Advances in Steel Structures - part 3 pdf

UNBRACED COMPOSITE FRAMES: APPLICATION OF THE WIND MOMENT METHOD D A Nethercot 1 and J S Hensman 2 ISchool of Civil Engineering, University of Nottingham, University Park, Nottingham NG

xx Contents

Analysis

Determination of Section Properties of Complicated Structural Members

Z.X Li, J.M Ko, T.H.T Chan and Y.Q Ni

Adaptive Finite Element Buckling Analysis of Folded Plate Structures

C.K Choi and M.K Song

Hoop Stress Reduction by Using Reinforced Rivets in Steel Structures

K.T Chau, S.L Chan and X.X Wei

1109

1117

1125

Safety Analysis and Design Consideration for Oil and Gas Pipelines

A.N Kumar

1133

Prediction of Residual Stresses: Comparison Between Experimental and Numerical

Results

Y Vincent, J.F Jullien and V Cano

1141

Soil Structure Interaction

Composite Foundation of Deep Mixing Piles for Large Steel Oil Tanks on Soft Ground

X Xie, X Zhu and Q Pan

An Analytical Study on Seismic Response of Steel Bridge Piers Considering Soil-

Structure Interaction

A Kasai and T Usami

1151

1157

Late Papers

Modelling Hysteresis Loops of Composite Joints Using Neural Networks

J.Y Wang, Y.L Wong and S.L Chan

New Design Methods for Concrete Filled Steel Tubular Columns

Y.C Wang

Keynote Paper

The Implications of the Information Society on the Practice of and Training for

Steelwork Construction

G Owens

1167

1175

1187

Keynote Papers

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UNBRACED COMPOSITE FRAMES: APPLICATION OF THE

WIND MOMENT METHOD

D A Nethercot 1 and J S Hensman 2 ISchool of Civil Engineering, University of Nottingham, University Park,

Nottingham NG7 2RD, UK 2Caunton Engineering Limited, Moorgreen Industrial Park, Moorgreen,

Nottingham NG16 3QU, UK

ABSTRACT

Proposals are given to extend the simplified design technique known as the Wind Moment Method

to cover a limited range of composite frames The range represents that of most interest in practice

in the UK Justification is by comparison with the findings from an extensive numerical study

K E Y W O R D S : Composite Construction, Connections, Frames, Joints, Steel Structures, Structural Design

I N T R O D U C T I O N

The Wind Moment Method (WMM) has long been established as a simple, intuitively based, design approach for unbraced frames More recently, it has been the subject of scientific study, designed to provide a more fundamental understanding of the link between actual frame behaviour and the inherent design simplifications This work has, until now, been restricted to bare steel construction

In a recent study Hensman, (1998), the authors have examined the case for an extension of the WMM to cover composite steel/concrete frames Although the approach adopted resembles that used for bare steelwork, a number of particular features have had to be addressed This paper summarizes the main outcomes from that study

The basis for the extension was numerical modelling, utilising the available body of knowledge on the performance of composite connections, the previous application of the WMM to bare steelwork and the capabilities of the ABAQUS package It was also found necessary to conduct a detailed examination of the role of column bases - a feature not previously addressed in WMM

D.A Nethercot and J.S Hensman

investigations Several of the findings therefore have relevance to potential improvements in the WMM for bare steel frames This paper covers: appraisal of the basic source data, outline of the numerical studies, presentation of the key findings and an indication of the resulting design approach This last item will be presented in a fashion suitable for direct use by designers in a forthcoming Steel Construction Institute Design Guide

KEY FEATURES OF THE WIND MOMENT METHOD

The approach was originally devised in the pre-computer era, when overall structural analysis of unbraced frames represented an extremely challenging and potentially tedious task It therefore sought an acceptable simplification so that the labour involved in the structural analysis might be minimised This was achieved by recognising that some simplification in the representation of the actual behaviour would be necessary Although it is now quite widely accepted that the true behaviour of all practical forms of beam to column connection in steel and concrete construction function in a semi-rigid and partial strength fashion- with the ideals of pinned and rigid only occasionally being approached- early methods of structural analysis could only cater for one or other of these ideals Thus the basic WMM uses the principle of superposition to combine the internal moments and forces obtained from a gravity load analysis that assumes all beams to be simply supported and a wind load analysis that assumes beam to column connections to be rigid with points of contraflexure at the mid-span of the beams and the mid-height of the columns as illustrated in Figure 1 This second assumption permits use of the so-called portal method of frame analysis

Once it became possible to conduct full range analyses of steel frames allowing for material and geometrical non-linear effects and including realistic models of joint behaviour, studies were undertaken to assess the actual performance of frames designed according to the WMM principles The findings permitted observations to be made of the two key behavioural measures:

9 That the load factor at ultimate was satisfactory

9 That drift limits at serviceability were achieved

This second point is of importance because, when estimating sway deflections at working load, the WMM normally involves taking the results of an analysis that assumes rigid connections and then applying a suitable scaling factor Important contributions in the area of bare steel construction are those of Ackroyd and Gerstle, (1982), Ackroyd, (1987), and Anderson and his co-workers at Warwick, Reading, (1989), Kavianpour, (1990), Anderson, Reading and Kavianpour,(1991) NUMERICAL APPROACH

All the numerical work was undertaken using the ABAQUS package Whilst this contained sufficient functionality to cover many of the necessary behavioural features, three items required particular attention:

9 Representation of the composite beams

9 Representation of composite beam to column connections

9 Inclusion of column base effects

4,4' 4'4, ,1,4, 4'4' 4'4' 4'4' 4'4' 4'4'4'

~4' 4'4' 4'4' 4'4' 4'4' 4'4' 4'4'4'

(a)

t

7" r 77

Unbraced Composite Frames: Application of the Wind M o m e n t M e t h o d

Figure 1 Superposition of gravity and lateral load analyses

For the first of these the approach previously utilised by Ye, Nethercot and Li, (1996), that is based

on moment curvature relationships developed by Li, Nethercot and Choo, (1993), was employed Since composite endplates were assumed for the beam to column connections, the work of Ahmed and Nethercot,(1997), in predicting moment-rotation response under hogging moment was directly employed

Data on the performance of composite beam to column connections under sagging i.e opening, moments was, however, almost non-existent Previous experience with the Wind Moment Method had, however, suggested that reversal in the sign of the rotation at any connection might be a rather unusual event An approximate model for composite connection behaviour under sagging moments was therefore devised by examining test data for such connections when subject to cyclic loading All previous studies of the WMM have assumed rigid i.e fully fixed column bases Enquiries among practitioners had, however, already revealed that such an option was not attractive In addition, there was a widely held belief that all practical forms of "pin" column bases were capable

of supplying quite significant amounts of rotational restraint Accordingly, all relevant information

on column base effects - particularly previous experimental studies - was carefully reviewed in an attempt to identify suitable minimum restraint levels likely to be supplied by notionally pinned bases, Hensman and Nethercot, (2000a) The findings were then incorporated in the full parametric study This point is regarded as particularly important as attempts to justify the WMM approach using truly pinned column bases, Hensman,(1998), had shown that it was almost impossible to satisfy realistic drift limitations due to the greatly enhanced overall frame flexibility resulting from the loss of column base restraint (as compared with the usual WMM assumption of fixed bases) It

is believed that the exercise should be r e p e a t e d - since bare steel columns were assumed throughout, it would merely be a case of conducting appropriate analyses on bare steel frames - as a way of similarly relaxing an unattractive restriction in the application of the WMM to bare steel construction

6 D.A Nethercot and J.S Hensman

Because of concem over the adequacy of the modelling of composite beam to column connections under sagging moments, particular attention was paid in an initial study, Hensman, J S (1998), to the occurrence (or not) of reversal in the sign of the connection rotations Initial studies using the sub-frame of Figure 2, that was specially configured to represent a typical intermediate floor in a more extensive structure, showed that for realistic arrangements of frame layout, member sizes and levels of gravity and wind loading reversal of rotations, even at the potentially most vulnerable windward connections was extremely unlikely It was therefore concluded that the full parametric study need not concern itself with further refinement of this feature

PARAMETRIC STUDY

Figure 3 illustrates the basic frame layouts considered and Tables 1 and 2 list the range of variables considered within the numerical study Although this was based on the equivalent set of restrictions given in Anderson, Reading and Kavianpour (1991) it has been adapted somewhat, both to recognise important differences between bare steel and composite construction e.g the likely use of longer span beams, and to reflect certain preferences from the industry and recent changes in the

UK design environment e.g issue of a new Code for wind loading A more detailed explanation of the arrangement of the study, including justification for decisions on joint types, load combinations etc., is available, Hensman and Nethercot (2000b) Full details of the 300 cases investigated covering 45 different frame arrangements, including summary results for each, are available in reference 1 In all cases the approach adopted was to first design the frame using the proposed WMM technique and then to conduct a full range computer analysis to check its condition at the SLS and ULS stages

MAIN FINDINGS

Undoubtedly the most significant overall outcome of the parametric study was the finding that every frame design using the proposed WMM approach was essentially satisfactory in terms of providing an adequate margin of safety against ULS load combinations This was despite the fact that the actual distributions of intemal forces and moments within the frames often differed significantly from those presumed by the WMM analyses Only in an extremely small number of cases was any degree of column overstress observed (and then less than 4%) - a comforting feature given that actual end restraint moments obtained from the rigorous analyses were often significantly higher than the assumed 10% of the WMM The actual values of up to 30% in certain cases might suggest that where gravity loads are high beam sections could be reduced by assuming a larger-say 20% - end restraint moment Before so doing, however, it would be important to check the effect

on overall lateral frame stiffness as it might well prove difficult to satisfy drift limitations with this inherently more flexible system

For all cases of frames designed for maximum gravity load and minimum wind load the SLS conditions were met However, if higher wind loads were introduced, particularly for frames with short bay widths, some difficulty in ensuring adequate serviceability performance might well be experienced

Unbraced Composite Frames." Application of the Wind Moment Method 7

A general discussion on the findings from the numerical study in terms of possible future modifications to the WMM and links between flame features and observed behaviour is available in Hensman and Nethercot (2000b)

Figure 2: Typical subframe arrangement used for preliminary study

(Beam spans vary between 6m and 12m)

8 D.A Nethercot and J.S Hensman

Figure 3 9 Schematic diagram of alternative flame layouts used in parametric study

Unbraced Composite Frames: Application of the Wind Moment Method

TABLE 1 RANGE OF VARIABLES CONSIDERED WITHIN THE PARAMETRIC STUDY

*' frames can have more than 4 bays, but a core of 4 bays is the maximum that can be considered to resist the applied wind load

,2 Wind loads = concentrated point load on plane frame at each floor level

TABLE 1 RELATIVE DIMENSIONS CONSIDERED WITHIN THE PARAMETRIC STUDY

Bay width: storey height

(bottom storey)

Bay width: storey height

(above bottom storey)

Greatest bay width:

Smallest bay width

R E C O M M E N D E D D E S I G N A P P R O A C H

The basic design approach is outlined in the chart of Figure 4 This presents all the relevant steps, including those intended to identify arrangements for which the W M M is not suitable Some key details for certain of the steps in the actual design procedure are discussed below

Once an initial frame arrangement has been decided upon, global analyses for the three load combinations:

9 1.4DE + 1.6IL + Notional Horizontal Forces

9 1.2(DL+IL+WL)

9 1.4 (DE+WE)

should be undertaken Notional horizontal forces should be taken as 0.5% of the factored dead + imposed load as specified by BS5950: Part 1 Pattern loading should be considered; it may well be