358 BUILDING LAWS 



surrounding the reinforcing members of reinforced-concrete 

 beams and girders shall be 2 in. on the bottom and 1^ in. on 

 the sides of the said beams and girders. The minimum 

 thickness of concrete under slab rods shall be 1 in. All 

 reinforcement in columns to have a minimum protection of 

 2 in. of concrete. 



"All the requirements herein specified for the protection 

 of steel and for fire-resisting purposes shall apply to rein- 

 forced-concrete flooring between rolled-steel beams, as well 

 as to reinforced-concrete beams and to entire structures in 

 reinforced concrete. Any concrete structure or the floor 

 filling in same, reinforced or otherwise, which may be erected 

 on a permanent centering of sheet metal, of metal lathing 

 and curved bars or a metal centering of any other form, 

 must be strong enough to carry its load without assistance 

 from the centering, unless the concrete is so applied as to 

 protect the centering as herein specified for metal rein- 

 forcement. 



"Exposed metal centering or exposed metal of any kind % 

 will not be considered a factor in the strength of any part 

 of any concrete structure, and a plaster finish applied over 

 the metal shall not be deemed sufficient protection unless 

 applied of sufficient thickness and properly secured, as 

 approved by the Chief of the Bureau of Building Inspection. 



"All concrete shall be mixed in a mechanical batch mixer 

 to be approved by the Bureau of Building Inspection, except 

 when limited quantities are required or when the condition 

 of the work makes hand mixing preferable; hand mixing to 

 be done only when approved by the Bureau of Building 

 Inspection. In all mixing the material shall be measured 

 for each batch. 



"When hand mixing is done under the aforesaid limita- 

 tions, the cement and fine gravel or coarse sand shall be 

 first thoroughly mixed dry and then made into a mortar by 

 gradually adding the proper amount of water. The crushed 

 stone or gravel shall be spread out to a depth not to exceed 

 6 in., in a tight box or upon a proper floor, and be sprinkled 

 with water as directed ; the mortar is then to be evenly spread 

 over the crushed stone, and the whole mass turned over a 



