Impact (Dancing) Loading on Timber Design 1

[dbnerds]

I have a liturgical client who occupies a large house of timber construction. He is requesting an analysis of the live load floor capacity. He shared that his congregation may occasionally dance and/or jump a bit. I should add that this congregation is small - occupying a room approx. 15'x20'

NDS provides Cd factors of 2.0 for IMPACT. This seems overly conservative for my purposes. I suspect this kind of load factor is more aimed at large mass impacts (e.g. cranes, forklifts). The next step down is SF 1.6 for seismic. The cyclic nature and possible random frequency seems to fit my "dancing model" more appropriately, but I still feel that 1.6 is a bit conservative for this scenario?

Any thoughts/experience of an appropriate safety factor?

thx

 

[woodman88]

dbnerds

"I've suggested a factor of 1.5 to the owner. The dancing load (at least each little impact) is to seismic and wind in duration, but I'm also trying to factor in the low chances that adjacent people's impacts (i.e. all parishioner's dancing along one given structural element) syncing together perfectly."


No No NO!!!

The impact is only allowed for a total of 2 seconds! NOT 2 seconds every year or two years. Only 2 seconds over the life time of the wood member. Your 1.5 factor for Cd gives a load that can be applied for less than 1 hour over the life time of the member.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[dbnerds]

Woodman,
you may misunderstand my 1.5 factor.

Its a mix of two concepts:
-it utilizes the 2.0 Cd (for impact)
-then lowers it a bit to take into the unliklihood of one structural element getting full on impact its entire length (similar to Live Load reduction) or you can also look at it like Load Combinations (in regards to likelihood of the loads acting simultaneously).

 

[woodman88]

On Duration of Load Cd from the Wood Handbook page 5-40 (A free download from

http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=p)

Page 5-40
"...Time under intermittent loading has a cumulative effect.
In tests where a constant load was periodically placed on a
beam and then removed, the cumulative time the load was
actually applied to the beam before failure was essentially
equal to the time to failure for a similar beam under the
same load applied continuously..."

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.