Sway of semi-rigid steel frames

Abstract

Lateral sway is most likely to control the design of semi-rigid steel frames where the frame arrangements do not include any form of bracing. This paper investigates the sway behaviour of semi-rigid regular steel frames, i.e. frames having the same arrangement of beam and column sections at all levels, and hence proposes some design charts for the prediction of sway that eliminate the need for doing any numerical modelling. Schueller’s equation has also been modified to incorporate connection flexibility in addition to its original rigid frame considerations. All the proposed methods have been validated using results obtained from numerical analysis.

Introduction

Limiting the lateral sway of building frames is an important design consideration, particularly in those cases for which so-called frame action is relied upon to provide the necessary lateral stiffness. For arrangements that do not include any form of bracing, it is the inherent bending stiffness of the beams and columns themselves that provides the necessary lateral stiffness. In recent years, there has been a growing realisation that a further structural component also contributes some degree of flexibility—both the connections between individual beams and columns and arrangements at the base of the columns where joints are made to the foundation system are now recognised as structural ‘members’ with their own strength and stiffness. Estimating the lateral sway of frame structures, including the making of realistic allowances for the contributions of the connections, is therefore an important design issue.

Techniques for analysing frames incorporating semi-rigid joint effects have been developed significantly during the last two decades. Much of this work has recently been reviewed [1], [2] and only a few particularly relevant contributions are covered herein. Huang and Morris [3] conducted a computer analysis on bare steel frames to study the effect of connection properties on lateral sway. The connection moment–rotation relationship was taken from test data. Colson and Bjorhovde [4] analysed a two storey, two-span bare steel frame assuming pin connections for the exterior beam-to-column connections and semi-rigid behaviour for the interior ones using the computer program PEP-Micro. Deirlein [5] studied semi-rigid bare steel frames using the computer program CU-STAND. All the beam-to-column connections were modelled using zero-length rotational springs to account for their non-linear moment–rotation behaviour. Li et al. [6] analysed a two-span, two storey semi-rigid bare steel frame using the general-purpose finite element software ABAQUS. They studied the effect of connection length and connection stiffness on the moments at different sections. Ye Mei-xin et al. [7] proposed a finite element model for composite frames using the general-purpose software ABAQUS.

It is, of course, possible to utilise a comparatively rigorous approach in which a computer-based frame analysis that explicitly allows for the contributions of the joints is undertaken. However, as with all computerised approaches, initial values must be assigned to the key member properties before such a study can be undertaken. It is, certainly, helpful if such values can be selected as being realistic in the sense that they are close to those likely to be the result of the final design decisions.

When checking deflections under serviceability conditions, it is important to recognise two features:

• Such calculations are normally undertaken assuming elastic behaviour.

• The essential design requirement is that the calculated deflection be not greater than the permissible limit, i.e. great precision in undertaking the analysis is not necessary unless the calculations produce a result that is very close to the limiting value.

Methods that utilise the principles of elastic analysis in an approximate fashion are therefore likely to be useful when attempting to arrive at an appropriate overall balance between member sizes and joint configuration.

Ammerman and Leon [8], Ahmed [9] and Ahsan [10] proposed some methods for sway prediction of semi-rigid frames. The proposals made by Ammerman and Leon and Ahmed were obtained using some specific test data and were applicable only to low-rise frames. Ahsan’s [10] proposed method is applicable only to medium-rise frames ranging from five to eight storeys high. So a more general approach is warranted to predict the sway of semi-rigid frames.

This paper investigates the sway behaviour of semi-rigid steel frames using the finite element (FE) package ANSYS V5.4 [11] to generate results that are then used as the basis for a simplified method for sway prediction. The proposed method is based on design charts obtained from a thorough investigation of the key parameters that affect the sway response of semi-rigid frames. Modifications are also made to the approach, originally proposed by Schueller [12], for estimating the lateral sway of frames with rigid joints, that permits explicit allowance to be made for the influence of the flexibility of the beam-to-column connections. Further extension of this approach to deal with the influence of column base effects is in progress.

The accuracy of the proposed methods is demonstrated by comparisons against a portfolio of numerical results for a representative set of structures. These have been obtained using the ANSYS package.