Lateral support of axially loaded columns in portal frame structures provided by sheeting rails

[摘要] Doubly symmetric I-section columns are often utilised in portal frame construction. The sheeting (orcladding) is carried by sheeting rails connected to the outer flange of these columns. Although it iscommon practice to include the sheeting rails in the longitudinal bracing system, by connecting thesheeting rail to the cross-bracing, designers must be wary because the connection between columnand 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-flexuralbuckling mode of failure can occur when the column is subjected to axial load only. It was seen thatthis phenomenon is provided for in BS 5950 [18], but is not present in many other design codes ofpractice, in spite of this phenomenon being relatively well known. In some cases the compressionresistance of a column can be significantly reduced when compared to that of a flexural buckledconfiguration.Previous work performed by Helwig and Yura [15] proposed specific column to sheeting railconnections which would allow for the sheeting rails to be used as elastic torsional braces andeffectively rigid lateral braces. However, it is the objective of this investigation to determine if it ispossible to include the eccentric sheeting rails into the bracing system, even when using a relativelysimple cleat connection with only two bolts onto the sheeting rail.The objective of the research was investigated by conducting experimental tests coupled with a seriesof detailed finite element analyses. The purpose of the experimental set-up was to investigate thebehaviour of a column laterally supported on one flange by a continuous sheeting rail and to compareit 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 finiteelement analyses. The evident conclusion that can be drawn is that, for the case of a continuoussheeting rail, connected to column simply by two bolts and a cleat, that sufficient torsional restraint isprovided 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 byenforcing the second flexural buckling mode instead of the first mode through utilising a continuoussheeting rail connected to a cross-bracing system as longitudinal bracing on the columns. This can beachieved without the need to provide any specific detailing to the column to sheeting rail connection. Itis however, recommended that further experimental work be conducted on varying lengths of columnin order to further validate the results of this work.

[发布机构] Stellenbosch University