5.
Determine base pressures.
B.4.2 Design Procedure for a Large Moment Base
Due to axial compression:
When the effective eccentricity is large (greater than ekern), there is a tensile force in the anchor rods due to the moment, see Figure B.2b. To calculate this force, the anchor rod force, T, and the length of bearing, A, must be determined, as shown in Figure B.3. By static equilibrium, the following equations can be derived.
f pb =
P P = A BN
where P = Pu for LRFD, Pa for ASD Due to applied moment: f pb =
T +P=
2 f p AB N ′ − A PA′ + M = 2 3
6 Pe M = S pl BN 2
where P = Pu for LRFD, Pa for ASD and M = Mu for LRFD, Ma for ASD. Combined pressure: f p(max) = f pa + f pb =
P 6e 1 + ≤ f p avail N BN
where P = Pu for LRFD, Pa for ASD LRFD
f p AB
where A′ = the distance between the anchor rod and the column center T = Tu for LRFD, Ta for ASD P = Pu for LRFD, Pa for ASD M = Mu for LRFD, Ma for ASD
ASD
f p avail = φ0.85 f c′
f p avail =
0.85 f c′ Ω
if fp(max) ≥ fp avail, adjust the base plate dimensions f p(max) = f pa − f pb =
P 6e 1− BN N
where P = Pu for LRFD, Pa for ASD. 6.
Determine pressure at m distance from fp(max). fpm = fp(max) – 2fpb(m /N)
7.
Determine Mpl at m. m m 2 m m 2 M pl = f p(max) − 2 f pb + 2 f pb N 3 N 2
8.
Determine required plate thickness. LRFD
treq =
4 M u pl φBFy
where φ = 0.90 and Ω = 1.67
ASD
treq =
4 M a pl Ω BFy Figure B.3. General definition of variables.
DESIGN GUIDE 1, 2ND EDITION / BASE PLATE AND ANCHOR ROD DESIGN / 57