Limit States Design Capacity of Standardised Steel ... - Science Direct

J. Construct. Steel Research 23 (1992) 227-253

Limit States Design Capacity of Standardised Steel Connections L. P h a m ", D. S. Mansell b & P. PaeverC aCSIRO Division of Building, Construction and Engineering, PO Box 56, Highett, Victoria 3190, Australia qnternational Development Technologies Centre, Faculty of Engineering, the University of Melbourne, Parkville, Victoria 3052, Australia ABSTRACT The range of standardised 'shear" (i.e. not moment-resisting) steel connections in Australia is described, and a model of each of the relevant limit states capacities is presented, consistent with current practice and with the new Australian Limit States Design Code AS4100. A computer package developed as a tutor/design program ( C T ) is used to calculate the capacities predicted by those models for a wide and representative range of members and connection components. Those capacities are contrasted with the working loads given in the previous (Working Stress Design )~c Code AS 1250; the safe working loads had been published by the Australian Institute of Steel Construction. The ratio of limit states design capacity to working stress design safe working load is in the range 1.25-2"32, which may be compared with the typical range of load factors (1"2-1"5)for design load levels.

NOTATION A Ac ae Zg

An As

Cross-section area of a component in compression Core area of a bolt (cross-section effective in shear) Edge distance (from the centre of the bolt to the nearest edge in the direction of the force between bolt and ply) Gross section of a component in tension Net section of a component in tension Stress area of a bolt (cross-section effective in tension)

:~ The acronyms LSD and WSD are used throughout this paper to abbreviate Limit States Design and Working Stress Design, respectively; they are sometimes also described as Load and Resistance Factor Design (LRFD) and Allowable Stress Design (ASD).

227 © 1992 Commonwealth Scientific and Industrial Research Organisation (CSIRO).

228 At

Av Aw Bp Bv Bw C

D d df e

To fuf

Lw f*a f*m

fy Lw gl, g3

Ip

/s M* Mb n N N* r/n Ns q ra rb T ta tp tt

L. Pham, D. S. Mansell, P. Paevere

Area of web in tension, participating in a block-shear limit state Area of web in shear, participating in a block-shear limit state Cross-section area of a web or end-plate Design bearing capacity at a hole in a web side-plate Design shear capacity of a bolt Design bearing capacity at a hole in a web Clearance between supported and supporting members Depth of section Depth of web between flanges Diameter of a bolt Eccentricity of applied force with respect to the heel of a cleat angle Specified minimum tensile strength of a plate or ply Specified minimum tensile strength of a bolt Specified minimum tensile strength of a weld Average shear stress on a section Maximum shear stress on a section Specified yield strength of a plate or ply Specified yield strength of an angle Specified yield strength of a bearing pad Yield strength of a beam's web Gauge of holes in a standardised cleat angle Length of cope, from the end of the web to the point where the cope radius starts Length of welded side of a bearing pad or flexible end-plate Length of a seat angle Design moment effect Nominal moment capacity (to be factored by 4) to give design moment capacity) Number of cleat angles Number of bolts in a connection Design axial load effect Number of shear planes in a bolt which include the thread Length of contact between seat angle and supported beam Design capacity of a weld Internal radius at the heel of an angle Radius of fillet between beam's flange and web Beam's flange thickness Thickness of a seat angle Thickness of an end-plate or bearing pad Throat size of a weld

Limits states capacity of standardised steel connections

tW

V*

Vm

VO W Vw

229

Thickness of a beam's web Design shear load effect Design capacity of a seat angle to resist the applied shear load by bending of the leg of the angle. Design shear capacity of a seat angle Design capacity of a cleat angle's leg which is attached to the supporting member Design capacity of a cleat angle's leg which is attached to the beam's web Design capacity of a bolt group Design local compression capacity of a beam's web Design compression capacity of a beam's web Design capacity of a cleat angle Design capacity of the fastener group (bolts or welds) on a seat angle Design bending capacity of a seat angle loaded with a specified eccentricity Design bearing capacity between an end-plate and a bearing pad Design capacity of a weld group Nominal shear capacity (to be factored by ~b to give design shear capacity) Design shear capacity of an end-plate with two rows of bolts Design shear capacity of a beam's web Design block-shear capacity of a web Design capacity of an end-plate weld Design capacity of a bearing pad weld Function of load and eccentricity, determined by assuming the plate is rigid and the bolts (welds) behave elastically until their capacity is reached Capacity factor

1 INTRODUCTION The Australian Institute of Steel Construction (AISC) published I a system of standardised connections in 1979 in two loose-leaf books. One contained the geometric and materials specification details, together with working stress design capacities. The other presented the analytical models used to compute those capacities. The decisions on standardisation drew

230


on the previous efforts of the American Institute of Steel Construction but followed the process through to the provision of tabulated capacities. In a review 2 of the first five years of application of this form of standardisation, two of the three authors originally involved in the two-part manual summarised the progress made in related schemes in the USA, Canada, Switzerland, Sweden and the UK. They pointed out that some of the features of the Australian fabrication industry made it more receptive to the concept of nationally standardised connections than has been the case elsewhere. Two years later, AISC published a third edition 3 in a single book containing the details and the capacities with a minimum of accompanying text, taking the opportunity to change some of the content in light of the industrial experience gained. Now that designers, in a different aspect of standardisation, are required to replace AS 12504 with AS 41005 and move entirely to limit states design, it becomes important to check the status of the standardised connections with respect to the new code and with respect to their apparent safety and economy.

2 STANDARDISATION OF CONNECTIONS In the third edition, AISC stated that its objective was '... to provide.., a rationalised approach to the design, detailing and fabrication of structural steelwork connections'. The benefits of doing so were then listed as follows: (i) provision to the structural designer of a range of safe and economic connections accompanied by load tables and other design data; (ii) elimination of the need for repetitive computation and detailing by structural draughtsmen and shop retailers; (iii) scope for the fabricator to produce connection components by production engineering methods, and to develop standard jigs and fixtures for assembly; (iv) advantages that can be expected to flow from industry rationalisation, such as better communication and better availability of materials and components. The standard components are: (a) universal sections to AS3679; 6 (b) fillet welds from rods classified E48xx or from wires classified W50xx (both having, for design purposes, a nominal ultimate tensile strength of 480 MPa), and with leg sizes of either 6 or 8 mm;


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(c) structural grade bolts (AS 12527) referred to as 8.8 bolts having, for design purposes, a nominal ultimate tensile strength of 830 MPa), with shank diameters of 20 mm, and inserted into 22 mm holes; (d) holes located at 70 mm pitch and 70, 90 or 140 mm gauge; (e) steel strip, either unholed, holed with standardised pitch and gauge, or custom-holed; (f) angles with the same holing options as for the strip. The designations of components and of connections are also standardised. They are unambiguous and therefore useful for contract purposes, but are unlikely to prove easy to commit to memory because of the amount of detailed information coded into them. The nomenclature for welds and bolts is brief, familiar and therefore easy. The symbols 6E48 and 8E48 readily identified E48 weldments with leg lengths of 6 and 8 mm, respectively. (The symbol E48 is assumed to embrace the equivalent semi-automatic W50 electrode.) Similarly, the code of 8"8/S for a bolt readily reminds the user of a bolt which is of grade 8.8 and installed snug tight. The codes for connections and components are necessarily longer, as Fig. 1 shows. The designations are: Strip S-XY/Z Angle A-XY/Z AS-X-Y Angle seat connections BP-X-Y-Z Bearing pad connections Flexible end-plate connections FE-X-YZ Angle cleat connections AC-X-Y-Z Web side-plate connections WP-X-Y-Z

Angle seat:

The illustration in Fig. 3 is designated AS-A-H70/28.8/S.

Bearing pad:

The illustration in Fig. 5 could be designated B P - S B90/C-S-B90/L~150-6E48 if the supported member is a 250UB.

Flexible end-plate: The illustration in Fig. 6 could be designated F E - S A90/5-SWC/120 if the number of rows of bolts is 5.

Web side-plate:

The illustration in Fig. 7 could be designated WP-A-SDO/5-DWC120 if the number of rows of bolts is 5.

Angle cleat:

The illustration in Fig. 8 could be designated A C - A - A KO/5--O if the number of rows of bolts is 5.

Fig. 1. Designations of standardised connections and components.

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The position of the generic symbol X may be occupied by a code letter specifying the size of a strip or an angle, or, in the case of a connection, it may contain a complete component designation. The position of a symbol Y may be occupied by a hole gauge in a strip or an angle, or, in the case of a connection, by a method of fastening, or a complete component designation, or a coping detail. The position of the symbol Z may be occupied by a code for the presence or absence of holes together with the component's length, or in the case of a connection, the weld details or coping details.

3 L I M I T STATES D E S I G N M O D E L S 3.1 Models The term 'model' is used here to refer to the combination of a conceptual model of what a particular limit state comprises and an analytical model of the statics of that limit state so that its capacity can be calculated. Because the behaviour of connections can be quite complex and influenced appreciably by local effects of limited predictability (such as lack of fit) and because of the resulting high cost and low reliability of attempts at precise analysis, design models usually involve considerable compromise between convenience and rigor. In that situation, it is important to recognise our dependence on the Static Theorem of Plasticity (or 'Lower Bound Theorem' as it is often called). The theorem may be found in textbooks on the theory of plasticity of materials, but it has been adapted 8 to the plastic design of flexural frames. A similar adaptation which is equally justified, although it does not appear explicitly in texts, is as follows: For a given connection, if there exists any distribution of force throughout the connection which is both safe (i.e. does not exceed the limit state capacity of any component) and statically admissible with a set of loads W (i.e. in equilibrium with W), the value of W must be less than or equal to the collapse load We. A vital assumption on which this theorem depends is that the behaviour is genuinely plastic and therefore that all components have sufficient

ductility. Reference will be made to this theorem in some of the discussion of models in the later sections of this paper.


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3.2 Components The chapter on connections in AS 4100 recommends that design models be based on the assumption that when load is to be shared by the parts of a fastener group, the connected plates are to be treated as though they are rigid. Making the further assumption that the deformations of a fastener are directly proportional to load leads to simple elastic analysis which satisfies the Lower Bound (or Static) Theorem of Plasticity, as long as the fasteners are ductile. Thus, LSD models of fastener groups can be very similar to those of WSD, except for the components' capacities. The Code has moved to ISO conventions on symbolism, so the models may be described as in the following sections. The starting point is the set of equations for the capacities of components, including the capacity factor ~. These equations, together with the values recommended for ~b, are given in Table 1, which is derived from a similar table published by the authors. 9 The symbols are defined in the Notation. The LSD expressions for capacity of the connected members include the following (where ~b is at present always 0-9):

(a) Web with uniform shear stress: V* ~ ~b × 0"6fyA~

(1) TABLE 1 Capacities of Components

Column 1 Component

Column 3 Capacity factor

Column 4 LSD capacity for M20 bolt (or 6E48 weld)

Column 5 WSD capacity for bolt (weld)

Column 4

d~xO'62fufn.Ac

0-8

89kN

45°kN

1"98

Bolt in tension

~ x fuf As

0.8

127 kN

88 kN

1.45

Ply in bearing

qSaetpfu