Lateral support of axially loaded columns in portal frame structures provided by sheeting rails
Date
2008-12
Authors
Louw, Graeme Scott
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
Doubly symmetric I-section columns are often utilised in portal frame construction. The sheeting (or
cladding) is carried by sheeting rails connected to the outer flange of these columns. Although it is
common practice to include the sheeting rails in the longitudinal bracing system, by connecting the
sheeting rail to the cross-bracing, designers must be wary because the connection between column
and sheeting rail will not prevent twisting of the columns cross-section. It has been shown ([11], [12],
[17]), that by including this eccentric restraint into the bracing of the column, that a torsional-flexural
buckling mode of failure can occur when the column is subjected to axial load only. It was seen that
this phenomenon is provided for in BS 5950 [18], but is not present in many other design codes of
practice, in spite of this phenomenon being relatively well known. In some cases the compression
resistance of a column can be significantly reduced when compared to that of a flexural buckled
configuration.
Previous work performed by Helwig and Yura [15] proposed specific column to sheeting rail
connections which would allow for the sheeting rails to be used as elastic torsional braces and
effectively rigid lateral braces. However, it is the objective of this investigation to determine if it is
possible to include the eccentric sheeting rails into the bracing system, even when using a relatively
simple cleat connection with only two bolts onto the sheeting rail.
The objective of the research was investigated by conducting experimental tests coupled with a series
of detailed finite element analyses. The purpose of the experimental set-up was to investigate the
behaviour of a column laterally supported on one flange by a continuous sheeting rail and to compare
it to the behaviour of a column laterally supported on both flanges by means of fly-braces (“kneebraces”).
The behaviour of the columns, as determined by the experimental tests, was validated by the finite
element analyses. The evident conclusion that can be drawn is that, for the case of a continuous
sheeting rail, connected to column simply by two bolts and a cleat, that sufficient torsional restraint is
provided to the column to prevent torsional-flexural buckling from being critical.
This result is helpful, as it means that the buckling capacity of a column can be increased four-fold by
enforcing the second flexural buckling mode instead of the first mode through utilising a continuous
sheeting rail connected to a cross-bracing system as longitudinal bracing on the columns. This can be
achieved without the need to provide any specific detailing to the column to sheeting rail connection. It
is however, recommended that further experimental work be conducted on varying lengths of column
in order to further validate the results of this work.
Description
Thesis (MScEng (Civil Engineering))--Stellenbosch University, 2008.
Keywords
Axially loaded columns, Portal frame structures, Sheeting rails, Lateral support, Theses -- Civil engineering, Dissertations -- Civil engineering