In this article we will discuss about the column connections and angle struts of compression members in steel structures.
A compression member is a very commonly encountered structural member whose function is to receive a compressive force. A compression member is known by various terms like column, stanchion, strut etc. Fig. 7.1 shows the various types of compression members. A column in a building is meant to support the gravity loads applied to the building frame. In a braced frame a brace is provided to provide lateral restraint to a column to resist horizontal forces due to wind or earthquakes. In a roof truss compression members are present as struts in the form of web members and top chord members.
Column Connections of Members:
Beam to Column Connections – Simple Connections:
ADVERTISEMENTS:
A beam can be connected to a column in many ways. Such connections are said to be simple connections, when the connections transmit negligible bending moment between the connected members. Such a connection has the rotational capacity to accommodate the necessary end rotation of the supported beam without transmitting any significant bending moment to the column.
The connection must have adequate strength to transmit the end shear reaction of the supported beam to the column. Rotation at the connected end is taken to be inelastic. These connections may be one sided connections or two sided connections.
One sided connection may be single-plate (shear tab) connection, single-angle connection or T-connection.
ADVERTISEMENTS:
In this case a steel plate is shop-welded to the column at one edge and is bolted to the web of the beam with one vertical row of bolts (Fig. 7.3).
This type of connection affords side erection of the beam providing sufficient clearance allowing adjustments for tolerance in the length of the beam.
ADVERTISEMENTS:
In this case one leg of an angle is welded or shop bolted to the column while the other leg of the angle is field bolted to one side of the web of the beam. In order to allow sufficient flexibility in the connection welding is done along the toe and across the bottom of the leg (Fig. 7.4).
iii. T – Connection:
ADVERTISEMENTS:
In this case a T – section is used for the connection. The flange of the T is welded or shop bolted to the column while the web of the T is field bolted to one side of the web of the beam (Fig. 7.5). This arrangement is found to be suitable to transmit large end reaction from the beam as, if needed it is possible to provide two rows of bolts in the web of the T.
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2. Two Sided Connection – Double Angle Connection:
In this case the connection is made using two angles. The two angles, one on each side of the web of the beam are connected by bolts. The toes of the outspread legs of the angles are shop welded to the column flange and to do this the bottom flange of the beam is coped to allow the beam to be knifed into position (Fig. 7.6).
Sometimes one or two beams may have to be supported over a column. Sometimes the end of a truss may have to be supported over a column. In such cases a steel plate is placed over the column and is welded to the flanges of the column. This plate provides a flat base over which the beams or the end of a truss can be supported. Such a plate is called a column cap (Fig. 7.7 (a, b)).
When rolled steel column sections are available in lengths less than the required lengths, it will be necessary to provide splicing. Sometimes the size of the column for an upper floor may be less than the size of the column for the lower floor. In such cases also at sections where the cross-section of the column changes suddenly, it will be necessary to provide splicing. Splicing consists of connection of the two components of a column by connecting plates.
Where the ends of the compression members are faced for bearing, they shall be spliced to hold the connected members accurately in position and to resist any tension when bending is present.
The ends of compression members faced for bearing shall invariably be machined to ensure perfect contact of surfaces in bearing.
Where such members are not faced for complete bearing the splices shall be designed to transmit all the forces to which they are subjected to.
Where ever possible, splices shall be proportioned and arranged so that, the central axis of the splice coincides as nearly as possible with the centroidal axes of the members joined in order to avoid eccentricity, but where eccentricity is present in the joint, the resulting stress be provided for.
Types of Column Splices:
The following types of column splices are used:
(i) Splices for Axial Load:
These are splices to transfer axial loads from one column to the other. These splices consist of plates provided over the flanges of the columns.
(ii) Splices for Moments:
When splices are expected to resist a bending moment, then such splices are provided in the form of plates over the flanges of columns. These splices are called flange splices. In this case, the splices are designed taking into account the direct load to be transmitted as well as the bending moment.
(iii) Splices for Horizontal Shear:
Splices meant to resist horizontal shear are provided in the form of plates over the webs of the columns. Such splices are called web splices.
Fig. 7.8 shows various types of splices provided for columns. Design of column splices is dealt with in a later article.
Column bases are provided to transmit axial load, horizontal load and moment to the concrete foundation.
The main types of column bases are the following:
(i) Slab base
(ii) Gusseted base
(i) Slab Base:
In this arrangement the column stands directly over a steel base plate which rests over a concrete foundation. The base plate is connected to the column flanges with connecting angles by welding or bolting. The column-base plate unit is fitted in the concrete foundation with foundation bolts.
(ii) Gusseted Base:
Gusseted base plates are used for columns carrying heavy loads. In this case fastenings are used to connect the base plate and the column in the form of gusset plates, angles etc.
Compression members of roof trusses are composed of single angles or double angles. These members may be continuous members (like the principal rafter of the roof truss) or discontinuous members (like the vertical and diagonal members of the truss).
The effective length KL of the compression members may be taken as 0.7 to 1.0 times the distance between centres of connections depending on the degree of end restraint provided. In the case of members of trusses for buckling in the plane perpendicular to the plane of the truss, the effective length, KL shall be taken as the distance between the centres of intersection.
Angle Struts of Compression Members:
Single Angle Struts:
The compressive force in single angle may be transferred either concentrically to its centroid through end gusset or eccentrically by connecting one of its legs to a gusset or adjacent member.
Concentric Loading:
When a single angle is concentrically loaded in compression, the design strength may be determined as in the case of axially loaded columns.
Angle Loaded through One Leg:
The flexural torsional buckling strength of a single angle loaded in compression through one of its legs may be evaluated using the equivalent slenderness ratio λe given by-
where,
K1, K2, K3 = Constants depending upon the end condition, as given in the table below-
where,
l = centre to centre length of the supporting member
rνν = radius of gyration about the minor axis
b1, b2 = width of the two legs of the angle
t = Thickness of the leg, and
ε = yield stress ratio (250/ƒy)0.5
For double angle discontinuous struts, connected back to back on opposite sides of the gusset or a section, by not less than two bolts or rivets in line along the angles to each end or by the equivalent in welding the load may be regarded as applied axially.
The effective length KL, in the plane of end gusset shall be taken as between 0.7 and 0.85 times the distance between intersections, depending on the degree of the restraint provided. The effective length KL, in the plane perpendicular to that of the end gusset, shall be taken as equal to the distance between centres of intersections. Design strength is calculated as in axially loaded columns.
Double angle discontinuous struts connected back to back, to one side of a gusset or section by one or more bolts or rivets in each angle or by the equivalent in welding, shall be designed as in the case of single angle struts.
Double angle continuous struts shall be designed as axially loaded compression members with effective length as 0.7 L to 1.0 L.
Compression Members Composed of Two Components Back to Back:
Compression members composed of two angles channels or tees back to back in contact or separated by a small distance, shall be connected together by riveting, bolting or welding so that the ratio of the most unfavourable slenderness of each member between the intermediate connections is not greater than 40 or 0.6 times the most unfavourable ratio of slenderness of the strut as a whole, whichever is less.
In no case shall the ends of the strut be connected with less than two rivets on bolts or their equivalent in welding and there shall be not less than two additional connections in between spaced equidistant along the length of the strut.
Where the members are separated back to back, the rivets or bolts through these connections shall pass through solid washers or packing in between. Where the legs of the connected angles or the connected tees are 125 mm wide or more, not less than two rivets or bolts shall be used in such connection one on line of each gauge mark.
Where these connections are made by welding, solid packing shall be used to effect the joining unless the members are sufficiently close together to permit direct welding, and the members shall be connected by welding along both pairs of edges of the main components.