CHAPTER III. 

 FLOORS. 



General One of the distinctive purposes of mill con- 



Construction struction is to obtain strength and stiffness with 

 a minimum amount of timber surface and cor- 

 ners exposed to attack by fire. Large supporting members 

 and flat, smooth, heavy floors provide this requirement. Large 

 timbers do not ignite readily, and if exposed to fire burn slowly 

 and with but slight penetration after a considerable period of 

 time. Flat, smooth surfaces possess this same resistance to 

 combustion, and may be reached easily by water from sprink- 

 lers or hose. While three general types of construction were 

 described briefly in Chapter I, each type is based upon these 

 fundamental principles and varies only according to the needs 

 of design in a given case. 



Heavy timber posts support large timber girders spaced 

 as far apart as the design will allow. These girders support a 

 thick main or carrying floor direct without aid of joists or 

 smaller timbers, except in semi-mill construction. The main 

 floor is covered with two or more layers of waterproof paper, 

 or similar material, to prevent leakage of water or passage of 

 dirt from the floor above. In usual practice a hardwood top 

 or wearing floor is laid on the paper and completes the total 

 thickness of floor. Edge-grain flooring of other woods may 

 also be used; while a new end-grain flooring of small blocks 

 glued to a solid base gives promise of success. 



The factor of safety used in calculations for size of girders 

 and columns is sufficient to allow serious charring by fire and 

 still leave strength to support the floor loads. Wood posts 

 have successfully withstood fire that would have entirely crip- 

 pled unprotected cast iron or steel columns. 



Beams of sufficient strength to carry the imposed loads 

 may deflect or vibrate under the action of machinery or impact; 

 therefore, the factors of weight, deflection, vibration and impact 

 should be considered in finding the dimensions of beams that 

 may be used in a given case. 



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