than in' a series of brick or segmental concrete arches. The reinforcement ,oyer the, support 

 would therefore be useless. That this is actually the case can. be clearly proven. It is ad- 

 mitted by all. close engineer-observers. Three distinctive, points can be cited to : substantiate 

 the 'argument. 



(1) The loads borne by these flat slabs are greatly in excess of those computed under con- e * * 

 tinuous beam formulae. So great is this difference that slabs which do not figure for_ a safe load 



of over 50 Ibs. per sq. ft. with a presumable factor of 'safety of 4 will actually carry over 1,500 

 Ibs. per-sq. ft. before failure is reached. If the sustained loads are used as a basis of computa- ; 

 tion for the stresses in the steel; they would imply that a stress existed .there of over 300,000 

 Ibs. per sq. in., which is absurd and unsupported by facts, as we know that the steel is not good 

 for more than 80,000 Ibs. per sq. in. This discrepancy between tests and theory is recognized 

 now by gome, .manufacturers of slab reinforcement, who do not issue tables for the, use of their 

 material, but instead, give a series of slabs and spans which" have been tested arid which are guar- 

 anteed to sustain a specified load under a stated factor of safety. The present New "York 

 City building code takes' advantage of this arch act-ion in -the slabs-, and will accept any sys- 

 tem of construction in chider concrete for any stated load which will sustain a test load ten 

 times greater. Notwithstanding this enormous' factor of safety,- the actual loads for which 

 given systems or styles are passed "are two to four times as great as 'would be permitted 

 under the common continuous beam formulae. 



(2) If the condition of a continuous beam existed there would be noted an exceedingly high 

 tensile stress over the support equaling the tensile stress at the center of the span. That this 

 stress does- not -exist -may be noted by any close observer of the' above-mentioned tests. Cracks 

 do not appear, at this point as under the slab at -.the, center of, the. span. Fine tension cracks are 

 sometimes seen, but only preliminary to failure when the loading has caused such a large .de- 

 flection in the middle of the span that wilj necessarily put tension in the upper portion near 

 the support, .This would be true in any arch that is wealc 'at the center. ..' 'this should not be 

 confounded, with .a true negative bending moment, such \as' ,'woul'd exist" 'in 'a contihu'dus 

 beam. ,A not uncommon form' of construction is given 'in 'the sketch. "This forni of' ^'(in- 

 struction, reinforced with a great many systems, has been tested' to destruction under 't'he 

 supervis'ion of. the building department of "New York City, and the results" of ' these' 'tests 

 are on reqord. , It will /be seen that it is the usual p'ract'ice "to^'rest the reirirqrcerrie'nt J on 

 the supporting 'beam .actually \ l / 2 inches "from" 'the extreme ' fibers , of the top: "As 'the 

 slab is commonly 4' inches 'thick, the 'lower 'inch serving only a's" fire cir rust protection, the effective 



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