(b) Forces and moment (d) Force on weld
FIGURE 13. (a) Stresses; (b) forces and moment; (c) stresses on
projection; (d) force on weld.
Calculation Procedure:
- Establish the dimensions of the grillage
Refer to Fig. 14. A load of this magnitude cannot be transmitted from the column to its
footing through the medium of a base plate alone. It is therefore necessary to interpose
steel beams between the base plate and the footing; these may be arranged in one tier or in
two orthogonal tiers. Integrity of each tier is achieved by tying the beams together by pipe
separators. This type of column support is termed a grillage. In designing the grillage, it
is assumed that bearing pressures are uniform across each surface under consideration.
The area of grillage required = load, kips/allowable stress, kips/in
2
= 2790/0.750 =
3720 in^2 (23,994 cm^2 ). Set A = 60 in (1524 mm) and B = 62 in (1574.8 mm), giving an
area of 3720 in^2 (23,994 cm^2 ), as required.
- Design the upper-tier beams
There are three criteria: bending stress, shearing stress, and compressive stress in the web
at the toe of the fillet. The concrete between the beams supplies lateral restraint, and the
allowable bending stress is therefore 24 kips/in
2
(165.5 MPa).
Since the web stresses are important criteria, a grillage is generally constructed of S
shapes rather than wide-flange beams to take advantage of the thick webs of S shapes.
The design of the beams requires the concurrent determination of the length a of the base
plate. Let/= bending stress;/,, = compressive stress in web at fillet toe; v = shearing
stress, P = load carried by single beam; S = section modulus of single beam; k = distance
from outer surface of beam to toe of fillet; tw = web thickness of beam; ^ 1 = length of
plate as governed by flexure; a 2 = length of plate as governed by compressive stress in
web.
Select a beam size on the basis of stresses/and/,, and then investigate v. The maxi-
(a) Stresses (c) Stresses on projection
