Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.109
2P u
qs � (9.88)
1.5B(L � 2e )
Reinforcement for Bearing. The bearing stress on the interface between a column
and a spread footing, pile cap, or drilled pier should not exceed the allowable stress
ƒ b given by Eq. (9.89), unless vertical reinforcement is provided for the excess.
FIGURE 9.43 Bearing stresses on column and pressure
against bottom of footing.
x
A2 A2
ƒb � 0.85�ƒ� c�A
; � 4 (9.89)
A
1 1
where � � strength-reduction factor � 0.70
ƒ�c � specified concrete compressive strength
A1 � loaded area of the column or base plate
A2 � supporting area of footing, pile cap, or drilled pier that is the lower
base of the largest frustrum of a right pyramid or cone contained
wholly within the footing, with A1 the upper face, and with side slopes
not exceeding 2 horizontal to 1 vertical (Fig. 9.43).
If the bearing stress on the loaded area exceeds ƒb, reinforcement must be provided
by extending the longitudinal column bars into the spread footing, pile cap, or
drilled pier or by dowels. If so, the column bars or dowels required must have a
minimum area of 0.005 times the loaded area of the column.
Provisions in the ACI 318 Building Code assure that every column will have a
minimum tensile capacity. Compression lap splices, which are permitted when the
column bars are always in compression for all loading conditions, are considered
to have sufficient tensile capacity so that no special requirements are needed. Similarly,
the required dowel embedment in the footing for full compression development
will provide a minimum tensile capacity.
Required compression-dowel embedment length cannot be reduced by end
hooks. Compression dowels can be smaller than column reinforcement. They cannot
be larger than No. 11 bars.
If the bearing stress on the loaded area of a column does not exceed ,
0.85�ƒ� c
column compression bars or dowels do not need to be extended into the footing,
pile cap, or pier, if they can be developed within 3 times the column dimension
(pedestal height) above the footing (Art. 9.49.8). It is desirable, however, that a
minimum of one No. 5 dowel be provided in each corner of a column.