148 COMBINED AND ECCENTRIC STRESSES 



Before calculating rhe stress due To wind moment, it will be necessary to de- 

 termine whether the end post is fixed or hinged- 



If the end post is fixed. the negative moment developed at the lower pin 

 will be M=^ = 5g00 / 226 =36/600 in Ibs. 



In order to obtain this condition of fixidity.the stress in the member must 

 develop a resisting moment equal in amount. 



Therefore the post may be considered fixed if 



-V ) n > Hd 



2 



or (95500 -8520) Q ^ 361600 



but 347120 < 561600 and the end post will not be fixed. 

 (While this is the usual solution, the resisting moment certainly reduces the 

 bending moment and the bending stress is less than computed below-) 



The maximum moment will then occur at the foot of the portal knee 

 brace and will be M =3ZOOX226 =723aoo in Ibs- 



Stress due to wind moment is a maximum in the leeward post and is 



723200X7 



(95500 1 12500) 555 z 



IOE. (OX280OOOOO 



f w = 22480 Ibs. 



5tress,fw,is compression on the windward side and tension on the lee- 

 ward side of the member- 



pression on the windward and tension on the leeward fibres. The 

 maximum fibre stress will come at the foot of the knee brace either on 

 the upper or lower fibres on the windward side of the post, depending 

 upon whether the stress due to weight is greater than the stress due 

 to eccentric loading, or the reverse. In this case the maximum stress 

 comes on the lower fibres of the windward side of the post. 



Combined Tension and Cross Bending. The stress due to 

 cross bending when the member is also subjected to direct tension is 

 given by the formula 



(75) 



the nomenclature being the same as in (74). The constant c is taken 

 equal to 10 where the ends are hinged. 



