Part 1 (1/2)

Son

by Edward Godfrey

Not o physicians had certain rules and practices by which they were guided as to when and where to bleed a patient in order to relieve or cure hiical, why have they been abandoned?

It is the purpose of this paper to show that reinforced concrete engineers have certain rules and practices which are noof former days If the writer fails in this, by reason of the uments on the other side of the questions he propounds, he will at least have brought out good reasons which will stand the test of logic for the rules and practices which he proposes to conde in the voluminous literature on this comparatively new subject

Destructive criticis journal Sohest benefit; when it succeeds in destroying error, it is reconstructive No reform was ever accomplished without it, and no refor up errors and faults is destructive criticism, we cannot have too ineering practice is to be purged of its inconsistencies and absurdities, it will never be done by dwelling on its excellencies

Reinforced concrete engineering has fairly leaped into prorowth, but it still wears soarments which it borrowed from sister for to it, and, while these were, perhaps, the best makeshi+fts under the circumstances, they fit badly and should be discarded It is some of thesepro Profession

[Illustration: FIG 1]

The first point to which attention is called, is illustrated in Fig 1

It concerns sharp bends in reinforcing rods in concrete Fig 1 shows a reinforced concrete design, one held out, in nearly all books on the subject, as a le, and then ain and run parallel with the top of the bea-chain or truss-rod around a queen-post The reinforcing rod is the hog-chain or the truss-rod Where is the queen-post? Suppose this rod has a section of 1 sq in and an inclination of 60 with the horizontal, and that its unit stress is 16,000 lb per sq in The forces, _a_ and _b_, are then 16,000 lb The force, _c_, must be also 16000 lb What is to take this force, _c_, of 16,000 lb? There is nothing but concrete At 500 lb per sq in, this force would require an area of 32 sq in Will son please state where this area can be found? It must, of necessity, be in contact with the rod, and, for structural reasons, because of the lack of stiffness in the rod, it would have to be close to the point of bend If analogy to the queen-post fails so completely, because of the almost coarentle curve of a radius twenty or thirty times the diameter of the rod, the side unit pressure will be from one-twentieth to one-thirtieth of the unit stress on the steel This being the case, and being a sihly understood, it is astounding that engineers should perpetrate the gross error ofrod under stress

The second point to which attention is called1 The rod marked 3 is also like the truss-rod of a queen-post truss in appearance, because it ends over the support and has the say ends with appearance, for the function of a truss-rod in a queen-post truss is not perfor rod in concrete, for other reasons than the absence of a post The truss-rod receives its stress by a suitable connection at the end of the rod and over the support of the bea rod, in this standard beam, ends abruptly at the very point where it is due to receive an ith, an eleth and safety of many a bea rod in a concrete bearip of the concrete; but these increments can only be imparted where the tendency of the concrete is to stretch

This tendency is greatest near the bottom of the beam, and when the rod is bent up to the top of the beareatest tendency to stretch The function of this rod, as reinforcee of the bea it up in this e reinforcement, as far as that rod is concerned, from the point of bend to the support

It is true that there is a shear or a diagonal tension in the beaonal portion of the rod is apparently in a position to take this tension This is just such a force as the truss-rod in a queen-post trussrod equipped to perform this office? The beam is apt to fail in the line, _A B_ In fact, it is apt to crack froe on this or alth dependent on the reinforcing steel Suppose such a crack should occur The entire strength of the bearip of the short end of Rod 3 to the right of the line, _A B_ The grip of this short piece of rod is so s the i that designers, having any care for the permanence of their structures, should consider for an instant such features of design,in which life and property depend on them

The third point to which attention is called, is the feature of design just mentioned in connection with the bent-up rod It concerns the anchorage of rods by the eth in concrete This rant violation of coineering works, where of all places better judg walls have been built, and described in engineering journals, in papers before engineering societies of the highest order, and in books enjoying the greatest reputation, which have, as an essential feature, a great nuth, and ht as well be of much smaller dimensions These rods are the vertical and horizontal rods in the counterfort of the retaining wall shown at _a_, in Fig 2 This retaining wall consists of a front curtain wall and a horizontal slab joined at intervals by ribs or counterforts The manifest and only function of the rib or counterfort is to tie together the curtain wall and the horizontal slab That it is or should be of concrete is because the steel rods which it contains, need protection It is clear that failure of the retaining wall could occur by rupture through the Section _A B_, or through _B C_ It is also clear that, apart fro which would produce this rupture is the pulling out of the short ends of these reinforcing rods

Writers treat the triangle, _A B C_, as a beale and a beaners seem to think that these rods take the place of so-called shear rods in a beam, and that the inclined rods are equivalent to the rods in a tension flange of a bea such results can be attained Any clear analysis leading to these conclusions would certainly be a valuable contribution to the literature on the subject It is scarcely possible, however, that such analysis will be brought forward, for it is the apparent policy of the reinforced concrete analyst to jump into the middle of his proposition without the encu the fact that this wall could fail, as stated, by rupture along either _A B_ or _B C_ It can be stated just as positively that a set of rods running from the front wall to the horizontal slab, and anchored into each in such a manner as would be adopted were these slabs suspended on the rods, is the only rational and the only efficient design possible This design is illustrated at _b_ in Fig 2

[Illustration: FIG 2]

The fourth point concerns shear in steel rods embedded in concrete For decades, specifications for steel bridges have gravely given a unit shear to be allowed on bridge pins, and every bridge engineer knows or ought to know that, if a bridge pin is properly proportioned for bending and bearing, there is no possibility of its being weak froh together to reduce the size of the pin to where its shear need be considered, because of the width required for bearing on the parts Concrete is about one-thirtieth as strong as steel in bearing There is, therefore, so any shear on steel rods eravity of the situation is not so much the serious manner in which this unit of shear in steel is written in specifications and building codes for reinforced concrete work (it does notin specifications for steelwork, because it is ignored), but it is apparent when designers soberly use these absurd units, and proportion shear rods accordingly

Many designers actually proportion shear rods for shear, shear in the steel at units of 10,000 or 12,000 lb per sq in; and the blaerous practice can be laid directly to the literature on reinforced concrete Shear rods are given as standard features in the design of reinforced concrete beaineering societies, a iven The stress, or shear per shear itudinal shear which would occur in the space froiven as to whether these bars are in shear or tension; in fact, either would be absurd and i some other part This is just a sample of the state of the literature on this important subject Shear bars will be taken up raphs

The fifth point concerns vertical stirrups in a beans of reinforcing concrete beams

Explanations of how they act are conspicuous in the literature on reinforced concrete by its total absence By stirrups arerod They are usually U-shaped and looped around the rod

It is a co the horizontal shear in a beae to the web take the horizontal shear or the increhtly driven into holes in a steel angle, with a loose vertical rod, 3/4 in in dia rod in a concrete bean in steel and reinforced concrete, as they are commonly practiced, is obtained

These stirrups can take but little hold on the reinforcing rods--and this h the medium of the concrete--and they can take but little shear Some writers, however, hold the opinion that the stirrups are in tension and not in shear, and soh to compare theine a Howe truss with the vertical tension members looped around the bottom chord and run up to the top chord without any connection, or hooked over the top chord; then compare such a truss with one in which the end of the rod is upset and receives a nut and large washer bearing solidly against the chord This gives a con in wood and reinforced concrete, as they are corip in the concrete is all that can be counted on, in any event, to take up the tension of these stirrups, but it requires an embedment of froth Take 30 to 50 dia end of these shearwill be left

In any case the point at which the shear ood for its full value, is far short of the centroid of compression of the beam, where it should be; in most cases it will be nearer the bottom of the bea their end connections near the bottom chord, would be equivalent to these shear members

The sixth point concerns the division of stress into shear members