Part 5 (1/2)

Mr Godfrey ood point that the accuracy of an elastic theory must be determined by the elastic deportment of the construction under load, and it seems to the writer that if authors of textbooks would pay some attention to this question and show by calculation that the elastic deport, the gross errors in the theoretical treatment of slabs in the majority of works on reinforced concrete would be reh he makes the excellent point noted, Mr Godfrey very inconsistently fails to do this in connection with his theory of slabs, otherwise he would have perceived the absurdity of anyato separate the construction into elementary beam strips This old-fashi+oned method was discarded by the practical constructor uarantee deflections of actual construction under severe tests Alulation li the deflection of concrete floors under test, and yet no co about calculating deflections

In the course of his practice the writer has been required to give surety bonds of frouarantee under test both the strength and the deflection of large slabs reinforced in multiple directions, and has been able to do so with accuracy byPoisson's ratio, and which are given in his book on concrete steel construction

Until the engineer payshis complicated theories with facts as detereneral a hier, and their respect for hily less, until such time as he demonstrates the applicability of his theories to ordinary every-day problems

PAUL CHAPMAN, assOC M AM SOC C E (by letter)--Mr Godfrey has pointed out, in a forcible n of reinforced concrete beaineer, however, has never used such methods of construction Mr

Godfrey proposes certain rules for the calculation of stresses, but there are no data of experiments, or theoretical demonstrations, to justify their use

It is also of the utmost importance to consider the elastic behavior of structures, whether of steel or concrete To illustrate this, the writer will cite a case which recently came to his attention A roof was supported by a horizontal 18-in I-beaes of which were coped at both ends, and two 6 by 4-in angles, 15 ft long, supporting the sa a frah the 18-in I-beam was not loaded to its full capacity, its deflection caused an outward flexure of 3/4 in and consequent dangerous stresses in the 6 by 4-in

angle struts The franed as a structure fixed at the base of the struts The importance of the elastic behavior of a structure is forcibly illustrated by coe which spans the East River with the expert reports on the salect of the elastic behavior of the structure in the contract drawings, and another cause, the average error in the stresses of 290 members was 18-1/2, with a maximum of 94 per cent

Mr Godfrey calls attention to the fact that stringers in railroad bridges are considered as sile knees at their ends can transfer practically no flange stress It is also to be noted that when stringers are in the plane of a tension chord, they are ths, and when in the plane of a coht clearance in order to prevent arch action

[Illustration: FIG 3]

The action of shearing stresses in concrete bea 3, where the bea to the left support,froonal The maximum intensity of stress probably would be in planes inclined about 45, since, considered independently, they produce the least deflection While the load, _W_, re but e, the concrete will carry a considerable portion of the bottoely increased, the coefficient of elasticity of the concrete in tension becomes small, or zero, if small fissures appear, and the concrete is unable to transfer the tension in diagonal planes, and failure results For a beale load, _W_, the failure would probably be in a diagonal line near the point of application, while in a uniforonal line near the support, where the shear is greatest

It is evident that the introduction of vertical stirrups, as at _b_, or the more rational inclined stirrups, as at _c_, influences the action of the shearing forces as indicated, the intensity of stress at the point of connection of the stirrups being high It is advisable to space the stirrups moderately close, in order to reduce this intensity to reasonable lional compression in the concrete acts in a plane inclined at 45, then the tension in the vertical stirrups will be the vertical shear ti of the stirrups divided by the distance, center to center, of the top and bottoes of the beam If the stirrups are inclined at 45, the stress in them would be 07 the stress in vertical stirrups with the sa up bottom rods sharply, in order to dispense with suspenders, is bad practice; the writer has observed diagonal cracks in the bea in New York City, which are due to this cause

[Illustration: FIG 4]

In several structures which the writer has recently designed, he has been able to dispense with stirrups, and, at the sa so a bar between them and those which re 4 The bend occurs at a point where the vertical component of the stress in the bent bars equals the vertical shear, and sufficient bearing is provided by the short cross-bar The bars which rehout the beam, are deflected at the center of the bea no shear at the center, the bars are spaced as closely as possible, and still provide sufficient room for the concrete to flow to the soffit of the beam Two orthe bars bent up froes By this syste a bar where the live load unit stress in adjoining bars is high, as their continual lengthening and shortening under stress would cause severe shearing stresses in the concrete surrounding the end of the short bar

[Illustration: FIG 5]

The bea 5 illustrates the principles stated in the foregoing, as applied to a heavier beam The duty of the short cross-bars in this case is perforitudinal rods and then continued up in order to support the bars during erection This beam, which supports a roof and partitions, etc, has supported about 80 of the load for which it was calculated, and no hair cracks or noticeable deflection have appeared If the ested by Mr Godfrey were a correct criterion of the actual stresses, this particular beam (and many others) would have shown many cracks and noticeable deflection The writer maintains that where the concrete is poured continuously, or proper bond is provided, the influence of the slab as a coe is an actual condition, and the stresses should be calculated accordingly

In the calculation of continuous T-beams, it is necessary to consider the fact that the ative moments is small because of the lack of sufficient compressive area in the stem or web

If Mr Godfrey will ns of practical engineers, he will find due provision ative moments It is very easy to obtain the proper a up the bars and letting thes 4 and 5 (taken froineer does not find, as Mr

Godfrey states, that the negative moment is double the positive moment, because he considers the live load either on one span only, or on alternate spans

[Illustration: FIG 6]

In Fig 6 a beae, a practice which Mr Godfrey condemns As the structure, which has about twenty si built, the writer would be thankful for his criticisitudinal steel in columns is worthless, but until definite tests are e, on both plain concrete and reinforced concrete coluned, the writer will accept the data of other experinized formulas

[Illustration: FIG 7]

Mr Godfrey states that the ”elastic theory” is worthless for the design of reinforced concrete arches, basing his objections on the shrinkage of concrete in setting, the unreliability of deflection foridity of the abut in air shrinks, whereas concrete setting in water expands, believes that if the arch be properly wetted until the setting up of the concrete has progressed sufficiently, the effect of shrinkage, on drying out, may be uarded against during the construction of an arch, the settle an uncertain element, should be a check on the accuracy of the calculations and the work should produce theoretically a certain deflection The unreliability of deflection formulas for beams is due mainly to the fact that the neutral axis of the beahout, and that the shearing stresses are neglected therein While there is necessarily bending in an arch ring due to tees soned arch, the compression exceeds the tension to such an extent that comparison to a beam does not hold true An arch should not be used where the abute should be built where a suitable anchorage cannot be obtained

The proper design of concrete slabs supported on four sides is a coned a floor construction, slabs, and beams, supported on four corners, which is si 7 is shown a portion of a proposed twelve-story building, 90 by 100 ft, having floors with a live-load capacity of 250 lb per sq ft For thein any panel the full load on that panel was considered, there being no live load on adjoining panels For thele line ”Checker-board”

loading was considered too improbable for consideration The flexure curves for beath), the tension rods in the longer bea placed underneath those in the shorter beams Under full load, therefore, approxiirder and the other half to the short-span girder The girders were the same depth as the beams For its depth the writer found this systeated

EP GOODRICH, M AM SOC C E--The speaker heartily concurs with the author as to the large nuineers and other practitioners who design and construct work in reinforced concrete It is exceedingly difficult for the hu them with others in past experience, but this association is apt to clothe the new idea (as the author suggests) in gar-bands,” and often go far toward strangling proper growth