Impact (Dancing) Loading on Timber Design
Impact (Dancing) Loading on Timber Design
(OP)
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
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






RE: Impact (Dancing) Loading on Timber Design
BA
RE: Impact (Dancing) Loading on Timber Design
Is your information your purely personal opinion, or do you have some information you're quoting?
RE: Impact (Dancing) Loading on Timber Design
BA
RE: Impact (Dancing) Loading on Timber Design
Otherwise, dancing and jumping are not a random loading, it is usually rhythmic, that’s one of the problems as relates to vibration and deflection of the floor system. Another is, that our typical 40lbs./sq.ft. LL for residential space is really not intended for a public gathering space, public congregating space. Take a look at ASCE 7 and see what it says about floor loads in churches, dance halls and public assembly spaces. This might give you some numbers to shoot for in this re-purposed residential bldg. While you might find that you can almost make the stresses check, you also have to make some judgement about how much like a trampoline you want this floor to act when this dancing and jumping is going on. And, if this happens a number of times per week, every week, one wonders when they might just start shaking the floor system apart. Can you stiffen the floor system from below in this assembly and jumping area? The rest of the house would probably be o.k.
RE: Impact (Dancing) Loading on Timber Design
However, if it's 2X8's @24, you have a trampoline - well at least for a short while.
Mike McCann
MMC Engineering
RE: Impact (Dancing) Loading on Timber Design
"We have analyzed your floor and find that it will support a live load of 40 psf in addition to the weight of construction materials. Occupants may move about over the floor but must not do so in an overly rhythmical fashion. Waltzing and fox trotting is deemed acceptable provided that not more than thirty persons (fifteen couples) are so engaged within a 15' x 20' area at any given time. Jitterbugging, religious fervor or other forms of rhythmic oscillations are strictly forbidden unless the owner can ascertain that the weight of the observers around the room is sufficient to dampen the vibrations caused by the dancers and/or movers (see equations on p. 25 of this report to determine weight required in various circumstances)".
BA
RE: Impact (Dancing) Loading on Timber Design
1607.9 Impact loads. The live loads specified in Sections 1607.3 through 1607.8 shall be assumed to include adequate allowance for ordinary impact conditions. Provisions shall be made in the structural design for uses and loads that involve unusual vibration and impact forces.
Mike mentions the "trampoline effect", that shouldn't be ignored. I used the same term in a report about 15 years ago. I was investigating the cracking of a concrete deck in a waste transfer station and witnessed it first hand. On the ground floor of the facility were rail tracks. Garbage was pushed through opening in the floor above to hopper cars; then an excavator would compact the trash. It was like being on a trampoline; the concrete was cracking. The designer used a 30% impact factor, when he should have used more on the order of 70%.
RE: Impact (Dancing) Loading on Timber Design
http://www.youtube.com/watch?v=CMCXjZ3xuEc
It's not wood construction, but it is dancing.
RE: Impact (Dancing) Loading on Timber Design
FYI,
secondary = timber 2x12, 16"oc, spanning 17'
primary = hot rolled W10x30, multiple/cont spans of 15'
congregation max = 20 ppl
As you can see, this is a small group of people worshiping, in a private (not public) setting. Its hard to call this a full fledged church, hence why I'm apprehensive to call it a "public assembly" and looking for a LL=100 psf. Its more akin to a large dinner party in a residence. I would say "dancing" is stretching what they will do (no square-dancing or pop-locking) - most likely some of the people clapping and bobbing they head and body to the rhythm.
I enjoy BA's report, LOL, good stuff.
I believe in my report I'll kind of go the opposite direction - advising (as I've done here) that there are no provisions/requirements/guidelines that specifically address impact of dancing. I'll provide some general feedback on research on the subject and provide my personal views on the subject. But that they are not set in stone and he should use my words as council and not the law, per se.
bridgebuster,
I agree/read your point on the live loads already have impact included. Thats what I would expect. However, NDS goes an extra step and address impact with different safety factors. A decreased live load vs. a safety factor descreasing stresses...we're speaking the same thing.
RE: Impact (Dancing) Loading on Timber Design
RE: Impact (Dancing) Loading on Timber Design
RE: Impact (Dancing) Loading on Timber Design
RE: Impact (Dancing) Loading on Timber Design
It has little to do with the magnitude of load and it is not something that the average structural engineer has been trained to evaluate.
BA
RE: Impact (Dancing) Loading on Timber Design
The combiniedload from people alone exceeded allowed vehicle loads, and caused a very visible "sag" in the usual arch between the two towers: The cables relaxed where the bridge was pulled "up" (flopping in the wind) and were extra tight (twanging" like guitar strings as one observer called it) where the bridge deck was pulled down from its usual position.
The Kansas City hotel lost its three suspended decks due to dancers - killed many, and other smaller decks have been pulled down by dancing people. Don't discount these loads just because it seems trivial or laughable for people to collapse a floor while they enjoy themselves.
RE: Impact (Dancing) Loading on Timber Design
Maybe like what happens starting from around 3:00 here ?
http://www.youtube.com/watch?v=LEqQJBGxcbg
RE: Impact (Dancing) Loading on Timber Design
i think most ppl here are getting caught up on the words "church" and "dancing". this scenario is similar words/terms to those only since they are a best fit -- no other descriptions come closer. this isn't a downtown church that has large services twice a week. its a large house who's inhabitants, plus some neighbors, come to worship in a living room. this isn't get-up-and-pound-your-feet dancing in unison, this is a few kids who may dance a little once they feel the spirit.
rarebugA, your note seems to disregard the monetary side of this issue. we only design strutures/elements for the natural loads they will encounter, plus the live loads that the client tells us they will encounter. to your wishes, how far do we go...perhaps the client may change that room to a library one day (yet he doesn't tell me) -- should I go ahead and design for "worst case" and tell him the floor should be rated for 250 psf?? then tell him he has to pay for new beams, posts, and foundations?
racookpe1978, most ppl know those stories and similar ones. are you actually suggesting that some engineers neglect to design properly "..because it seems trivial or laughable for people to collapse a floor while they enjoy themselves"?? otherwise i don't see the point of your post?
the point of my post was to ask what people felt may be an appropriate safety factor for dancing impact on timber floor -- yet its meandered to a (quasi) combative narrative that I'm somehow improperly disregarding the effects these loads have on a structure??? this has been most often carried out by reminding readers of this thread of dreadful stories of failures, yet not contributing to the original post's request.
please focus a bit.
RE: Impact (Dancing) Loading on Timber Design
RE: Impact (Dancing) Loading on Timber Design
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.
RE: Impact (Dancing) Loading on Timber Design
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.
structSU10, do you think your living room floor is rated for 75-100 PSF? Shouldn't it be if you have some people dancing in your living room?
RE: Impact (Dancing) Loading on Timber Design
"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.
RE: Impact (Dancing) Loading on Timber Design
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).
RE: Impact (Dancing) Loading on Timber Design
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.