Civil Engineering Reference
In-Depth Information
flooring systems or as complete multi-storey framing systems
with acoustic and thermal performance requirements inte-
grated in the wall and floor construction build-ups. These cold
formed systems follow closely the finishes' build-up and thick-
nesses of characteristic walling and floor dimensions used in
traditional masonry construction.
In some circumstances, hot rolled sections with composite
decks and down stand composite beams, or special fabricated
sections can be used such as:
■
Slimflor
- where a deck up to 225 mm deep spans onto special
shallow fabricated sections providing a minimum floor depth of
300 mm.
■
Slimflor
- where precast slabs span onto special fabricated sec-
tions sized to fit within a minimum depth of 200 mm (+tolerance/
camber).
■
Slimdek -
where a deck up to 225 mm deep spans onto rolled
asymmetrical flanged beams (ASB sections).
Figure 18.2 Cost-effective detailing exploiting hollow sections,
simple joints and corrosion addressed with galvanising
18.1.4 Summary, new-build
Structure needs to be appropriate for the application and not
necessarily lowest cost. The initial steel frame design is often
costed more accurately than the stage of building detail devel-
opment justifies. Building details need developing with the
other team members. If there is inadequate time for develop-
ment of design details, cost provisions for details to be resolved
should be incorporated in the initial estimate.
The challenge for the designer is to avoid the temptation to
over-optimise structure.
Figure 18.1
shows an example of a single
column used at an expansion joint position that led to a severely
compromised detail instead of simply using a pair of columns.
Hollow sections are more costly than open sections but their
use can provide benefits in detailing, durability and cost as
shown in
Figure 18.2
.
Secondary cladding elements often use aluminium or stain-
less steel, which are costly relative to steel. When included in
cost planning rates used for elevations/cladding there may not
be adequate provision for interfacing with a 'low cost' struc-
ture. An unexpected cost penalty might result on the overall
project. Alternatively, if cladding costs do not incorporate sec-
ondary structure, the provision of 'secondary steelwork', iden-
tified late, is sometimes added to the mainframe costs causing a
cost overrun on the structural costs. Ambiguity is best avoided
by clarifying scope and cost of secondary steelwork with gen-
eric cladding details and spans defined at early costing stage or
by making cost provision for design development risk.
The structural designer needs to be active in the develop-
ment of construction details at an early stage. Forming the
contextual background to the structural designer's choice of
section sizes and design topology is an important task com-
municated by details, drawings and written structural phil-
osophy statements. Designers typically estimate that 5-8%
of the overall steel member weight is adequate provision for
connections. Some plan arrangements should signal potential
problems with complex costly arrangements and be avoided.
For example,
Figure 18.3
shows an eight-member junction at
a shallow roof apex. If architectural roof detailing had been
more practical using either a flat roof and high performance
membrane or simple roof falls this might have been avoided.
The risk of unexpected costs emerging as the design develops
can thus be reduced.
The designer needs to be conversant with the advantages
and disadvantages of steel construction. Steel is unaffected by
creep, shrinkage, pre-stress losses or time-dependent loading.
This makes deflection prediction straightforward which is par-
ticularly useful where transfer structures are required.
However, steel structure is relatively low mass. This requires
attention to deflection compatibility with finishes and satisfac-
tory dynamic performance.
Figure 18.1 An ill-thought-out connection at an expansion joint
compromising headroom. A twin column arrangement was the
obvious cost effective and structurally better solution
