JOINTS AND CONNECTIONS 151 



saving can be made by using ribs, each rib being considered as a 

 cantilever carrying a part of the load, shear being duly taken into 

 account. No casting should be less than f in. thick and sharp 

 corners should be avoided. 



25 000 

 The washers at Ui and LI = ' - 71.5 sq. in. If extended 



OOO 



across the chord the dimensions will be 8 in. x 9 in. and this is 

 the best size to make them, for if square or round they will pro- 

 ject beyond the edges, thereby decreasing the bearing area and 

 increasing the stress on the wood. The thickness will be com- 

 puted by the formula used in the case of the washers at joints 

 Ui and Lj. 



The size of the lower chord will now be computed. The maxi- 

 mum tensile stress is 45,000 Ibs. The allowable fiber stress is 



XK rjnn 



1600 Ibs. The area = - - = 28.1 sq. ins. The width will be 



lOvU 



assumed at 8 ins., from which will be subtracted 2 ins. on account 

 of the hole for the largest vertical rod, which leaves a net width of 



28 1 



6 ins. The depth = ^- = 4.7 ins. If it will be possible to use a 



single piece of timber for the bottom chord we can use a 5 ins. x 8 

 ins. stick. It is not possible that a single stick can be obtained and 

 we will take it for granted some splicing will be necessary, which 

 will call for two lines of f-in. bolts going through the sides of the 

 chord. This makes the thickness 4.7 + (2 x $) = 6.2 ins. Using 

 commercial size timbers, this will make the chord 8 ins. wide and 



7 ins. deep. It is usually best to have the depth equal or exceed 

 the width and we will make the chord 8 ins. x 8 ins. 



The maximum stress in the upper chord is 40,000 Ibs. and the 



Af\ f 



area - -: r- - 33.3 sq. ins. Assuming a width of 8 ins. and sub- 



1 \r(J 



33 3 

 tracting 2 ins. for the hole for the rod the depth = ^- - 5.55 ins. 



A shallow beam has a tendency to deflect unduly and if it is under 

 compression the deflection will be increased. It will be advisable, 

 therefore, to increase the depth and as this will decrease the breadth 

 the minimum advisable thickness should be found. 



The allowable maximum compressive fiber stress is based on 

 a length not exceeding 15 times the least thickness. When the 

 length is greater the fiber stress is decreased. One-fifteenth of 



