natural frequency limits for structures
 

natural frequency limits for structures
i have recently been investigating the design of a pair of long span (100m) trusses for an aircraft hanger roof. the trusses are arched and braced similarly to the sydney harbour bridge in appearance and have a convex roof hung off the bottom cord.
what i would like to know is what an appropriate limit for this structure's natural frequeny would be.
the australian wind code recommends a dynamic analysis for nat. freq's less than 1hz. the structures current nat. freq is about 1.2hz.
where can i find some guidance for a structure like this? i don't want the next tacoma narrows on my hands!
dl.
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hi, davidlewis.
i am assuming that you are based in australia (from your reference to sydney harbour bridge and as code).
i suggest that you should contact professor bill melbourne at monash university at (03) 9902 6000 and discuss your problem with him. i have always found him to be very helpful in such matters.
1.a 100 metre long truss is not long span!
2.a truss structure will not experience a tacoma narrows
type phenomenon!!
3.a 100 metre long truss structure should be designed for
a gust wind of 15 seconds averaging period (my opinion,this
should be as dictated by your code).
4. if it is a wind-prone area, with large wind speeds, a check
for wind induced vortex shedding on individual structural
elements is advisable. preferable to use tubular members
for the truss members (reduces drag forces due to wind,
and gives best section for buckling strength). if welded
tubular joints are provided, diameter of the tube should be
greater than 6% of the length between supporting nodes.
(my conservative thumb rule, could be smaller for less
windy regions).
i have not checked for tubulars with ends connected to
chords by a gusset plate. probably 8% of dia would be
reasonable.
hope this helps,
m. hariharan