Some Mooted Questions in Reinforced Concrete Design | Page 4

that can be counted on, in any
event, to take up the tension of these stirrups, but it requires an
embedment of from 30 to 50 diameters of a rod to develop its full
strength. Take 30 to 50 diameters from the floating end of these shear
members, and, in some cases, nothing or less than nothing will be left.
In any case the point at which the shear member, or stirrup, is good 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
beam. In a Howe truss, the vertical tension members having 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.
Briefly stated, the common method is to assume each shear member as
taking the horizontal shear occurring in the space from member to
member. As already stated, this is absurd. If stirrups could take shear,
this method would give the shear per stirrup, but even advocates of this
method acknowledge that they can not. To apply the common analogy
of a truss: each shear member would represent a tension web member
in the truss, and each would have to take all the shear occurring in a
section through it.
If, for example, shear members were spaced half the depth of a beam
apart, each would take half the shear by the common method. If shear
members take vertical shear, or if they take tension, what is between
the two members to take the other half of the shear? There is nothing in
the beam but concrete and the tension rod between the two shear
members. If the concrete can take the shear, why use steel members? It
is not conceivable that an engineer should seriously consider a tension
rod in a reinforced concrete beam as carrying the shear from stirrup to
stirrup.
The logical deduction from the proposition that shear rods take tension
is that the tension rods must take shear, and that they must take the full
shear of the beam, and not only a part of it. For these shear rods are
looped around or attached to the tension rods, and since tension in the

shear rods would logically be imparted through the medium of this
attachment, there is no escaping the conclusion that a large vertical
force (the shear of the beam) must pass through the tension rod. If the
shear member really relieves the concrete of the shear, it must take it all.
If, as would be allowable, the shear rods take but a part of the shear,
leaving the concrete to take the remainder, that carried by the rods
should not be divided again, as is recommended by the common
method.
Bulletin No. 29 of the University of Illinois Experiment Station shows
by numerous experiments, and reiterates again and again, that shear
rods do not act until the beam has cracked and partly failed. This being
the case, a shear rod is an illogical element of design. Any element of a
structure, which cannot act until failure has started, is not a proper
element of design. In a steel structure a bent plate which would
straighten out under a small stress and then resist final rupture, would
be a menace to the rigidity and stability of the structure. This is exactly
analogous to shear rods which cannot act until failure has begun.
When the man who tears down by criticism fails to point out the way to
build up, he is a destructive critic. If, under the circumstances,
designing with shear rods had the virtue of being the best thing to do
with the steel and concrete disposed in a beam, as far as experience and
logic in their present state could decide, nothing would be gained by
simply criticising this method of design. But logic and tests have shown
a far simpler, more effective, and more economical means of disposing
of the steel in a reinforced concrete beam.
In shallow beams there is little need of provision for taking shear by
any other means than the concrete itself. The writer has seen a
reinforced slab support a very heavy load by simple friction, for the
slab was cracked close to the supports. In slabs, shear is seldom
provided for in the steel reinforcement. It is only when beams begin to
have a depth approximating one-tenth of the span that the shear in the
concrete becomes excessive and provision is necessary in the steel
reinforcement. Years ago, the writer recommended that, in such beams,
some of the rods be curved up toward the ends of the span and

anchored over the support. Such reinforcement completely relieves the
concrete