Civil Engineering Reference
In-Depth Information
5.4.5
Outline sequence for design of a composite braced frame
This over-view is intended to provide an introduction to the subject; it is
not comprehensive. Its scope is limited to regular multi-storey braced
frames of the type used in the examples in Chapters 3, 4 and 5. It is
assumed that the detailing will provide the required resistance to fire, and
that the following decisions are made at the outset:
•
number of storeys, storey heights, column positions, layout and spans
of beams;
•
use of floors to transfer lateral forces to bracing elements;
•
type and location of bracing elements;
•
imposed floor loadings and wind loading;
•
type(s) of beam-to-column joints (pinned, semi-rigid, or rigid);
•
nominal eccentricity for any nominally-pinned joints to columns;
•
strengths of materials and densities of concretes to be used.
Ultimate limit states
(1)
Design the floor slabs (concrete or composite), spanning between
the beams.
(2)
Find imposed-load reductions (if any) for the beams.
(3)
Do preliminary designs for beams, neglecting interaction with
internal columns, as these have little influence, even if joints are
rigid. Use of nominally-pinned joints to external columns enables
their influence on beam design also to be ignored. Include the
flexibility of any semi-rigid joints in analyses of continuous beams.
(4)
Find imposed-load reductions for columns, and do preliminary
designs. Check that columns are not so slender that their imperfec-
tions should be included in global analysis.
(5)
Consider creep and cracking of concrete, and find elastic stiffnesses
for all beams and columns.
(6)
Do elastic first-order global analyses for all frames, for gravity
loads only, to find action effects in beams and columns. Moments
in beams may be redistributed, within permitted limits. Neglect
frame and member imperfections.
(7)
Check beam designs. Revise if necessary.
(8)
Increase bending moments in each column to allow for second-
order effects within the column length and for column imperfections.
Check column designs and revise if necessary.
(9)
Allow for frame imperfections by notional horizontal forces.
Compare these with forces from wind, and decide whether, for
horizontal loading, the leading variable action should be imposed
gravity loading or wind loading.
