How to Design Laced and Battened Columns ? | Civil Engineering

In this article we will discuss about how to design laced and battened columns.

To carry heavy loads a built-up column also called an open web column is provided. Such a column may consist of different components like, two channels, four angles, two I-sections etc. If an arrangement to connect or stitch together the individual components of such a column is not made then each component will act as a separate or independent unit.

In such case the design strength of the column will be the sum of the design strengths of the individual components which will definitely be much less than the design strength of the composite unit acting as a single unit. By providing an arrangement to connect the individual components, full use of the strength of the material is utilized.

Connecting the individual components assists in holding the components in their respective positions. The components of a built- up column can be connected by- (i) lacing and (ii) battening and accordingly such columns are called laced columns and battened columns respectively.

Laced Columns:

In this case the components of the composite section are connected by lacing. Lacing consists of connecting the components of the column by a system of generally flat plates. (In some cases angles and channels are also used as lacings). Lacing plates may be 50 mm to 75 mm wide and 8 mm to 10 mm thick.

General Requirements for Lacing (IS 800):

(i) Compression members comprising two main components laced and tied, should where practicable have a radius of gyration about the axis perpendicular to the plane of lacing not less than the radius of gyration about the axis parallel to the plane of lacing.

(ii) As far as possible the lacing system shall be uniform throughout the length of the column.

(iii) Single laced systems on opposite faces of the components being laced together shall preferably be in the same direction so that one is the shadow of the other instead of being mutually opposed in direction.

(iv) The effective slenderness ratio (KL/r)_e of laced columns shall be taken as 1.05 times the (KL/r)_o, the actual maximum slenderness ratio, in order to account for shear deformation effects.

(v) Width of lacing bars – In bolted/riveted construction, the minimum width of lacing bars shall be three times the nominal diameter of the end bolt/rivet.

(vi) Thickness of lacing bars – The thickness of flat lacing bars shall not be less than one-fortieth of its effective length for single lacing and one-sixtieth of the effective length for double lacings.

(vii) Rolled sections or tubes of equivalent strength may be permitted instead of flats for lacings.

(viii) Angles of inclination – Lacing bars, whether in double or single or double system shall be inclined at an angle not less than 40° nor more than 70° to the axis of the built-up member.

(ix) Spacing – The maximum spacing of lacing bars, whether connected by bolting, riveting or welding, shall also be such that the maximum slenderness ratio of the components of the main member (a₁/r₁) between consecutive lacing connections is not greater than 50 or 0.7 times the most unfavourable slenderness ratio of the member as a whole, whichever is less, where a₁ is the unsupported length of the individual member between lacing points, and r₁ is the minimum radius of gyration of the individual member being laced together.

Design of Lacings:

The lacing shall be proportioned to resist a total transverse shear V_t, at any point in the member, equal to at least 2.5 per cent of the axial force in the member and shall be divided equally among all transverse lacing systems in parallel planes.

For members carrying calculated bending stress due to eccentricity of loading, applied end moments and/or lateral loading, the lacing shall be proportioned to resist the actual shear due to bending in addition to that specified above.

The slenderness ratio KL/r, of the lacing bars shall not exceed 145. In bolted/riveted constructions, the effective length of lacing bars for the determination of the design strength shall be taken as the length between the inner end fastener of the bars for single lacing and as 0.7 of this length for double lacing effectively connected at inter sections. In welded construction, the effective lengths shall be taken as 0.7 times the distance between the inner ends of welds connecting the single lacing bars to the members.

Attachments to Main Members:

The bolting riveting or welding of lacing bars to the main members shall be sufficient to transmit the force calculated in the bars. Where welded lacing bars overlap the main members, the amount of lap measured along either edge of the lacing bar shall be not less than four times the thickness of the bar or the thickness of the element of the members to which it is connected, whichever is less. The welding should be sufficient to transmit the load in the bar and shall in any case, be provided along each side of the bar for the full length of lap.

End Tie Plates:

Laced compression members shall be provided with tie plates at the ends of the lacing systems and at intersections with other members/stays and at point, where the lacing systems are interrupted.

Battened Columns:

Compression members composed of two main components battened should preferably have the individual members of the same cross-section and symmetrically disposed about their major axis. Where practicable, the compression members should have a radius of gyration about the axis perpendicular to the plane of the batten not less than the radius of gyration about the axis parallel to the plane of the batten.

The battens should be placed opposite to each end of the member and at points where the member is stayed in its length and as far as practicable, be spaced and proportioned uniformly throughout. The number of battens shall be such that the member is divided into not less than three bays within its actual length from centre to centre of end connections.

The effective slenderness ratio (KL/r)_c of battened columns, shall be taken as 1.1 time (KL/r)_o, the maximum actual slenderness ratio of the column, to account for shear deformation effects.

Design of Battens:

Battens shall be designed to carry the bending moments and shear forces arising from transverse shear force V_t equal to 2.5 per cent of the total axial force in the whole compression member, at any point in the length of the member divided equally between parallel planes of the battens. Battened member divided equally between parallel planes of the battens.

Battened member carrying calculated bending moment due to eccentricity of axial loading, calculated end moments or lateral loads parallel to the plane of battens, shall be designed to carry actual shear in addition to the above shear. The main members shall also be checked for the same shear force and bending moments as for the battens.

Battens shall be of plates, angles, channels or I-sections and at their ends shall be riveted, bolted or welded to the main components so as to resist simultaneously a shear V_b = (V₁C)/N5 along the column axis and a moment M = (V₁C)/2N at each connection.

where,

V₁ = Transverse shear force as defined above

C = Distance between centre to centre of battens, longitudinally

N = Number of parallel planes of battens, and

S = Minimum transverse distance between the centroid of the rivet/bolt group/welding connecting the batten to the main member.

Tie Plates:

Tie plates are members provided at the end of battened and laced members, and shall be designed by the same method as battens. In no case shall a tie plate and its fastening be incapable of carrying the forces for which the lacing or batten has been designed.

Size:

When plates are used for battens, the end battens and those at points where the member is stayed in its length shall have an effective depth, longitudinally, not less than the perpendicular distance between the centroids of the main members.

The intermediate battens shall have an effective depth of not less than three quarters of this distance, but in no case shall the effective depth of any batten be less than twice the width of one member in the plane of the battens.

The effective depth of a batten shall be taken as the longitudinal distance between outermost bolts, rivets or welds at the ends. The thickness of batten or the tie plates shall be not less than one-fiftieth of the distance between the innermost connecting lines of rivets, bolts or welds, perpendicular to the main member.

The requirement of bolt size and thickness of batten specified above does not apply when angles, channels or I-sections are used for battens with their legs or flanges perpendicular to the main member. However it should be ensured that the ends of the compression members are tied to achieve rigidity.

Spacing of Battens:

In battened compression members where the individual members are not specifically checked for shear stress and bending moments, the spacing of battens, centre to centre of its end fastenings shall be such that the slenderness ratio (KL/r) of any component over that distance shall be neither greater than 50 nor greater than 0.7 times the slenderness ratio of the member as a whole about its Z-Z (axis parallel to the battens).

Attachment to Main Members:

Welded Connections:

Where tie or batten plates overlap the main members, the amount of lap shall be not less than four times the thickness of the plate. The length of weld connecting each edge of the batten plate to the member shall in aggregate be not less than half the depth of the batten plate. At least one-third of the weld shall be placed at each end of this edge. The length of weld and the depth of batten plate shall be measured along the longitudinal axis of the main member.

In addition, the welding shall be returned along the other two edges of the plates transversely to the axis of the main member for a length not less than the minimum lap specified above.