292 



responds. The formulas for fluid pressure are more simple than 

 those in which a number of factors must be used, so the material 

 is assumed to act as a fluid having a weight per cubic foot very 

 much less than the actual weight of the material. 



In Fig. 185 three problems in the design of lintels over open- 

 ings are shown. At A the lower opening is spanned by a lintel 

 which carries a load indicated by the shaded triangle, which is 

 equilateral. The reason this triangular load is carried is that 



the brick work bond 

 will have strength 



enough to assume a 

 form resembling an 

 arch. The bending 

 moment due to a tri- 

 angular load is 

 wL 2 



M = 



Fig. 185 Lintels over Openings 



The point of the 

 arch over the lower 

 opening is below the bottom of the upper opening a depth equal 

 to, or greater than, one-fourth the span of the upper opening. 

 In the upper opening there is a lintel to carry the coping wall. 

 A sixty degree line drawn from each upper corner will intersect 

 the coping wall; therefore the lintel must be figured to carry all 

 the load above it, within the shaded area. 



At B is shown another case. The sides of an equilateral tri- 

 angle will intersect the bottom of the upper opening so it is com- 

 mon to assume the sloping lines at the -side to connect the corners 

 of the openings as shown. The triangle over the top of the upper 

 opening is more than one-fourth the span below the top of the 

 wall, so as some arch action can take place the lintel is assumed 

 to carry only the triangular portion of the wall. The author 

 would not so design the two lintels. The lintel over the bottom 

 opening would be designed to carry all the load between the 

 dotted vertical lines. 



At C a similar condition is found. The lintel over the lowest 

 opening should be designed to carry all the load between two 

 vertical lines extended from the upper corners to the top of the 

 wah 1 . It is not safe to assume the sloping form of the broken 

 wall opening unless there is a bridging across sufficient to form 



