Part 8 (1/2)

In his advocacy of placing steel to sie, the author is more nearly correct than in other issues he makes Undoubtedly, an attempt should be n the bars over the supports, when, if pliable enough, they will assume a natural droop which is practically ideal; or, if too stiff, they may be bent to conform approximately to this position In slabs, too, the reinforceths covering several spans, and, where ends occur, by generous lapping In beams the problem is somewhat more complicated, as it is impossible, except rarely, to bow the steel and to extend it continuously over several supports; but all or part of the horizontal steel can be bent up at about the quarter point, carried across the supports into the adjacent spans, and anchored there by bending it down at about the sa the ends

[Illustration: PLATE III--JUNCTION OF BEAM AND WALL COLUMN

REINFORCEMENT IN PLACE IN BEAM, LINTEL, AND SLAB UP TO BEAM NOTE END ANCHORAGE OF BEAM BARS]

It is seldom necessary to adopt the scheme proposed by the author, na washer and a nut to hold the washer in place, although it is sometimes expedient, but not absolutely necessary, in end spans, where prolongation into an adjacent span is out of the question In end spans it is ordinarily sufficient to give the bars a double reverse bend, as shown in Plate III, and possibly to clasp hooks with the horizontal steel If steel be placed in this manner, the catenary curve will be practically approxith of e In this case, also, there is rity of the bond; in fact, if there were no bond, the structure would still develop h the deflection under heavy loading reater

The writer once had an experience which sustains this point On peeling off the for to thetogether of the bars in the bottom, coupled with a little too stiff a mixture, the bearip the steel in the portion between the points of bending up, or for about the middle half of the member; consequently, it was decided to test this bea load was first applied and no deflection, cracking, or slippage of the bars was apparent; but, as the loading was continued, deflection set in and increased rapidly for s, a number of fine cracks opened up near the mid-section, which extended to the neutral plane, and the steel slipped just enough, when drawn taut, to destroy what bond there was originally, owing to the contact of the concrete above At three times the live load, or 450 lb per sq ft, the deflection apparently reached aabout 5/16 in for a clear distance, between the supports, of 20 ft; and, as the load was increased to 600 lb per sq ft, there was no appreciable increase either in deflection or cracking; whereupon, the owner being satisfied, the loading was discontinued The load was reduced in a load (450 lb) and left on over night; the next e, the beam was declared to be sound When the load was removed the beam recovered all but about 1/8 in of its deflection, and then repairs were ht expanded h nearly three years have elapsed, there have been no unfavorable indications, and the owner, no doubt, has eased his ard to thecan only be explained by the catenary action of the main steel, and some truss action by the steel which was horizontal, in conjunction with the U-bars, of which there were plenty As before noted, the clear span was 20 ft, the width of the bay, 8 ft, and the size under the slab (which was 5 in thick) 8 by 18 in The reinforcement consisted of three 1-1/8-in round medium-steel bars, with 3/8-in U-bars placed the effective depth of the member apart and closer toward the supports, the first two or three being 6 in apart, the next two or three, 9 in, the next, 12 in, etc, up to a hout the mid-section, of 15 in Each U-bar was provided with a hook at its upper end, as shown in Plate III, and engaged the slab reinforcement, which in this case was expanded metal Two of the 1-1/8-in bars were bent up and carried across the support At the point of bending up, where they passed the single horizontal bar, which was superimposed, a lock-bar was inserted, by which the pressure of the bent-up steel against the concrete, in the region of the bend, was taken up and distributed along the horizontal bar This feature is also shown in Fig 14 The bars, after being carried across the support, were inclined into the adjacent span and provided with a liberal, well-rounded hook, furnishi+ng efficient anchorage and provision for reverse stresses This was at one end only, for--to ; consequently, the benefit of continuity was denied The bent-up bars were given a double reverse bend, as already described, carrying the them in the hook of the horizontal bar This apparently stiffened up the free end, for, under the test load, its action was si the value of this

The writer has consistently followed this ood results, and believes that, in some measure, it approximates the truth of the situation Moreover, it is economical, for with the bars bent up over the supports in thisprovided, it is possible to remove the forms with entire safety much sooner than with the ordinary methods which are not as well stirruped and only partially tied across the supports It is also possible to put the structure into use at an earlier date Failure, too, by the premature removal of the centers, is almost impossible with this method These considerations more than compensate for the trouble and expense involved in connection with such reinforcement The writer will not attempt here a theoretical analysis of the stresses incurred in the different parts of this bea and instructive

[Illustration: FIG 14]

The concrete, with the reinforce on the steel as a saddle, furnishi+ng it with a rigid jacket in which to work, and itself acting only as a stiff floor and a protecting envelope Bond, in this case, while, of course, an adjunct, is by no enerally the case with beams unrestrained in any way and in which the reinforcee, in which case a continuous bond is apparently--at any rate, theoretically--indispensable

An exan, where provision for reverse stresses was ale, in Philadelphia, which collapsed while the operation of casting the roof was in progress, in the su world is fairly familiar with the details of this disaster, as they were noted both in the lay and technical press In this structure, not only were U-bars almost entirely absent, but the few main bars which were bent up, were stopped short over the support

The result was that the ties between the rib and the slab, and also across the support, being lacking, some of the beams, the forms of which had been reht, and, later, when the roof collapsed, owing to the deficient bracing of the centers, it carried with it each of the four floors to the base way abruptly over the supports Had an adequate tie of steel been provided across the supports, the collapse, undoubtedly, would have stopped at the fourth floor So h only half of it had fallen, it was ordered to be entirely demolished and reconstructed

The cracks in the beaht alone, were , and illuminative of the action which takes place in a concrete beale of approxie of the support to the bottoonal steel, crossing this plane of weakness, more emphatically de line of thought was suggested:

Should not the concrete in the region above the supports and for a distance on either side, as encoarded as abundantly able, of and by itself, and without reinforcing, to convey all its load into the colu to be considered in the truncated portion intersected? Not even the bending should be considered, except in the case of relatively shallow e-shaped section to slip out on the 45 planes, thereby requiring sufficient reinforce of these planes of principal weakness to take the co to shove it out

This reinforcement, of course, should be anchored securely both ways; ina suspensory, and, in the other direction, by prolonging it past the supports, the concrete, in this case, along these planes, being assumed to assist partly or not at all

This would seened in this manner and checked by co continuity of action, are found to agree fairly well Hence, the following stateh steel is provided, crossing nore of the support upward and outward, to care for the component of the load on the portion included within a pair of these planes, tending to produce sliding along the same, and this steel is adequately anchored both ways, there will be enough reinforcement for every other purpose

In addition, U-bars should be provided for practical reasons

The weak point of beams, and slabs also, fully reinforced for continuity of action, is on the under side adjacent to the edge of the support, where the concrete is in compression Here, too, the a no slab to assist it, as is the case within the middle section, where the compression is in the top Over the supports, for the width of the coluth, for here the steel has a leverage equal to the depth of the colue and for at least one-tenth of the span out, conditions are serious The usual ion is to subpose brackets, suitably proportioned, to increase the available coe of the steel, at the sa the intensity of compression Brackets, however, are frequently objectionable, and are therefore very generally oners, no especial co ineers arenearly always included True, if brackets are o which horizontal bars ion, but sufficient additional compressive resistance is rarely afforded thereby

Perhaps the best way to overco to brackets, is to increase the co horizontal steel through it Thisscraps of iron or bits of expandedits resistance The experies of the City of New York, on the value of nails in concrete, in which results as high as 18,000 lb per sq in were obtained, indicate the availability of this device; the writer has not used it, nor does he know that it has been used, but it seems to be entirely rational, and to offer possibilities

Another practical test, which indicates the value of proper reinforcee warehouse in Canada, the floor was designed, according to the building laws of the town, for a live load of 150 lb per sq ft, but the restrictions beingthe lever arm of the steel to 75 of the effective depth, this was about equivalent to a 200-lb

load in the United States The structure was to be loaded up to 400 or 500 lb per sq ft steadily, but the writer felt so confident of the excess strength provided by his uarantee the structure, designed for 150 lb, according to the Canadian laws, to be good for the actual working load Plain, round, medium-steel bars were used A 10-ft panel, with a bea the top coat), with 1/2-in round bars, 4 in on centers, was loaded to 900 lb per sq ft, at which load no measurable deflection was apparent The writer wished to test it still further, but there was not enough ce The load, however, was left on for 48 hours, after which, no sign of deflection appearing, not even an incipient crack, it was re was 14 by 20 ft The beam was continuous at one end only, and the slab only on one side In other parts of the structure conditions were better, square panels being possible, with reinforcement both ways, and with continuity, both of bea co was as before indicated The enorth of the structure, as proved by this test, and as further demonstrated by its use for nearly two years, can only be explained on the basis of the continuity of action developed and the great stiffness secured by liberal stirruping Steel was provided in theto the rule, (_w_ _l_)/8, the span being taken as the clear distance between the supports; two-thirds of the steel was bent up and carried across the supports, in the case of the beams, and three-fourths of the slab steel was elevated; this, with the lap, really gave, on the average, four-thirds as much steel over the supports as in the center, which, of course, was excessive, but usually an excess has to be tolerated in order to allow for adequate anchorage

Brackets were not used, but extra horizontal reinforceular horizontal steel, was laid in the bottoly, was satisfactory The columns, it should be added, were calculated for a very low value, so like 350 lb per sq in, in order to compensate for the excess of actual live load over and above the calculated load

This piece of as done during the winter, with the te below zero over night The precautions observed were to heat the sand and water, thaw out the concrete with live stea or before it was settled in place, and as soon as it was placed, it was decked over and salamanders were started underneath Thus, a job equal in every respect to eather installation was obtained, it being possible to reht

[Illustration: PLATE IV, FIG 1--SLAB AND BEAM REINFORCEMENT CONTINUOUS OVER SUPPORTS SPAN OF BEAMS = 14 FT SPAN OF SLABS = 12 FT

SLAB, 6 IN THICK]

[Illustration: PLATE IV, FIG 2--REINFORCEMENT IN PLACE OVER ONE COMPLETE FLOOR OF STORAGE WAREHOUSE SLABS, 14 FT SQUARE REINFORCED TWO WAYS NOTE CONTINUITY OF REINFORCEMENT AND ELEVATION OVER SUPPORTS

FLOOR DESIGNED FOR 150 LB PER SQ FT LIVE LOAD TESTED TO 900 LB PER SQ FT]

In another part of this job (the factory annex) where, owing to the open nature of the structure, it was impossible to house it in as well as the warehouse which had bearing walls to curtain off the sides, less fortunate results were obtained A teht of nearly 50, followed by a spell of alternate freezing and thawing, effected the ruin of at least the upper 2 in of a 6-in slab spanning 12 ft (which was reinforced with 1/2-in round bars, 4 in on centers), and the re 4 in was by no ht that this particular bay would have to be replaced Before deciding, however, a test was arranged, supports being provided underneath to prevent absolute failure But as the load was piled up, to the extent of nearly 400 lb per sq ft, there was no sign of giving (over this span) other than an insignificant deflection of less than 1/4 in, which disappeared on re the load This slab still performs its share of the duty, without visible defect, hence it must be safe The question naturally arises: if 4 in of inferior concrete could , what ood concrete in the other slabs?

The reinforcing in the slab, it should be stated, was continuous over several supports, was proportioned for (_w_ _l_)/8 for the clear span (about 11 ft), and three-fourths of it was raised over the supports