Civil Engineering Reference
In-Depth Information
The lower bound values of the expressions for simplified design of beams
failing either in shear or flexure may be written as follows: Beams with web
openings failing in shear: from Eqn (3.24)
where
x
1
=performance factor or safety factor for beams failing in shear=
0.75.
Beams failing in flexure: from Eqn (3.40)
where
x
2
is the performance factor or safety factor for beams failing in
flexure =0.85.
As well as the performance factor (or safety factor), the partial safety factors
for loading and materials as suggested in this section will have to be used.
3.17
Recommendations for lever arm (Z)
Beams with solid webs
For a preliminary design, an approximate value of
Z
is necessary which
cannot be obtained from the Eqn (3.46) without full knowledge of details of
the beams in advance. So, for the preliminary design of beams a value of
Z
=0.7
D
is recommended.
The value of
Z
suggested by Kong
et al.
(1975) was 0.6
D
and that by
CEB-FIP (1970) was 0.2 (
L
+ 2
D
) for 1
2, which comes to 0.7
D
also
for
L
/
D
ratio 1.5 (Ray, 1980; 1983). So, the design bending moment should
not exceed:
£
L/D
£
0.7
A
s
(
f
sy
/
g
m
)
D
(3.47)
Beams with openings in webs
Eqn (3.47) is applicable for beams with web openings also, but a capacity
reduction factor of 0.65 is recommended (Ray, 1980; 1982) —i.e. the design
bending moment should not exceed the limit:
3.18
Design example
For comparison the data assumed for the design example are the same as
those used by Kong and his associates (1975). In working out the example,
