With worked examples dynamic loading due vortex shedding
Steel Structures
Design Manual To AS 4100First Edition
CONTENTS
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Contents vii
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This book introduces the design of steel structures in accordance with AS 4100, the Australian Standard, in a format suitable for beginners. It also contains guidance and worked examples on some more advanced design problems for which we have been unable to find simple and adequate coverage in existing works to AS 4100.
The book is based on materials developed over many years of teaching undergraduate engineering students, plus some postgraduate work. It follows a logical design sequence from problem formulation through conceptual design, load estimation, structural analysis to member sizing (tension, compression and flexural members and members subjected to combined actions) and the design of bolted and welded connections. Each topic is introduced at a beginner’s level suitable for undergraduates and progresses to more advanced topics. We hope that it will prove useful as a textbook in universities, as a self-instruction manual for beginners and as a reference for practitioners.
viii
Preface ix
Brian Kirke
Iyad Al-JamelJune 2004
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Im = I of the member under consideration
Iw = warping constant for a cross-section
Ix = I about the cross-section major principal x-axis
Iy = I about the cross-section minor principal y-axis
J = torsion constant for a cross-section
ke = member effective length factor
kf = form factor for members subject to axial compression kl = load height effective length factor
kr = effective length factor for restraint against lateral rotationl = span; or,
= member length; or,
= segment or sub-segment length
le /r = geometrical slenderness ratio
lj = length of a bolted lap splice connection
Mb = nominal member moment capacity
Mbx = Mb about major principal x-axis
Mcx = lesser of Mix and Mox
Mo = reference elastic buckling moment for a member subject to bendingNc = nominal member capacity in compression
Ncy = Nc for member buckling about minor principal y-axis Nom = elastic flexural buckling load of a member
Nomb = Nom for a braced member
Noms = Nom for a sway member
Ns = nominal section capacity of a compression member; or = nominal section capacity for axial load
Nt = nominal section capacity in tension
Ntf = nominal tension capacity of a bolt
N* = design axial force, tensile or compressivenei = number of effective interfaces
Q = nominal live load
Rb = nominal bearing capacity of a web
Rbb = nominal bearing buckling capacity
Rby = nominal bearing yield capacity
Rsb = nominal buckling capacity of a stiffened web Rsy = nominal yield capacity of a stiffened web r = radius of gyration
ry = radius of gyration about minor principle axis.
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xiv Notation
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Why enclose the space? To protect people or goods? From what? Burglary? Heat? Cold? Rain? Sun? Wind? In some situations it may be an advantage to let the sun shine in the windows in winter and the wind blow through in summer (Figure1.1). These considerations will affect the design.
Architects rather than engineers are usually responsible for the problem formulation and conceptual design stages of buildings other than purely functional industrial buildings. However structural engineers are responsible for these stages in the case of other industrial
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The following decisions need to be made:
1. Who is responsible for which decisions?
Besides buildings, other types of structure are required for various purposes, for example to hold something vertically above the ground, such as power lines, microwave dishes, wind turbines or header tanks. Bridges must span horizontally between supports. Marine structures such as jetties and oil platforms have to resist current and wave forces. Then there are moving steel structures including ships, trucks and railway rolling stock, all of which are subjected to dynamic loads.
Once the designer has a clear idea of the purpose of the structure, he or she can start to propose conceptual designs. These will usually be based on some existing structure, modified to suit the particular application. So the more you notice structures around you in everyday life the better equipped you will be to generate a range of possible conceptual designs from which the most appropriate can be selected.
1.3 CHOICE OF MATERIALS
Steel is roughly three times more dense than concrete, but for a given load-carrying capacity, it is roughly 1/3 as heavy, 1/10 the volume and 4 times as expensive. Therefore concrete is usually preferred for structures in which the dead load (the load due to the weight of the structure itself) does not dominate, for example walls, floor slabs on the ground and suspended slabs with a short span. Concrete is also preferred where heat and sound insulation are required. Steel is generally preferable to concrete for long span roofs and bridges, tall towers and moving structures where weight is a penalty. In extreme cases where weight is to be minimised, the designer may consider aluminium, magnesium alloy or FRP (fibre reinforced plastics, e.g. fibreglass and carbon fibre). However these materials are much more expensive again. The designer must make a rational choice between the available materials, usually but not always on the basis of cost.
Figure1.5 Composite construction: steel beams supporting concrete slab in Sydney Airport car park
1.4.2 Live loads (imposed actions) are loads due to people, traffic etc. that come and go. Although these do not depend on member cross sections, they are less easy to estimate and we usually use guidelines set out in the Loading Code AS 1170.1
1.4.3 Wind loads (wind actions) will come next. These depend on the geographical region – whether it is subject to cyclones or not, the local terrain – open or sheltered, and the structure height.
1.6 MEMBER SIZING, CONNECTIONS AND DOCUMENTATION
After the analysis has been done, we can do the detailed design – deciding what cross section each member should have in order to be able to withstand the design axial forces, shear forces and bending moments. The principles of solid mechanics or stress analysis are used in this stage. As mentioned above, dead loads will depend on the trial sections initially assumed, and if the actual member sections differ significantly from those originally assumed it will be necessary to adjust the dead load and repeat the analysis and member sizing steps.
To design effectively it is necessary to know something about the properties of the material. The main properties of steel, which are of importance to the structural designer, are summarised in this chapter.
2.2 STRENGTH, STIFFNESS AND DENSITY
Structural steels are ductile at normal temperatures under normal conditions. This property has two important implications for design. First, high local stresses due to concentrated loads or stress raisers (e.g. holes, cracks, sudden changes of cross section) are not usually a major problem as they are with high strength steels, because ductile steels can yield locally and relive these high stresses. Some design procedures rely on this ductile behaviour. Secondly, ductile materials have high “toughness,” meaning that they can absorb energy by plastic deformation so as not to fail in a sudden catastrophic manner, for example during an earthquake. So it is important to ensure that ductile behaviour is maintained.
The factors affecting brittle fracture strength are as follows:
(6) Internal stress due to welding contraction.
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