Civil Engineering Reference
In-Depth Information
(10)
Do preliminary designs for bracing elements, to find their
stiffnesses.
(11)
Do elastic first-order global analyses for the complete structure,
for horizontal loads plus appropriate gravity loads, to find lateral
deflections. The arrangement of imposed loading should be that
which gives maximum side-sway.
(12)
Repeat (11) with all beam-column intersections displaced laterally
by the amounts found in step (11). Check that the increases in the
action effects that govern design of members are all less than 10%.
(This condition for the use of first-order global analyses is assumed
to be satisfied.)
(13)
Check the design of the bracing elements, taking account of imper-
fections and second-order effects. (The example in this chapter
does not include this.)
Design for serviceability limit states
Re-analyse the frames for unfactored vertical loads to check deflections
and susceptibility to vibration. Detail reinforcement to control crack widths,
as necessary.
5.5
Example: composite frame
5.5.1
Data
To enable previous calculations to be used, the structure to be designed
has a composite slab floor that spans 4.0 m between two-span continuous
composite beams with spans of 9.5 m. There are nine storeys, each with
floor-to-floor height of 4.0 m, as shown in Fig. 5.1. The outer columns are
assumed to be nominally pinned at ground level, and the internal columns
to be nominally pinned at basement level, 4.0 m below the beam at ground
level. For simplicity, it is assumed that the roof has the same loading
and structure as the floors, though this would not be so in practice. The
building stands alone, and the horizontal span of its floors between lateral
restraints is 28 m, as explained earlier.
The materials and loadings are as used previously, Sections 3.2, 3.11
and 4.6, and the composite floor is as designed in Section 3.4. The two-
span composite beams are as designed in Section 4.6, with nominally-
pinned connections to the external columns (Section 5.10), except that
they are not continuous over a central point support. There is instead a
composite column at mid-length of each beam, to which each span is
connected by a 'rigid' and 'full-strength' joint. These terms are defined in
Section 5.3.2.
