Theatre Design & Technology - Spring 1988 - 22

Tec~nical Report
Fred M. Wolff, editor

Truss Batten Bovving
BY DONALD E. KIMBALL, JR.

russ battens are known to have
fouling problems caused by horizontal bowing of the truss. When
this occurs, the top chord bows
horizontally and the truss appears
to roll over, as shown in Figure 2. Tllis
results in overstress and interference
with other nearby objects.
An example of this bowing phenomenon was found recently in a theatre equipped with 80-foot-Iong battens suspended by W' wire rope at 14foot spacings. These trusses were load
tested by placing hanging weights in
the center region of the truss. Under a
250-pound load, bowing was clearly
visible. At a load of 500 pounds, the
truss had bowed laterally 9W' in the
center relative to the two ends and was
visibly rolled over. When the truss was
loaded to its full arbor capacity of
2,300 pounds, the lateral bowing was
23". Later analysis revealed that these
trusses would be subjected to lateral
bowing due to buckling under almost
all load configurations except for a
load uniformly distributed over the
full length.

n the case of
new batten design/ alternatives
to truss battens
should also be
investigated.
pounds of load. However, if the pipe
coluI11.l1 was 30' tall, with no bracing
between its ends, it would fail- the
nliddle bowing out (buckling) at an
estimated load 'of only 850 pounds, a

97% reduction in strength. This same
30' colunm could be made to hold the
28,OOO-pound load if laterally braced
at 6-8" intervals, that is, if the middle
was prevented from bowing out.
l.n a truss batten, the top chord,
which goes into compression when a
load is placed at midspan, is like a
slender coluI11.l1 in that it has no lateral
bracing in the horizontal plane. Conversely, if loads are placed at the ends
of the truss with no load in the center,
the bottom chord goes into compression and it also has no horizontal bracing. In either case, the truss capacity
can be significantly less, due to the
buckling of the compression chord.

TRUSS BATTEN DESIGN

Truss battens can and should be designed so that buckling-type failure
does not occur. Conversely, for a particular truss batten already designed
or built, the buckling load should be
calculated so that the truss's safe loading rating can be established. In order

THE BOWING
PHENOMENON

l.n engineering terminology, the bowing phenomenon is called buckling.
Buckling is most often associated
with columns but also occurs in
trusses because the top (or bottoml
chord of a truss is under compression
like a vertical column (while the opposite chord is in tension) and behaves
as though it were a horizontal column.
l.n order to gain an appreciation of
the buckling phenomenon, consider a
1W'-diameter pipe column that is only
6" to 8" tall. This pipe column could be
compressed until the yield strength of
the metal was reached, at about 28,000
20

TD&T·

SPRING 1 BBB

Typical truss batten

FIGURE 1

--~.

- 1 1/2· STD. PIPE

f-o"+

._~

STD. PIPE OR

ROUND BAR
CABLE~

'CROSS SECTION

BOTTOM CHORD

Table of Contents for the Digital Edition of Theatre Design & Technology - Spring 1988