226 



PRACTICAL STRUCTURAL DESIGN 



curved members SW and NW have no stress except under wind. 

 Another form of this roof dispenses with the vertical tie PQ and 

 the analysis resembles that for the scissors truss. Nothing is 

 gained by doing without the vertical tie, but, on the contrary, 

 the roof is bettered by having the tie. 



In this truss all that portion above joints 5 and 10 is treated as 

 a separate roof, resting on the frame represented by joints 1, 5, 

 10, 14. The dotted lines from 5 to 1 and from 10 to 14 represent 

 in direction the thrust of the roof and it is necessary to have 



o.,r 



Fig. 150. Hammer Beam Truss. 



buttresses to carry it, or divide it at the lower joints into a ver- 

 tical component which the wall will carry and a diagonal com- 

 ponent acting along the roof of an aisle in the building. The 

 aisle roof in turn will deliver its load to walls or buttresses. 



In the force diagram, to the right, the loads are laid off ver- 

 tically, beginning with joint 4, represented by the load be, the loads 

 on joint 2 and joint 13 going vertically into the wall or column. 

 One-half the load at joint 4 is carried by the lower joint, so on 

 the load line from a point midway between b and c draw a dotted 

 line parallel with the line on the truss diagram from 5 to 1. This 

 intersects the horizontal reaction line at w, and the length xw 

 gives the amount of horizontal thrust on the support at 5th 

 lowest joint. 



There is no stress in rw and ow. These members are stressed 

 in tension by the small upper truss of which they are the tie, 

 but they are stressed an equal amount in compression, due to 

 the thrust against the walls, and one stress neutralizes the other. 



