Building Design and Construction Handbook - Merritt - Ventech!

9.154 SECTION NINE
that should be furnished for beams, one-way slabs, and two-way slabs, except for
two-way flat plates, is
A � 0.004A (9.135)
s
For two-way flat plates, where tension stress in the concrete under service loads is
not greater than 2 �ƒ�, c bonded reinforcement is not required in positive moment
areas. When the tension stress in the concrete under service load exceeds 2 �ƒ�, c
the minimum amount of bonded reinforcement provided in positive moment areas
should be
Nc 0.5ƒy A � (9.136)
s
where A � area of that part of the cross section between the flexural tension face
and centroid of gross section
N c � tensile force in the concrete under actual dead load plus live load
ƒ y � yield strength of bonded reinforcement, but not more than 60,000 psi
In the negative moment regions of two-way flat plates at column supports, the
minimum amount of bonded reinforcement provided in the top of the slab in each
direction should be
A � 0.00075A (9.137)
s cƒ
where A cƒ � larger gross cross-sectional area of the slab-beam strips of the two
orthogonal equivalent frames intersecting at a column of a two-way
flat plate
The Code requires the bonded reinforcement computed by Eq. (9.137) to be distributed
between lines that are 1.5h outside opposite faces of the column support,
and that at least four bars be provided in each direction spaced not over 12 in,
where h is the depth of the flat plate. Requirements are included in the Code for
minimum lengths and extensions of the bonded reinforcement computed by Eqs.
(9.135), (9.136) and (9.137).
Grouting of Tendons. When posttensioned tendons are to be bonded, a cement
grout is usually injected under pressure (80 to 100 psi) into the space between the
tendon and the sheathing material of the duct. The grout can be inserted in holes
in the anchorage heads and cones, or through buried pipes. To ensure filling of the
space, the grout can be injected under pressure at one end of the member until it
is forced out the other end. For long members, it can be injected at each end until
it is forced out a vent between the ends.
Grout provides bond between the posttensioning tendons and the concrete member
and protects the tendons against corrosion.
Members should be above 35�F in temperature at the time of grouting. This
minimum temperature should be maintained until field-cured 2-in cubes of grout
reach a minimum compressive strength of 800 psi.
Tendon Sheaths. Ducts for grouted or unbonded tendons should be mortar-tight
and nonreactive with concrete, tendons, or filler material. To facilitate injection of
the grout, the duct should be at least 1 ⁄4 in larger than the diameter of a single
posttensioning tendon. For multiple strand, bar or wire tendons, the duct should
have an internal area at least twice the gross area of the prestressing steel.