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

 

[BAretired]

A normal residential floor is not suitable for dancing or jumping. Tell your client to ask his congregation to refrain from dancing or they may end up in the basement.

BA

 

[dbnerds]

BAretired:
Is your information your purely personal opinion, or do you have some information you're quoting?

 

[BAretired]

Personal opinion. But if you intend to follow through, check the floor for vibrations.

BA

 

[dhengr]

Some people claim that praying lightens one’s load. So, if the congregation prays hard enough whilst dancing and jumping, you may have to install hold-downs on that floor system, or the ceiling above.

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.

 

[msquared48]

If the floor is 2X10's @ 16 or better spanning 10 feet or so, no worries. Should be OK.

However, if it's 2X8's @24, you have a trampoline - well at least for a short while.

Mike McCann
MMC Engineering

 

[BAretired]

How would the final report read?

"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

 

[bridgebuster]

As ususal BA is on the money. ASCE 7 and IBC 2012 recommend 100 psf live load for dance halls. In addition IBC states:

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%.

 

[Archie264]

I've posted this before:

http://www.youtube.com/watch?v=CMCXjZ3xuEc

It's not wood construction, but it is dancing.

 

[dbnerds]

I appreciate all the feedback everyone.

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.

 

[structSU10]

One thing though is what if this 'church' or whatever it is gets a huge surge in people? I doubt they will come back to ask about loads on a floor and such, and it could become an isssue. When people behave rhythmically is when issues to arise - usually people load floors and none of them are in sync, so no one frequency of load is present. When people dance, you get a specific frequency, generally, so it should be examined.

 

[shobroco]

Why is it so common to ignore physical realities? The room is 15x20,x 40psf = 12,000lb. 200lb/person = 60 people & we're asked if it's okay for 20? Put 60 200 lb. people in a 15X20 room & I want to see them twist & shout, never mind putting 150 people (100psf) in that room. Give your head a shake, you guys.

 

[RareBugTX]

Some time ago I do not know if it was here or somewhere else, I saw a video of a floor collapse in, I believe, Turkey. It was a wedding, everybody was dancing and jumping.....The entire floor collapsed, Plenty killed. Dont waste time trying to justify a structure for a use that was not designed for. Reinforce it! Period.

 

[BAretired]

I remember one of my professors telling my class how the armed forces break step when crossing a bridge. It is not that their weight is so huge, but when they are marching or running in step, they create a vibration which can have a devastating effect on the bridge if their rhythm happens to correspond with the natural frequency of the structure.

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

 

[racookpe1978]

We were in California in Vallejo when the Golden Gate Bridge was opened for foot traffic Road was closed to vehicles, and people "only" were walking across from both sides to celebrate.

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.

 

[Tmoose]

Maybe like what happens starting from around 3:00 here ?

http://www.youtube.com/watch?v=LEqQJBGxcbg

 

[dbnerds]

shobroco is in line with my thoughts on the situation. if you follow his math, with 60 ppl in that room -- they each have 5sf to occupy. it won't happen.

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.

 

[structSU10]

It does come down to your judgement. I would analyze the floor and find, as is, what the allowable uniform load would be. I would use the LL duration factor, and maybe a check with a higher allowance for wind or something, and see what the numbers tell me. When someone jumps you can easily achieve 3-4 times their weight as a force, so from that, I would still feel more comfortable with 75-100 PSF capacity of the floor.

 

[woodman88]

The Cd (Duration of load) factor applies to the total length of time the load will be applied to lumber. Impact (2.0) is a 2 second maximum duration of the load. Wind (1.6) is a ten minute maximum duration of the load, etc. over the life of the wood member. Neither should be applied to a "people" load. See the link below.

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]

WikiEngineer is new to me, thanks Woodman.

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?