Steel Designer''''s Manual Part 2 doc

(a) (b)
(c) (d(
potentol duct zone
taper beam -
duct
duct
zone
zone
(extract)
(supply)
VAV box
ceiling grid
duct
zone
(supply)
Factors influencing choice of form 51
Fig. 2.7 Integration of services: (a) separated (traditional); (b) integrated (shallow floor
‘Slimdek
®
’ system); (c) integrated (long span ‘primary’ beam – stub girder); (d) inte-
grated (long span ‘secondary’ beams)
Fig. 2.8 Tapered beams and services
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01
deflection
Ms2 moment of
resistance
of connection

-I-
T
4-
02<01
-H h-
4-
bending moment = ——
(a)
bending moment = —— - Ms 2
lb)
03<02
RH
Ms3> Ms21
T
cl
The overall depth may also be reduced by using higher-strength steel, but this is
only of advantage where the element design is controlled by strength. The stiffness
characteristics of both steels are the same: hence, where deflection or vibration
govern, no advantage is gained by using the stronger steel.
Recently, shallow floor systems have been developed for spans up to about 9m
which allow integration of services within the slab depth. Structural systems range
from conventional fabricated beams using precast units to proprietary systems using
new asymmetric rolled beams and deep metal decking. These approaches can form
the basis of energy-saving sustainable solutions.
Semi-continuous braced frames can provide an economic balance between the
primary benefits associated with simple or continuous design alternatives. The
degree of continuity between the beams and columns can be chosen so that complex
stiffening to the column is not required. Methods of analysis have been developed
for non-composite construction to permit calculation by hand. It is possible to
achieve reduced beam depths and reduced beam weights.

52 Multi-storey buildings
Fig. 2.9 Floor depth: (a) simple; (b) semi-continuous; (c) continuous
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External wall construction
The external skin of a multi-storey building is supported off the structural frame.
In most high quality commercial buildings the cost of external cladding systems
greatly exceeds the cost of the structure. This influences the design and construc-
tion of the structural system in a number of ways:
•
The perimeter structure must provide a satisfactory platform to support the
cladding system and be sufficiently rigid to limit deflections of the external wall.
•
A reduction to the floor zone may significantly reduce the area and hence cost
of cladding.
•
Fixings to the structure should facilitate rapid erection of cladding panels.
•
A reduction in the weight of cladding at the expense of cladding cost will not
necessarily lead to a lower overall construction cost.
Lateral stiffness
Steel buildings must have sufficient lateral stiffness and strength to resist wind and
other lateral loads. In tall buildings the means of providing sufficient lateral stiff-
ness forms the dominant design consideration. This is not the case for low- to
medium-rise buildings.
Most multi-storey buildings are designed on the basis that wind and/or notional
horizontal forces acting on the external cladding are transmitted to the floors, which
form horizontal diaphragms transferring the lateral load to rigid elements and then
to the ground. These rigid elements are usually either braced-bay frames, rigid-

jointed frames, reinforced concrete or steel–concrete–steel sandwich shear walls.
Low-rise unbraced frames up to about six storeys may be designed using the sim-
plified wind-moment method. In this design procedure, the frame is made statically
determinate by treating the connections as pinned under vertical load and rigid
under horizontal loads. This approach can be used on both composite and non-
composite frames, albeit with strict limitations on frame geometry, loading patterns
and member classification.
British Standard BS 5950 sets a limit on lateral deflection of columns as height/300
but height/600 may be a more reasonable figure for buildings where the external
envelope consists of sensitive or brittle materials such as stone facings.
Accidental loading
A series of incidents in the 1960s culminating in the partial collapse of a system-
built tower block at Ronan Point in 1968 led to a fundamental reappraisal of the
approach to structural stability in building.
Traditional load-bearing masonry buildings have many in-built elements provid-
ing inherent stability which are lacking in modern steel-framed buildings. Modern
structures can be refined to a degree where they can resist the horizontal and
Factors influencing choice of form 53
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vertical design loadings with the required factor of safety but may lack the ability
to cope with the unexpected.
It is this concern with the safety of the occupants and the need to limit the extent
of any damage in the event of unforeseen or accidental loadings that has led to the
concept of robustness in building design.Any element in the structure that supports
a major part of the building either must be designed for blast loading or must be
capable of being supported by an alternative load path. In addition, suitable ties
should be incorporated in the horizontal direction in the floors and in the vertical
direction through the columns. The designer should be aware of the consequences

of the sudden removal of key elements of the structure and ensure that such an
event does not lead to the progressive collapse of the building or a substantial part
of it. In practice, most modern steel structures can be shown to be adequate without
any modification.
Cost considerations
The time taken to realize a steel building from concept to completion is generally
less than that for a reinforced concrete alternative. This reduces time-related build-
ing costs, enables the building to be used earlier, and produces an earlier return on
the capital invested.
To gain full benefit from the ‘factory’ process and particularly the advantages
of speed of construction, prefabrication, accuracy and lightness, the cladding and
finishes of the building must have similar attributes. The use of heavy, slow and in
situ finishing materials is not compatible with the lightweight, prefabricated and fast
construction of a steel framework.
The cost of steel frameworks is governed to a great extent by the degree of sim-
plicity and repetition embodied in the frame components and connections.This also
applies to the other elements which complete the building.
The criterion for the choice of an economic structural system will not necessar-
ily be to use the minimum weight of structural steel. Material costs represent only
30–40% of the total cost of structural steelwork.The remaining 60–70% is accounted
for in the design, detailing, fabrication, erection and protection. Hence a choice
which needs a larger steel section to avoid, say, plate stiffeners around holes or
allows greater standardization will reduce fabrication costs and may result in the
most economic overall system.
Because a steel framework is made up of prefabricated components produced in
a factory, repetition of dimensions, shapes and details will streamline the manufac-
turing process and is a major factor in economic design (Fig. 2.10).
Fabrication
The choice of structural form and method of connection detailing have a significant
impact on the cost and speed of fabrication and erection. Simple braced frameworks

with bolted connections are considered the most economic and the fastest to build
for low- to medium-rise buildings.
54 Multi-storey buildings
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Economy is generally linked to the use of standard rolled sections but, with the
advent of automated cutting and welding equipment, special fabricated sections are
becoming economic if there is sufficient repetition.
The development of efficient, automated, cold-sawing techniques and punching
and drilling machines has led to the fabrication of building frameworks with bolted
assemblies. Welded connections involve a greater amount of handling in the fabri-
cation shop, with consequent increases in labour and cost.
Site-welded connections require special access, weather protection, inspection
and temporary erection supports. By comparison, on-site bolted connections enable
the components to be erected rapidly and simply into the frame and require no
further handling.
The total weight of steel used in continuous frames is less than in semi-
continuous or simple frames, but the connections for continuous frames are more
complex and costly to fabricate and erect.On balance, the cost of a continuous frame
structure is greater, but there may be other considerations which offset this cost
differential. For example, in general the overall structural depth of continuous
frames is less.This may reduce the height of the building or improve the distribution
of building services, both of which could reduce the overall cost of the building.
Corrosion protection to internal building elements is an expensive and time-
consuming activity. Experience has shown that it is unnecessary for most internal
locations and consequently only steelwork in risk areas should require any protec-
tion. Factory-applied coatings of intumescent fire protection can be cost-effective
and time-saving by removing the operation from the critical path.
Construction

A period of around 8–12 weeks is usual between placing a steel order and the arrival
of the first steel components on site. Site preparation and foundation construction
generally take a similar or longer period (see Fig. 2.11). Hence, by progressing fab-
rication in parallel with site preparation, significant on-site construction time may
be saved, as commencement of shop fabrication is equivalent to start-on-site for an
in situ concrete-framed building. By manufacturing the frame in a factory, the risks
Factors influencing choice of form 55
Fig. 2.10 Structural costs: (a) economic and (b) uneconomic layouts
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____________________________________________
4 8 12162024283236404448525660646872768084889296100104108
EXCAVATE & CONSTRUCT FOUNDATIONS
IT
Excavate &
Prepare
for
piling
Bored
Piling
•
o
Construct Lower
Basement &
Foundations
• — '
a)
UNDERGROUND
DRAINS & GROUND

SLAB 0
-
Construct Basement & Drains (Non-critical)
• — '
Perimeter
Bays
Ground Floor (Non-critical)
' .
U-
ERECT STEELWORK
' — '
E
DECKING & COMPOSITE SLABS
—
0
FIRE PROOFING
(sprayed
on site)
U —
CLADDING PANELS
External & Sunscreens & Internal Atrium
— —
WATERPROOFING
Roof and Balconies
U — U
—
BRICKWORK PARTITIONS & INTERNAL FINISHI
SERVICES
—
COMMISSIONING

—
EXTERNAL WORKS & GRANITE
— —
CLEAR SITE
' —
4 weeks
possible
time-saving
Eusing
off-site fire
protection
56 Multi-storey buildings
Fig. 2.11 Typical progress schedule (in weeks)
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n 11
Li
n
:'
U__[[I
a__U__[[I
of delay caused by bad weather or insufficient or inadequate construction resources
in the locality of the site are significantly reduced.
Structural steel frameworks should generally be capable of being erected without
temporary propping or scaffolding, although temporary bracing will be required,
especially for welded frames.This applies particularly to the construction of the con-
crete slab, which should be self-supporting at all stages of erection. Permanent metal
or precast concrete shutters should be used to support the in situ concrete.
In order to allow a rapid start to construction, the structural steelwork frame

should commence at foundation level, and preference should be given to single
foundations for each column rather than raft or shared foundations (Fig. 2.12).
Speed of erection is directly linked to the number of crane hours available. To
reduce the number of lifts required on site, the number of elements forming the
framework should be minimized within the lifting capacity of the craneage provided
on site for other building components. For similar-sized buildings, the one with the
longer spans and fewer elements will be the fastest to erect. However, as has been
mentioned earlier, longer spans require deeper, heavier elements, which will
increase the cost of raw materials and pose a greater obstruction to the distribution
of building services, thereby requiring the element to be perforated or shaped and
hence increasing the cost of fabrication.
Columns are generally erected in multi-storey lengths: two is common and three
is not unusual. The limitation on longer lengths is related more to erection than to
restrictions on transportation, although for some urban locations length is a major
consideration for accessibility.
To provide rapid access to the framework the staircases should follow the erec-
tion of the frame. This is generally achieved by using prefabricated stairs which are
detailed as part of the steel frame.
The speed of installation of the following building elements is hastened if their
connection and fixing details are considered at the same time as the structural steel
frame design. In this way the details can be either incorporated in the framework
or separated from it, whichever is the most effective overall: it is generally more
efficient to separate the fixings and utilize the high inherent accuracy of the frame
Factors influencing choice of form 57
Fig. 2.12 Columns on large diameter bored piles
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to use simple post-fixed details, provided these do not require staging or scaffold-
ing to give access.

Finally, on-site painting extends the construction period and provides potential
compatibility problems with following applied fire protection systems. Painting
should therefore only be specified when absolutely necessary.
2.3 Anatomy of structure
In simple terms, the vertical load-carrying structure of a multi-storey building
comprises a system of vertical column elements interconnected by horizontal beam
elements which support floor-element assemblies. The resistance to lateral loads is
provided by diagonal bracing elements, or wall elements, introduced into the verti-
cal rectangular panels bounded by the columns and beams to form vertical trusses,
or walls. Alternatively, lateral resistance may be provided by developing a continu-
ous or semi-continuous frame action between the beams and columns. The flexibil-
ity of connections should be taken into account in the analysis.All structures should
have sufficient sway stiffness, so that the vertical loads acting with lateral displace-
ments of the structure do not induce excessive secondary forces or moments in the
members or connections. A building frame may be classed as ‘non-sway’ if the sway
deformation is sufficiently small for the resulting secondary forces and moments
to be negligible. In all other cases the building frame should be classed as ‘sway-
sensitive’. The stiffening effect of cladding and infill wall panels may be taken into
account by using the method of partial sway bracing. The floor-element assemblies
provide the resistance to lateral loads in the horizontal plane.
In summary, the components of a building structure are columns, beams, floors
and bracing systems (Fig. 2.13).
2.3.1 Columns
These are generally standard, universal column, hot-rolled sections. They provide a
compact, efficient section for normal building storey heights. Also, because of the
section shape, they give unobstructed access for beam connections to either the
flange or web. For a given overall width and depth of section, there is a range of
weights which enable the overall dimensions of structural components to be nomi-
nally maintained for a range of loading intensities.
Where the loading requirements exceed the capacity of standard sections, addi-

tional plates may be welded to the section to form plated columns, or fabricated
columns may be formed by welding plates together to form a plate-column (Fig.2.14).
The use of circular or rectangular tubular elements marginally improves the load-
carrying efficiency of components as a result of their higher stiffness-to-weight ratio.
Concrete filling significantly improves the axial load-carrying capacity and fire
resistance.
58 Multi-storey buildings
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floor
primary beam
column
bracing
(a)
(b)
(d)
p
Anatomy of structure 59
Fig. 2.13 Conventional steel frame components
Fig. 2.14 Types of column: (a) plated (by addition of plates to U.C. section); (b) universal;
(c) tubular; (d) fabricated plate
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2.3.2 Beams
Structural steel floor systems consist of prefabricated standard components, and
columns should be laid out on a repetitive grid which establishes a standard struc-
tural bay. For most multi-storey buildings, functional requirements will determine
the column grid which will dictate spans where the limiting criterion will be stiff-

ness rather than strength (Fig. 2.15).
Steel components are uni-directional and consequently orthogonal structural
column and beam grids have been found to be the most efficient.The most efficient
floor plan is rectangular, not square, in which main, or ‘primary’, beams span the
shorter distance between columns and closely-spaced ‘secondary’ floor beams span
the longer distance between main beams. The spacing of the floor beams is con-
trolled by the spanning capability of the concrete floor construction (Fig. 2.16).
60 Multi-storey buildings
Fig. 2.15 Typical floor layout
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I I
I
I
1
— —
I
primary beams
secondary beams
I
I
I I
(a)
2.4-3.0 m spocing
(b)
.
I
Composite trusses
Tapered fabricated beam

I
Stub girder
I
Castellated or cellular beams and shallow metal deck
Plain composite beams and shallow metal deck
Simple beams with precast slabs
Slim floor and deep metal deck
I I
5 10
Beam span (m)
15 20
Fig. 2.16 Beam and shallow deck layout: (a) inefficient; (b) efficient
Having decided on the structural grid, the designer must choose an economic
structural system to satisfy all the design constraints. The choice of system and its
depth depends on the span of the floor (Fig. 2.17). The minimum depth is fixed by
practical considerations such as fitting practical connections. As the span increases,
the depth will be determined by the bending strength of the member and, for longer
spans, by the stiffness necessary to prevent excessive deflection under imposed load
or excessive sensitivity to induced vibrations (Fig. 2.18). For spans up to 9m, shallow
beams with precast floors or deep composite metal deck floors can be used to
Anatomy of structure 61
Fig. 2.17 Span ranges for different composite floor systems
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1000
E
E
0
=

C-)
=
500
Vibration
Deflection
Strength
Minimum
0 5 10 15 20
Beam span (m)
minimize the floor zone. Simple universal beams with precast floors or composite
metal deck floors are likely to be most economic for spans up to 12m. A range of
section capacities for each depth enables a constant depth of construction to be
maintained for a range of spans and loading. As with column components, plated
beams and fabricated girders may be used for spans above 10–12m. They are par-
ticularly appropriate where heavier loading is required and overall depth is limited.
For medium to lightly loaded floors and long spans, beams may also take the form
of castellated beams fabricated from standard sections, cellular sections or plates.
Above 15m, composite steel trusses may be economic. As the span increases, the
depth and weight of the structural floor increase, and above 15m spans depth pre-
dominates because of the need to achieve adequate stiffness.
Castellated and cellular beam sections
Castellated beams (Fig. 2.19(a)) have been used for many years to increase the
bending capacity of the beam section and to provide limited openings for services.
These openings are rarely of sufficient size for ducts to penetrate without sig-
nificant modification, which increases fabrication cost. The cellular concept is a
development of castellated beams that provides circular openings and greater shear
capacity. Since their introduction in 1990, they have proved to be increasingly
popular for longer span solutions where services and structure have to be integrated.
Bespoke openings for services can be cut in the webs of universal beams and
fabricated plate girders.

62 Multi-storey buildings
Fig. 2.18 Structural criteria governing choice
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—
(a) castellated beam
(b) cellular beam
(
C
(c) beam with web openings
Fabricated plate girders
Conventional universal beams span a maximum of about 15m. Recent advances in
automatic and semi-automatic fabrication techniques have allowed the economic
production of plate girders for longer span floors. Particularly if a non-symmetric
plate girder is used, it is possible to achieve economic construction well in excess of
15m (Fig. 2.20). Such plate girders can readily accommodate large openings for
major services. If these are in regions of high stress, single-sided web stiffening may
be used. Away from regions of high stress, stiffening is usually not required.
The use of intumescent paints, applied offsite, is becoming increasingly popular.
One major fabricator is now offering an integrated design and fabrication service
for customized plate girders which can achieve a fire resistance of 2 hours when
applied as a single layer in an off-site, factory-controlled process.
Taper beams
Taper beams (Fig.2.20) are similar to fabricated plate girders except that their depth
varies from a maximum in mid-span to a minimum at supports, thus achieving a
highly efficient structural configuration. For simply-supported composite taper
beams in buildings the integration of the services can be accommodated by locat-
ing the main ducts close to the columns. Alternative taper beam configurations can
be used to optimize the integration of the building services.

Anatomy of structure 63
Fig. 2.19 Beams with web openings for service penetrations
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IVZf/Z f/4
V,W/ 7Z7W4
1wJ /f//p
equal flanges
unequal flanges
fish belly
linear taper
horizontal/taper
optimum spans 10—18 m
haunched taper
Composite steel floor trusses
Use of composite steel floor trusses as primary beams in the structural floor system
permits much longer spans than would be possible with conventional universal
beams.The use of steel trusses for flooring systems is common for multi-storey build-
ings in North America but seldom is used in Britain. Although they are consider-
ably lighter than the equivalent universal beam section the cost of fabrication is very
64 Multi-storey buildings
Fig. 2.20 Fabricated plate girders and taper beams
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Vierendeel panel
AJ
A
much greater, as is the cost of fireproofing the truss members. For maximum

economy, trusses should be fabricated from T-sections and angles using simple
welded lap joints. The openings between the diagonal members should be designed
to accept service ducts, and if a larger opening is required a Vierendeel panel can
be incorporated at the centre of the span. Because a greater depth is required for
floor trusses, the integration of the services is always within the structural zone (Fig.
2.21).
Stub girder construction
Stub girders were developed in North America in the 1970s as an alternative form
of construction for intermediate range spans of between 10 and 14m.They have not
been used significantly in the UK. Figure 2.22 shows a typical stub girder with a
bottom chord consisting of a compact universal column section which supports the
secondary beams at approximately 3-metre centres. Between the secondary beams
a steel stub is welded on to the bottom chord to provide additional continuity and
to support the floor slab. The whole system acts as a composite Vierendeel truss. A
disadvantage of stub girders is that the construction needs to be propped while the
concrete is poured and develops strength. Arguably, a deep universal beam with
large openings provides a more cost-effective alternative to the stub girder because
of the latter’s high fabrication content.
Anatomy of structure 65
Fig. 2.21 Composite truss
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3000 3000
•
3000
3000
bottom stub
chord
:slob

stub
bottom chord
_______ (universal column)
2.3.3 Floors
These take the form of concrete slabs of various forms of construction spanning
between steel floor beams (Fig. 2.23). The types generally found are:
•
in situ concrete slab cast on to permanent profiled shallow or deep metal decking,
acting compositely with the steel floor beams;
•
precast concrete slabs acting non-compositely with the floor beams:
•
in situ concrete slab, with conventional removable shuttering, acting compositely
with the floor beams;
•
in situ concrete slab cast on thin precast concrete slabs to form a composite slab,
which in turn acts compositely with the floor beams.
The most widely used construction internationally is profiled shallow metal decking.
Composite action with the steel beam is normally provided by shear connectors
welded through the metal decking on to the beam flange. Shallow floor systems
using deep metal decking are gaining popularity in the UK although precast con-
crete systems are still used extensively. Composite action enables the floor slab to
work with the beam, enhancing its strength and reducing deflection (Fig. 2.24).
Because composite action works by allowing the slab to act as the compression
flange of the combined steel and concrete system, the advantage is greatest when
the beam is sagging. Consequently composite floor systems are usually designed as
simply supported.
66 Multi-storey buildings
Fig. 2.22 Stub girder
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—
precast
—L
concrete
shutter
welded shear stud
(e)
deep
metal
deck
,— asymetric slimfior'
J beam (ASB)
I—
•1
k—deep
metal
deck
grouted joint
unit
(a)
///////7
(b)
(C)
(d)
shallow
metal
deck
Shallow metal deck floor construction

Experience has shown that the most efficient floor arrangements are those using
shallow metal decking spanning about 3–4.5m between floor beams. For these spans
the metal decking does not normally require propping during concreting and the
Anatomy of structure 67
Fig. 2.23 Floor construction: (a) precast (non-composite); (b) in situ (composite); (c) in
situ/precast (composite); (d) in situ/shallow metal decking (composite); (e) Slim-
floor – in situ/deep metal decking (composite); (f) Slimdek
®
– in situ/deep metal
decking (composite)
Fig. 2.24 Metal deck floor slabs
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(a)
(b)
concrete thicknesses are near the practical minimum for consideration of strength
and fire separation.
Steel studs are welded through the decking on the flange of the beams below to
form a connection between steel beam and concrete slab. Concrete, which may be
either lightweight or normal weight, is then poured on to the decking, usually by
pumping, to make up the composite system. Shallow metal decking acts both as per-
manent formwork for the concrete and as tensile reinforcement for the slab. There
are many types of steel decking available (Fig. 2.25(a)) but perhaps the most com-
monly used is the re-entrant profile type, which provides a flat soffit and facilitates
fixings for building services and ceilings.
Some of the advantages of composite shallow metal deck floor construction are:
•
Steel decking acts as permanent shuttering, which can eliminate the need for slab
reinforcement and, due to its high stiffness and strength, propping of the con-

struction while the wet concrete develops strength.
•
Composite action reduces the overall depth of structure.
•
It provides up to 2 hours fire resistance without additional fire protection and 4
hours with added thickness or extra surface protection.
•
It is a light, adaptable system that can be easily manhandled on site, cut to
awkward shapes and drilled or cut out for additional service requirements.
•
Lightweight construction reduces frame loadings and foundation costs.
•
It allows simple, rapid construction techniques.
Figure 2.26 illustrates alternative arrangements of primary and secondary beams for
a deck span of 3m.
68 Multi-storey buildings
Fig. 2.25 Metal deck profiles: (a) shallow deck (50–100 mm); (b) deep deck (150–250mm)
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11111:1 Jooo
99
3OOO
(a)
(b)
Deep metal deck, shallow floor construction
Deep metal decks are normally used to create shallow floors e.g. Slimflor
®
and
Slimdek

®
construction. The deep metal deck extends the span capability up to 9m;
however, the deck and/or support beams may require propping during concreting.
An additional tensile reinforcement bar is provided within the ribs of the deep
decking to improve the load carrying capacity and fire resistance of the floor slab.
Although there are several variants internationally,Slimflor
®
construction in the UK
comprises a universal column with a plate welded to the underside supporting the
deep metal decking. Shear studs are shop-welded to the top flange of the beam to
form a connection between the steel beam and concrete slab. Slimdek
®
construc-
tion is a technologically advanced solution comprising a rolled asymmetric beam
(ASB) which supports the deep metal decking directly.Shear connection is achieved
through the bond developed between the steel beam and concrete encasure. These
features reduce material and fabrication content. Partial concrete encasement of the
steel beam provides up to 1 hour inherent fire resistance.
The range of deep metal deck profiles (Fig. 2.25(b)) is more limited than for
shallow decks, and those available carry similar attributes and advantages. Some
additional advantages of Slimflor
®
and Slimdek
®
construction are:
•
The shallow composite slab achieves excellent load capacity, diaphragm action
and robustness
•
There are fewer frame components to erect, saving construction time

•
The shallow floor construction allows more floors for a given building height or
reduces the cost of cladding and vertical services, lift shafts, etc.
•
It provides up to 1 hour inherent fire resistance and up to 2 hours using passive
fire protection to the beam soffit only
•
Large openings for vertical services can be formed in the floor slab without the
need for secondary framing
Anatomy of structure 69
Fig. 2.26 Alternative framing systems for floors: (a) long span secondary beams; (b) long
span primary beams
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Ii
RHSFB edge beam
5 to 9 m
tie
member
,i.
ASB
-Ff1 column (UC or SHSI
5 to 9 m
'1 deep
deck
fr span
— — I
•
Services can be integrated between the decking ribs passing through web-

openings in the beam
•
ASBs achieve composite action without the need for shear studs
•
Rectangular hollow section edge beams provide good torsional resistance and
maintain the shallow floor depth
•
Floor slabs can be used for fabric energy storage forming part of an environ-
mentally sustainable building solution.
Figure 2.27 illustrates a typical beam layout at the building perimeter.
Precast floor systems
Universal beams supporting precast prestressed floor units (Fig. 2.28) have some
advantages over other forms of construction. Although of heavier construction
than comparable composite metal deck floors, this system offers the following
advantages.
•
Fewer floor beams since precast floor units can span up to 6–8m without
difficulty.
•
No propping is required.
•
Shallow floor construction can be obtained by supporting precast floor units on
shelf angles or on wide plates attached to the bottom flanges of universal columns
acting as beams (Slimfloor).
•
Fast construction because no time is needed for curing and the development of
concrete strength.
70 Multi-storey buildings
Fig. 2.27 Slimdek
®

floor arrangement
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6000 6000
H—
/9o00
_T
/i'
/:-
elements
precast floor
elements
.
•
ceiling line
On the other hand the disadvantages are:
•
Composite and diaphragm action is not readily achieved without a structural
floor screed.
•
Heavy floor units are difficult to erect in many locations and require the use of
a tower crane, which may have implications for the construction programme.
2.3.4 Bracings
Three structural systems are used to resist lateral loads: continuous or wind-moment
frames, reinforced concrete walls and braced-bay frames (Fig. 2.29). Combinations
of these systems may also be used.
Anatomy of structure 71
Fig. 2.28 Precast concrete floors
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U
U
U
El
/\
><
><
><
><
eccentric cross
K
Continuous construction
Continuous frames are those with rigid moment-resisting connections between
beam and columns. It is not necessary that all connections in a building are detailed
in this way: only sufficient frames to satisfy the performance requirements of the
building.
The advantage of a continuous frame is:
•
Provides total internal adaptability with no bracings between columns or walls
to obstruct circulation.
72 Multi-storey buildings
Fig. 2.29 Bracing structures: (a) continuous frame; (b) reinforced concrete wall; (c) braced
bay frames
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additional bracing/rigid
frame required

However, the disadvantages are:
•
Increased fabrication for complex framing connections
•
Increased site connection work, particularly if connections are welded
•
Columns are larger to resist bending moments
•
Generally, less stiff than other bracing systems.
Wind-moment frames are limited in application.
Shear walls
Reinforced concrete walls constructed to enclose lift, stair and service cores gener-
ally possess sufficient strength and stiffness to resist the lateral loading.
Cores should be located to avoid eccentricity between the line of action of the
lateral load and the centre of stiffness of the core arrangement. However, the core
locations are not always ideal because they may be irregularly shaped, located at
one end of the building or are too small. In these circumstances, additional braced
bays or continuous frames should be provided at other locations (Fig. 2.30).
Although shear walls have traditionally been constructed in in situ reinforced
concrete they may also be constructed of either precast concrete or brickwork.
Anatomy of structure 73
Fig. 2.30 Core locations: (a) efficient; (b) inefficient
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The advantages of shear walls are:
•
The beam-to-column connections throughout the frame are simple, easily fabri-
cated and rapidly erected.
•

Shear walls tend to be thinner than other bracing systems and hence save space
in congested areas such as service and lift cores.
•
They are very rigid and highly effective.
•
They act as fire compartment walls.
The disadvantages are:
•
The construction of walls, particularly in low- and medium-rise buildings, is slow
and less accurate than steelwork.
•
The walls are difficult to modify if alterations to the building are required in the
future.
•
They are a separate form of construction, which is likely to delay the contract
programme.
•
It is difficult to provide connections between steel and concrete to transfer the
large forces generated.
Recent developments in steel–concrete–steel composite sandwich construction
(Bi-steel
®
) largely eliminate these disadvantages and allow pre-fabricated and fully
assembled lift shafts to be erected simultaneously with the main steel framing.
Steel–concrete–steel construction can also be used for blast-resistant walls and
floors.
Braced-bay frames
Braced-bays are positioned in similar locations to reinforced concrete walls, so they
have minimal impact upon the planning of the building. They act as vertical trusses
which resist the wind loads by cantilever action.

The bracing members can be arranged in a variety of forms designed to carry
solely tension or alternatively tension and compression. When designed to take
tension only, the bracing is made up of crossed diagonals. Depending on the wind
direction, one diagonal will take all the tension while the other remains inactive.
Tensile bracing is smaller than the equivalent strut and is usually made up of flat-
plate, channel or angle sections. When designed to resist compression, the bracings
become struts and the most common arrangement is the ‘K’ brace.
The advantages of braced-bay frames are:
•
All beam-to-column connections are simple
•
The braced bays are concentrated in location on plan
•
The bracing configurations may be adjusted to suit planning requirements (eccen-
tric bracing)
74 Multi-storey buildings
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