50 BULLETIN 801, U. S. DEPARTMENT OF AGRICULTURE. 



most dangerous use of steel from a fire hazard standpoint is the use 

 of steel rods in truss beams. There is a temptation to use this con- 

 struction for increasing the sj)an of beams and so reducing the 

 number of columns, particularly in the compress compartment.^ 



SEMI-SLOW-BURNING CONSTRUCTION. 



A modified type of slow-burning construction for cotton ware- 

 - houses is in very general and satisfactory use. This type of build- 

 ing is exactly like the slow-burning construction already described 

 except that the exterior walls are of light frame and the division 

 fire walls are modified in such manner as to form an effective fii'e 

 break in these inflammable exterior walls. Also the omission of the 

 durable top floor and waterproof paper over the heavy planking is 

 permissible and in keeping with the cheapness of this construction. 

 This combination may be called semislow-burning construction. It 

 is frequently spoken of as ^' wood-end " construction, though this is 

 a rather indefinite term. A general view of such a building is 

 shown in Plate III, figure 1, and is referred to in the beginning of 

 this discussion. The value of the semi-slow-burning construction lies 

 in the economical restriction of cotton subject to a single fire. Pro- 

 Added the design of the division fire wall and the wood end is cor- 

 rect, this construction is very satisfactory for buildings properly 

 separated and not seriously exposed to other fire hazard. This de- 

 sign is especiallj^ economical of construction in communities where 

 lumber is readily available and masonry materials are scarce. 



EXTERIOR WALLS. 



The exterior walls constructed of light frame afford advantages 

 in some cases to firemen, since the boards can be pulled off readily, 

 thus providing openings through which hose streams may be played 

 effectively upon the fire. The disadvantage of the wood-end wall 



1 ^Usually the steel rod ecjuivalent to 1 incli square is subject to a stress^ 

 of 16,000 pounds wMcli it readily withstands at a normal temperature, but 

 with ordinary temperatures reached in a fire this strength is reduced quickly 

 to so low a value as to make failure certain and prompt. 



Steel or wrought-iron timber hangers or post caps are less objectionable 

 than steel truss rods, as a partial yielding of the timber hanger does not alter 

 as seriously the mechanical stress which remains equal to the sheer load of the 

 beam. 



I If it is necessary to secure spans longer than is economical with timbers, 

 trusses may be used provided all members (both tension and comprt?siou) are 

 of wood having the dimensions of no side less tlian 6 inches and are such that 

 charring of all sides to a depth of one-half inch will not reduce the strength 

 of the member more than 20 per cent. Steel or iron should be used only for 

 bolts enclosed or insulated by the wooden members. 



