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When designing our bespoke buildings we use AutoCAD software. It is a software application
Where necessary the steps in the design of portals, are given below:
a) Determine possible loading conditions.
b) Compute the factored design load combination(s).
c) Estimate the plastic moment ratios of frame members.
d) Analyse the frame for each loading condition and calculate the maximum required
plastic moment capacity, Mp
e) Select the section, and
f) Check the design for other secondary modes of failure (IS: 800-
The design commences with determination of possible loading conditions, in which
decisions such as, whether to treat the distributed loads as such or to consider them as
equivalent concentrated loads, are to be made.
In step (b), the loads determined in (a) are multiplied by the appropriate load factors to
assure the needed margin of safety. This load factor is selected in such a way that the real factor of safety for any structure is at least as great as that decided upon by the designer.
The step (c) is to make an assumption regarding the ratio of the plastic moment capacities of the column and rafter, the frame members.
(i) Determine the absolute plastic moment value for separate loading conditions.
(Assume that all joints are fixed against rotation, but the frame is free to sway). For
beams, solve the beam mechanism equation and for columns, solve the panel (sway)
mechanism equation. These are done for all loading combinations. The moments thus
obtained are the absolute minimum plastic moment values. The actual section moment
will be greater than or at least equal to these values.
(ii) Now select plastic moment ratios using the following guidelines.
In the step (d) each loading condition is analysed by a plastic analysis method for
arriving at the minimum required Mp. Based on this moment, select the appropriate
sections in step (e). The step (f) is to check the design according to secondary
design considerations discussed in the following sections (IS: 800-
The 'simple plastic theory' neglects the effects of axial force, shear and buckling on the
member strength. So checks must be carried out for the following factors.
a) Reductions in the plastic moment due to the effect of axial force and shear force.
b) Instability due to local buckling, lateral buckling and column buckling.
c) Brittle fracture.
d) Deflection at service loads.
In addition, proper design of connections is needed in order that the plastic moments can be developed at the plastic hinge location
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