Steel Structure Element Design1.IntroductionStructural design forms part of a process of creating an artefact. In the narrowest sense, structural design is concerned with how the structure behaves under loading. But the scope of structural design is much wider. In the broadest sense, the aim of structural design should be to provide, with due regard to economy, a structure capable of fulfilling its intended function and sustaining the specified loads for its intended life. The design should facilitate safe fabrication, transport, handling and erection. It should also take account of the needs of future maintenance, final demolition, recycling and reuse of material & other sustainability issues.As an introduction to steel structural design, this course covers simple structural analysis, and calculations to check whether a structural member has sufficient capacities to resist the forces in the member. The member capacity check calculations will be performed according to Eurocode 3.2.Limit statesIn Eurocode 3 (EC3), as in other modern codes of practice for structural design, a structure is checked for different limit states. A limit state is simply a condition that if exceeded would result in loss of intended function of the structure. EC3 covers the following limit states: Ultimate limit states (ULS)z Strength, Stability, Fatigue, Brittle fractureServiceability limit states (SLS)z Deflection, vibration, wind induced oscillation, durabilityStructural integrity: accidental load, progressive collapseULS is concerned with structural safety under the expected design loads. SLS is concerned with function of the structure under normal use. Structural integrity is concerned with prevention of structural collapse under accidental actions.This course will be concerned with ULS and SLS deflection check.3.Loads and load combinationsA structure will be subjected to a variety of loads (called actions in Eurocode terminology). These actions can be permanent throughout the structure life, or variable. They should be combined to give design loads. For using Eurocodes, the relevant code for determining loads and their combinations is Eurocode 0 (BS EN 1990). A future course (Design 4 in year 4) will provide more detailed introduction to loads and their combinations. In this course, we will deal with the simple case of self-weight (symbol G, permanent action, also called Dead Load) combined with imposed load (symbol Q, variable action, also called Live Load) applied on floors.Loads are represented by their characteristic values. Because of uncertainty in quantifying loads, safety factors (γ factors) should be used to multiply the characteristic values of loads to give the design values of loads.For ULS design checks, the safety factor for permanent action (G) is γG = 1.35 and the safety factor for variable action is γQ = 1.5. So the design load for ULS is 1.35G+1.5Q.For SLS design deflection check, because SLS refers to normal use of the structure, the safety factor is 1.0. So if the design check is for deflection under variable load, then the design load is Q. If the design check is for deflection under combined permanent and variable loads, then the design load is G+Q.4. Structural analysisBefore member capacity check isundertaken, a structural analysisis performed to obtain the forces in the structural members from the loads acting on the structure.This course will only deal withsimply supported structures, suchas that shown. In this structure, the load applied on the floor slabis distributed to secondary beamsby the one-way spanning floorslab. The secondary beams aresupported by primary beams and distribute the floorloads to the primary beams. The primary beams are supported by columns and distribute their loads tothe columns. The columns transmit the loads to the foundation. The load path is shown by the red arrows.In the structure shown in the figure, the load on a secondary beam is uniformly distributed, obtained by multiplying the applied floor load (kN/m 2) by the slab width supported by the secondary beam.The loads on a primary beam are concentrated at the locations of the secondary beams, at each location equal to the total reaction force from the supported secondary beams at that location. The load on a column isin compression, equal to the total load transmitted by allthe connected beams, including both primary beams and secondary beams. A beam reaction force (shown as arrowin the figure) does not act at the centre of the column.Therefore, it will generate a bending moment in thecolumn. If the bending moments generated by all thebeams connected to the column about an axis of the column are not balanced, the column is subject to a net unbalanced bending moment about this axis. The column is said to be under combined axial compression andbending. The distance from the centre to the end reaction of each beam is approximately half the dimension of the P rimary beam Secondary beam Slab ColumnElevation view of a connection M= UnbalancedBM from beams M in columncolumn (D in the figure) plus 100mm. The value of 100mm is the approximate connection dimension as shown in the figure. It should be pointed out that the bending moment from a beam is caused by the beam’s reaction force acting at an eccentricity from the centre of the column. The unbalanced bending moment should be distributed to the columns. If there is only one column connected to the beams (e.g. top floor), then this column takes all the unbalanced bending moment. If there are two columns (above/below the beams), then it may be accepted that the unbalanced bending moment is equally shared by these two columns. See figure above.5.Material propertiesHaving determined the forces in a structural member, the capacities of the structural member should be calculated to check that the forces can be resisted by the structural member. To calculate the capacities of a structural member, its mechanical properties should be known. For the design check calculations of this lecture, the strength and Young’s modulus of steel and connection components are required. The Young’s modulus of steel can be taken as 205000 N/mm2. The strength of steel depends on its grade. S275 and S355 grades are commonly used. In this designation, “S” means “Structural steel”. 275 and 355 refer to the nominal yield stress of steel, being 275N/mm2 and 355 N/mm2 respectively. Normal carbon steel also has an ultimate tensile stress (UTS). Their values are listed in Table 3.1 of the design code (see Eurocode 3 extract). The strength of steel is slightly affected by the thickness of steel as a result of some impurities in the steel. These values are also listed in Table 3.1. Bolts are used to make connections. Bolt designation is Grade x.y. Multiplying the first number (“x”) by 100 gives the UTS of the bolt in N/mm2. Multiplying the UTS of bolt by “y” 10th gives the design yield stress of the bolt. For example, Grade 10.9 bolts has 1000 N/mm2 UTS and 900 N/mm2 yield stress. Mechanical properties of other bolts are given in Table 3.1 of EC3 Part 1.8 (see Eurocode 3 extract).There are uncertainties in strength of materials. Therefore, safety factors for material should also be applied. In the Eurocode 3 extract, two values are used, 1.05 and 1.25. The lower value is used if the design check is for a ductile failure mode (steel sections). The higher value is used if the design check is for a brittle failure mode (bolts/weld).6.Structural elementsThis course will deal with the following structuralelements: member in tension, member in compression,member in bending, member in combined bending andaxial compression, and simple beam to columnconnections7.Section sizesA variety of different steel section shapes and sizesmay be used in a steel framed building. See figure. Inthis course, Universal Beam (UB) and UniversalColumn (UC) sections will be used. A steelbeam/column section is designated AAAxBBBxCCC,where AAA is the approximate section height, BBB theapproximate section width and CCC the weight in kg permetre of the section. For example 762x267x197UB refers to aUniversal Beam section about 762mm in height (h), 267mm inoverall width (b) and 197 kg/m in weight.Each cross-section has two principal axes, the y-y axis and z-zaxis. The y-y axis is often referred to as the major axis and thez-z axis the minor axis. See figure to the right.8.Introduction to EurocodesChecking whether a structural member has sufficient capacity is generally done according to a code of practice. Eurocode is a set of structural design codes of practice that is now adopted by countries in the European Union. These codes supersede their national codes of practice. The Eurocode for Steel Structural Design is Eurocode 3 (BS EN 1993). There are many parts of Eurocode 3 dealing with different aspects of steel structural design. In this course, aspects of Eurocode 3 Part 1.1 (BS EN 1993-1-1) and Eurocode 3 Part 1.8 (BS EN 1993-1-8) will be applied. Eurocode 3 Part 1.1 is for the design of main structural members and Eurocode 3 Part 1.8 is for the design of steel connections. An extract of Eurocode 3 Part 1.1 and Eurocode 3 Part 1.8 is provided for this course. The main objective of this steel structural course is to develop an understanding of different steel structural phenomena and then to apply Eurocode 3 Part 1.1 and Eurocode 3 Part 1.8 to the design of the structural elements mentioned in Section 6. Although not necessary, you are encouraged to download BS EN 1993-1-1 and BS EN 1993-1-8 from the University Library (>electronic resources>electronic database>British Standards Online).9.Summary of design check steps for different types of structural elementAs mentioned in Section 2, Limit State design is adopted. When under load, a structural member may reach its limit states in different ways. A structural member may fail in different ways (modes). Therefore, for Ultimate Limit State (ULS) design checks, the first thing to do is to identify the failure modes of the structural member. Afterwards, design calculations are carried out to ensure that under each failure mode, the structural member has sufficient capacity to resist the applied loads. There are many Serviceability Limit States (SLS). This course will only consider beam deflection.Instruction on how to carry out detailed design calculations will be provided in the lectures. Worked examples will also be provided. This note will only explain the failure modes and list the design checks required for the different types of structural elements in Section 6. To thoroughly understand how to carry out detailed design calculations, read this section in conjunction with my Eurocode 3 Extract notes (with explanations) and Worked Example sheets. In the notes below, red italic text refers to the Clause numbers in Eurocode 3 for detailed calculation equations. Eurocode 3 Part 1.1 will be referred, unless stated otherwise as Eurocode 3 Part 1.8 clause.9.1Member in tensionA member in tension will fail either due to excessive deformations when the stress has reached the yield stress, or fracture when the stress has reached the Ultimate Tensile Stress. The following design checks should be carried out:•Gross cross-section yield capacity: clause 6.2, first paragraph•Net cross-section fracture capacity: clause 6.2, second paragraph9.2Member in compressionWhen a structural member is under compression, cross-section yield may happen and buckling can also occur. Buckling can happen locally or globally. Therefore, the following design checks should be carried out:•Local buckling (check flange/web width/thickness ratio to achieve at least Class 3: Clause 5.5, Table 5.2•Global buckling (including consideration of cross-sectional yield): Clause 6.3.1.1, Clause 6.2.4(2) and Tables 6.1/6.2.9.3Member in bending and shearWhen a beam is under load, there will be bothshear force and bending moment in the beam.Under bending, part of the beam will be undercompression. This compression can cause the beamto buckle locally or globally. For beam, the globalbuckling mode is referred to as Lateral TorsionalBuckling (LTB, see figure), whereby the beamtwists and deforms laterally. LTB occurs if thebeam is allowed to deform laterally or twist (i.e.without lateral or torsional restraint). If the beam iscompletely restrained torsionally and laterally, it isnot necessary to carry out LTB check.For ULS, the following design checks should be carried out:•Local buckling (check flange/web width/thickness ratio to achieve at least Class 2 or better): Clause 5.5, Table 5.2•Cross-section bending moment capacity at maximum bending moment: Clause6.2.5(2)•Cross-section shear capacity at maximum shear: Clause 6.2.6(2)•Combined bending moment and shear: Clause 6.2.8(3)•Lateral torsional buckling for the unrestrained length of the beam: Clause 6.3.2.2For SLS design checks, the maximum beam deflection under SLS loading should be checked.9.4Member in combined bending and axial compressionWhen a member in under combined bending and axial compression, all the failure modes identified for member in compression and member in bending should be checked. The ULS design checks are as follows:•Check for local buckling (flange/web width to thickness ratio) as for columns to achieve at least Class 2: Clause 5.5, Table 5.2•Check cross-section for maximum combined axial compression and bending: Clause6.2.1(7)• Check flexural bending under combined axial compression/bending moments: Clause6.3.3.(1)• Check combined flexural and lateral torsional buckling: Clause 6.3.3.(2)9.5 Flexible endplate connectionA flexible (or partial depth) endplate connection is made bywelding an endplate to the end of the beam and then connect theendplate to the column (either flange or web) using bolts in pre-drilled holes in the endplate and the flange/web of the column.See figure to the right.A flexible endplate connection is considered as a simple connecti transmitting a shear force from the beam to the column, as shown in the figure. This force is transmitted from the beam to the column via a number of connection components and each of these connection components should be checked to have sufficient resistance. The components along the load transmitting path, and therefore the design checks, are:From end of on. It is capable of only beam to the weld via the length of the beam web that is welded to theld to the endplate: the shear strength of weld should be checked: Part 1.8 dplate to the bolts: the shear strength of the endplate should be checked: the column flange or web: bolt capacity (shear/bearing) should be .6 Fin plate connectionin plate connection is another type of simple connection. It ishe components along the load transmitting path, and therefore the design checks, are: lts ear force shear: Partor all connections, in addition to capacity check, the connection dimensions should also be C Wang, November 2010 endplate: the shear resistance of the beam web length should be checked: Clause6.2.6(2)From we Clause 4.5;From the en Clause 6.2.6(2);From the bolts to checked: Part 1.8 Table 3.4. e 9F made by welding a fin plate to the column flange or web and thenbolt the fin plate to the web of the beam through pre-drilled holes inthe fin plate and beam web, see figure to the right. The connectiontransfers a shear force from the beam to the column. The shearforce acts along the bolt line as shown.T From beam web to fin plate via bolts: bolt group capacity (shear resistance of bo and bearing resistance of bolts on beam web/fin plate): Part 1.8 Table 3.4;From fin plate to weld: fin plate under combined shear and bending (note sh acting at eccentricity e in figure): Cl. 6.2.6(2), Cl. 6.2.5(2), Cl. 6.2.8(3)From weld to column: weld stress under combined bending moment and 1.8 Clause 4.5F checked to ensure that the bolts are not too close nor too far apart, and that the distance from bolts to the edge of the connected plates are not too short or too long, see Part 1.8 Table 3.5.Y。
