES 59 



will be commercial 6 ins. x 6 ins. wood, after surfacing. The pieces 

 therefore can be 6 ins. square. The lines of stress pass through 

 the centers of the pieces, otherwise some twisting and bending 

 strains will be set up. Twisting is called torsion in such cases. 



The actual construction of such a bracket frame is shown in 

 Fig. 51, the stresses for which are given in Fig. 50. The lower 

 piece rests on an angle at the wall and a plate may be placed 

 between the end and 

 the wall if the pressure 

 exerted is greater than 

 the wall can stand. 

 The area of the plate 

 will be such that the 

 pressure will be distri- 

 buted to an extent cal- Fig. 51 Design for Sidewalk Canopy 

 culated to keep the Vl/ 



allowable compressive load on the wall within proper limits. The 

 load P represents the reaction at the outer end of the frame, 

 the reaction at the wall end being carried on the angle support. 

 The diagonal is a rod which will be about 1 in. in diameter if of 

 >tcd, for steel can be stressed to 16,000 Ibs. per square inch in 

 tension. To anchor the rod in the wall a bolt 1 in. in diameter 

 extends into the far side and there a plate is fixed or it may be 

 anchored in a concrete block in the wall. 



The circumference of a circle is 3.1416 x the diameter. The 

 circumference of a 1-in. rod is 3.1416 ins., so for each inch in length 

 there is an area of 3.1416 sq. ins. An adhesion of concrete to 

 steel of 75 Ib. per square inch is customary, so the adhesion per 

 inch of length of the anchor rod in the concrete = 3.14 x 75 - 236 



12 369 

 Ibs. The total length of the rod = ' = 52.41 ins. Instead of 



using one straight rod with a ring in the end it can be made 

 U-shaped, each leg embedded in the concrete 27 ins. A much 

 lower stress may be obtained by running the tie rod higher, an 

 angle of about 45 degrees being good. 



This example was worked to illustrate the simplicity of such 

 computations and to show that all lines of stress must pass through 

 points. In Fig. 51 the tie rod is shown to run quite a distance 

 into the bottom member in order to have all the forces acting 

 properly. In practical work the vertical bolt suspending the 



