ZAMIL STEEL BUILDINGS
DESIGN MANUAL
4. Main framing design
PEB DIVISION
4.1.2.7.. Optimization
The following rules should be followed to produce the most economical frame profiles. These rules result in
lighter frames while satisfying fabrication, shipping and erection limitations.
1. Minimize number of splices in the columns and rafters by providing maximum possible lengths
regardless of the material savings that can be produced otherwise. Section lengths should be
multiple of 3m i.e., 3m, 6m, 9m and 12m in order to reduce the scrap.
2. In case of different bay spacing avoid using more than 3 frames.
3. Different frame should be adopted if saving of 5% on all frames with a minimum of 1.0ton is
ascertained.
4. When different frames have to be used due to different bay spacing, maintain the same web cuts for
all such frames.
5. Minimize the number of different flange widths in a frame. Maximum different widths of flanges in all
the frames should preferably be less than three.
6. As much as possible maintain uniformity in the base plate detail and anchor bolt sizes for all the
frames.
7. Try
at the locations where the bending moment is least and/or where the depth
y to locate the splices
p
is least in a frame.
8. Try
for the controlling
y to follow the shape
p of bending
g moment diagram
g
g load combination in the
configuration of the frame by maintaining the stress unity check ratios closer to 1.
Controlling Deflections:
Lateral Sway h:
If lateral sway h exceeds the prescribed limit (normally EH/45) then check the EH/Width ratio. If H/B > 0.75
then fixing the base would result in more economical frame. If H/B<0.75 then increase the web depth at
knee of both column and rafter (difference between knee depth of column and rafter < 200mm)
In multispan frames before going for the option of fixing the exterior column at base, check whether fixing
the tops of interior columns control the lateral sway. If not then fix the exterior column bases.
Vertical Deflection v:
If Vertical deflection v exceeds the prescribed limit (normally Span/180),
Span/180) increase the web depth at knee of
both column and rafter (difference between knee depth of column and rafter < 200mm). A slight increase in
the rafter depth at ridge will also help control the vertical deflection.
Stress Unity Checks:
Combined Stress Unity Check:
f a f bx f by
+
+
1 .0
Fa Fbx Fby
Where fa, fbx and fby are actual axial, major axis bending and minor axis bending stresses respectively. Fa,
Fbx and Fby are corresponding allowable stresses. If combined stress unity ratio exceeds 1.0, check whether
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ZAMIL STEEL BUILDINGS
DESIGN MANUAL
PEB DIVISION
4. Main framing design
vertical deflection/lateral sway limits are satisfied. If not first control the deflection/lateral sway. If deflections
are under control and still the section fails in combined unity check then check the allowable stresses:1. If allowable stresses of top and/or bottom flanges are much lower than 0.6Fy then it implies that the
member is not properly braced then try one of the following:
For rafters and exterior columns (with sheeted side walls) adding flange braces(1) with roof purlins or
wall girts will adjust the allowable stresses for the unbraced flange.
For exterior columns (without sheeted side walls) then providing strut tubes adequately connected to
bracing system at an appropriate height would reduce the unbraced length and adjust the allowable
stress.
For interior I-section columns they can also be braced by means of strut tubes if allowed and
adequately connected to bracing system.
For interior I-section columns that brace points can not be added in the design then stress ratios can
be improved by increasing flanges width or by minor adjustment in the flange thickness.
For columns connected with mezzanine beams/joists columns are considered braced at mezzanine
level.
For columns supporting top running crane beam the columns are considered laterally braced at the
level of carne beam top flange(2).
2. If allowable stresses are sufficiently high and still the section is failing in unity check, then unity check
ratio can be improved by increasing the following in the given order:
Increasing the web depth
Increasing the flange width
Increasing the flange thickness
Shear Stress Unity Check:
If Shear stress unit ratio fv/Fv > 1.0 increase web thickness.
Slenderness Ratio Check:
The slenderness ratio (KLu)/r of a member under compression must be less than 200. Usually in the minor
axis this ratio may exceed the limit. Increasing the flange width in comparison to web depth would improve
the slenderness ratio.
K = Effective length factor
Lu = Unbraced Length
r = Radius of gyration (I/A)
Note: While optimizing keep in mind to satisfy clauses from 4.1.2.1 to 4.1.2.6
(1)
(2)
Refer for clause 4.3.1. for flange brace requirements
Refer to clause 7.2.3.1. for connection details
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