Page 212 - Civil Engineering Formulas

CONCRETE FORMULAS 147

computed from
V u
A vf (5.107)
f y
where V is the design shear, kip (kN), at the section; f is the reinforcement
y
u
yield strength, but not more than 60 ksi (413.7 MPa); and , the coefficient of
friction, is 1.4 for monolithic concrete, 1.0 for concrete placed against hardened
concrete, and 0.7 for concrete placed against structural rolled-steel members.
The shear-friction reinforcement should be well distributed across the face of the
crack and properly anchored at each side.

SPIRALS

3
This type of transverse reinforcement should be at least /8 in (9.5 mm) in
diameter. A spiral may be anchored at each of its ends by 1 /2 extra turns of
1
the spiral. Splices may be made by welding or by a lap of 48 bar diameters,
but at least 12 in (304.8mm). Spacing (pitch) of spirals should not exceed 3 in
1
(76.2 mm), or be less than 1 in (25.4 mm). Clear spacing should be at least 1 /3
times the maximum size of coarse aggregate.
The ratio of the volume of spiral steel/volume of concrete core (out to out of
spiral) should be at least
A g f c
s 0.45 1 (5.108)
A c f y

where A gross area of column
g
A core area of column measured to outside of spiral
c
f spiral steel yield strength
y
f c 28-day compressive strength of concrete
BRACED AND UNBRACED FRAMES

As a guide in judging whether a frame is braced or unbraced, note that the com-
mentary on ACI 318–83 indicates that a frame may be considered braced if the
bracing elements, such as shear walls, shear trusses, or other means resisting
lateral movement in a story, have a total stiffness at least six times the sum of
the stiffnesses of all the columns resisting lateral movement in that story.
The slenderness effect may be neglected under the two following conditions:
For columns braced against sidesway, when

kl u M 1
34 12 (5.109)
r M 2