50 BULLETIN 891, U. S. DEPARTMENT OE AGBICTJI/FUKE. 
most dangerous Hse 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 span of beams and so reducing the 
number of column-, particularly in the compress compartment. 1 
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 fire 
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 fi 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- -low-burning construction lies 
in the economical restriction of cotton subject to a single fire. Pro- 
vided the design of the division fire wail 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 especially 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 equivalent to 1 inch square is subject to a stress 
of 16.000 pounds which 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. 
If it is necessary to secure spans longer than is economical with timbers, 
trusses may be used provided all members (both tension and compresion) are 
of wood having the dimensions of no side less than 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 bv the wooden members. 
