Laced and Battened columns

Raju SharmaAssistant ProfessorChandigarh UniversityMohali, Chandigarh

Laced and Battened columnsIntroduction

Lacings

Perforated cover plates are also used for this purpose. However IS 800 do not give any specification for the design of such plates.Lacings Rolled steel flats and angles are used for lacing. One can use single lacing or double lacing system.The object of providing lateral system is to keep the main members of the column away from principal ones. In doing so, the lacings are subjected to shear forces due to horizontal forces on columns.

Instead of lacing one can use battens to keep members of columns at required distancesBattensDesign of Laced ColumnsBatten plates should be provided symmetricallyAt both ends batten plates should be provided. They should be provided at point where the member is stayed in its length.The number of battens should be such that the member is divided into not less than three bays as far as they should be spaced and proportioned uniformly throughout.Battens shall be of plates, angles, channels, or I-sections and at their ends shall be riveted, bolted or weldedBy providing battens distance between the member of columns is co maintained that ryy >rxxDesign of Battened ColumnsThe effective slenderness ratio of battened columns shall be taken as 1.1 times the maximum actual slenderness ratio of the column, to account for shear deformation.The vertical spacing of battens, measured as centre to centre of its end fastening, shall be such that the slenderness ratio of any component of column over that distance shall be neither greater than 50 nor greater than 0.7 times the slenderness ratio of the member as a whole above its z-z axis.Battens shall be designed to carry the bending moments and shear force arising from transverse shear force vt equal to 2.5% of the total axial load.

In case columns are subjected to moments also , the resulting shear force should be found and then the design shear is sum of this shear and 2.5% of axial loadThe design sher and moments for battens plates is given by.Vb = Vt x C/NS and M = Vt x C/ 2N at each connection.Vt = transverse shear force C = distance b/w Centre to Centre of battens longitudinallyN= number of parallel planes S = minimum transverse distance b/w the centroid of the fasteners connecting batten to the main member. The effective depth of end battens (longitudinally), shall not be less than the distance b/w the centroids of main members.Effective depth of intermediate battens shall not be less than th of the above distance.In no case the width of battens shall be less than twice the width of one member in the plane of the batten. It is to be noted that the effective depth of the batten shall be taken as the longitudinal distance b/w the outermost fasteners.The thickness of battens shall be not less than 1/50th of the distance between the innermost connecting lines of rivets, bolts or weldsThe length of the weld connecting batten plate to the member shall not be less than half the depth of batten plate. At least one third of the weld shall be placed at each end of this edge.

Connecting two pieces of sections to get the required length of column is called column splicing. In multistory building, the section of the column may be changed from storey to storey for economy. This also creates the need for splicing. In such cases column is preferably spliced at the point of inflection, which is usually 150 to 300 mm above the floor line. There are two distinct types of compression splices:Those having ends cut by ordinary methodThose having the ends cut and milled.If the ends are not milled, the splice plates and their connections to the column are designed to transmit all forces. The column having milled ends, the ends areColumn Splice Placed firmly in contact with each other and hence considerable load is transferred by bearing. The connections and splice plates are designed for only 50% of axial load.The situation in which various types of column splices are used.When the columns are of the same size, milled ends are provided.When columns are of slightly different sizes, filler plates are used. Load is transferred partially by bearing.When the columns are of considerably different sizes, bearing plates are used.The following procedure may be used in the design of column splices.(1) Column splice plates may be assumed to act as short columns of zero slenderness ratio i.e. assume fcd = fy/1.1 and calculate required area.(2)Width of splice Plate is taken equal tot hat of flange of column and the required thickness calculated.(3)For the selected diameter of the bolts, the bolt is computed and the number required is found.(4) If the moment and shear force are also acting in addition to axial load splice plates are provided to flange as well as to web.Splice plate attached to flange are designed to resist additional axial load equal to M/a,

Design of column splicesWhere a is the distance between centre to centre of flange splice plates. The web splice plates are designed to resist maximum shear force.When bearing plates are to be provided to join two columns of unequal sizes the following steps are to be used for the design of bearing plates.(i) bearing plates may be assumed as short beam to transmit the axial load to the lower column.(ii) Axial load of the column is assumed to be taken by flanges only. Thus the load transfer is shown in fig below.Hence maximum moment in bearing plate =(p/2)xaThe thickness of bearing plate required t is given by(1/6)bt fbs =MFbs = design bending stress = (fy/mo) = 250/1.1 = 227.27 N/mmp/2 p/2aaP