LACED
AND
BATTENED COLUMNS
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LACED COLUMNS
BATTENED COLUMNS
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LACED (Section: 7.6) AND BATTENED COLUMNS (Section: 7.7)
: Designed to achieve maximum radius of zyration by placing sections
away from principal axis without increasing the area of the section.
LACED COLUMNS
BATTENED COLUMNS
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LACINGS:
1. Single lacing system
2. Double lacing system
By keeping the main members
of the columns away from the
principal axes, the lacings are
subjected to shear force due to
horizontal forces on columns
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Design of Laced Columns (As per IS 800 -2007)
1. As far as possible, the lacing system should be uniform throughout.
2. In single laced system the direction of lattices on opposite faces should be
shadow of the other. It should not be mutually opposite.
3. In bolted construction, the minimum width of lacing bars shall be three times the
nominal diameter of the bolt.
4. The thickness of flat lacing bars shall not be less than 1/40 th of its effective
length for single lacing and 1/60 th of the effective length for double lacings.
5. Lacing bars shall be inclined at 40°- 70° to the axis of built up member.
6. The distance between the two main members should be kept so as to get ryy> rxx
where, ryy is the radius of zyration about weaker axis and rxx is the radius of
zyration of stronger axis of individual member.
7. Maximum spacing of lacing bars shall be such that the maximum slenderness of
the main member between consecutive lacing connection is not greater than 50
or 0.7 time the most unfavourable slenderness ratio of the member as a whole.
8. The lacing shall be designed to resist transverse shear stress Vt = 2.5% of axial
force in columns. If there are two transverse parallel system then each system
has to resist shear force= Vt/2
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9. If the column is subjected to bending also, Vt = Bending shear + 2.5 % of
axial force.
10. Effective length of single laced system is equal to the length between the
inner end fastener. For welded joints and double laced case, effectively
connected at intersection , effective length may be taken as 0.7 times the
actual length.
11. The slenderness ratio KL/r for lacing bars should not exceed 145.
12. Laced compression members should be provided with end tie plates.
13. The effective SR of laced columns shall be taken as 1.05 times the actual
maximum SR, in order to account for shear deformation effects.
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Problem:
Design a laced column with two channels back to back of length 10 m to carry
an axial factored load of 1400 kN. The column may be assumed to have
restrained in position but not in direction at both ends (i.e., hinged ends).
Izz i.e., here Ixx =10008x104 mm4
ryy = 28.3 mm
Iyy = 430.6x104 mm4
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(For the built up section)
(Table 9, For Class of buckling: C and Grade of Steel Fe 250)
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< 0.7x 76.87 = 53.81 O.K.
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Design of Battens(As per IS 800 -2007)
: Instead of lacing one can use battens to keep members
of columns at required distances.
IS 800 -2007 specifies the following rules for the design
of battened columns (Cl. 7.7)
1. Battened should be provided systematically.
2. At both ends batten plates should be provided.
3. The number of battens should be such that the member
is divided into not less than three bays. As far as
possible they should be spaced and proportioned
uniformly throughout.
4. Battens shall be of plates, angles, channels or Isections and at their ends shall be bolted or welded.
5. By providing battens, distance between the members of
columns is so maintained that ryy > rzz.
6. The effective SR of the battened columns shall be
taken as 1.1 times the maximum actual SR of the
column to account for shear deformation.
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7. The vertical spacing of battens measured as centre to centre of its end
fastening shall be such that the SR of any component of column over that
distance shall be neither greater than 50 or 0.7 times the SR of the member
as a whole about the z-z axis.
8. Battens shall be designed to carry the bending moments and shear forces
arising from transverse shear force Vt equal to 2.5 % of the total axial
force.
9. In case columns are subjected to moments also, the resulting shear force
should be found and then design shear is sum of this shear and 2.5 % of
axial load.
10. The design shear and
moments for batten plates is
given by:
𝑽𝒕 𝑪
𝑽𝒕 𝑪
𝑽𝒃 = 𝑵𝑺
and 𝑴 = 𝟐𝑵
at each
connection.
Where,
N = No. of parallel planes
S = Minimum transverse distance
between the centroids of bolt
groups connecting members
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11. The effective depth of end battens (longitudinally) shall not be less than the
distance between the centroids of main members.
12. Effective depth of intermediate battens shall not be less than ¾ th of above
distance.
13. In no case the depth 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 a batten shall be taken as the longitudinal distance between the
outermost fasteners,
14. The thickness of battens shall be not less than 1/50 th of the distance
between the innermost connecting lines of bolts or welds.
15. The 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.
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Problem: Design the built-up column of previous problem using battens.
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Also,
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i.e., 131.2 N/mm2
= 0.6x131.2 = 78.72 N/mm2
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