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Although
such a practice is not economical, the error is on the side of safety; and it
makes some allowance for the fact that a mixture which is nominally richer may
not have any greater strength than the values used for the 1:3:5 mixtures, on
account of defective workmanship or inferior cement or sand. Some of the
constants for use with 1:3:5 mixture and 1:2:4 mixtures will now be worked out.
For the 1:3:5 mixture, r = 12; c = 2,000; and we shall assume s = 55,000. On
the basis of such values, the economical percentage of steel is .84 per cent.
Under these conditions, k will always be .395; and x will equal .141 d.
Therefore the term (d - x) will always equal .859 d, or, say, .86 d, which is
close enough for a working value. Since the above values for c and s represent
the ultimate values, the resulting moment is the ultimate moment, which we
shall call 310. Therefore, for 1:3:5 concrete, we have the constant values: M0
= .0084 x b d >< 55,000 >< .86d similarly we can compute a
corresponding value for 1:2:4 concrete, using the values previously allowed for
this grade: M0=565 bd7. Concrete flooring with a live-load capacity of 150
pounds per square foot, is to be constructed on 1-concrete beams spaced 6 feet
center to center, using 1:3:5 concrete. What thickness of concrete slab will be
required,-and how much steel must be used? Using the approximate estimate,
based on experience, that such a concrete slab will weigh about 50 pounds per
square foot, we can compute the ultimate load by multiplying the live load,
150, by four, and the dead load, 50, by two, and obtain a total ultimate load
of 700 pounds per square foot. A strip 1 foot wide and 6 feet long (between the
concrete beams) will therefore carry a total load of 700 >< 6 = 4,200
pounds. Considering this as a simple concrete beam, we have: Placing this
numerical value of M. = 397 b d2, as in Equation 23, we have 37,800 = 397 b d2.
In this case, b = 12 inches. Substituting this value of b, we solve for d2, and
obtain d2 = 7.93, and ci = 2.82 inches. Allowing an
extra inch below the steel, this will allow us to use a 4-inch I concrete slab.
Theoretically we could make it a little less. Practically this figure should be
chosen. The required steel, from Equation 23, equals .0084 bd. Taking b = 1, we
have the required steel per inch of width of the concrete slab = .0084 X 2.82 =
.0237 square inch. If we use --inch square bars which have a cross-sectional
area .25 of .25 square inch, we may space the bars = 10 inches. This reinforcement
could also be accomplished by using *-inch square bars, which have an area of
.1406. The spacing may therefore be 1406= 6.0 inches. As referred to later,
there should also be a few bars laid perpendicular to the main reinforcing
bars, or parallel with the I-concrete beams, so as to prevent shrinkage. The
required amount of this steel is not readily calculable. Since the I-concrete
beams are 6 feet apart; if we place two lines of *-inch square bars spaced 2
feet apart, parallel with the I-concrete beams, there will then be reinforcing
steel in a direction parallel with the I-concrete beams at distances apart not
greater than 2 feet, since the I-concrete beams themselves will prevent shrinkage
immediately around them. The working unit-compressions for even the best grade
of concrete are seldom allowed exceeding 600 pounds per square inch. An inspection
of Fig. 93 will show that the curve from the point o to the point indicating a
pressure of 600 pounds, although really a parabola, is so nearly a straight
line that there is but little error in considering it to be straight. On this
account, many formulae for the strength of reinforced concrete have been
developed on the basis of a uniform modulus of elasticity for the concrete.
This is virtually the same as assuming that q equals zero in Equation 16. The
other equations which are derived from equations involving q, must also be
correspondingly modified.
Are You in Newfields New
Hampshire? Do You Need Concrete Cutting?
We Are Your Local
Concrete Cutter
Call 603-622-4441
We Service Newfields NH and all surrounding Cities & Towns