Reduced Floor Load for an Assembly Space

[H57]

ASCE 7 shows that assembly areas have a designed live load of 100 psf. I have been asked to review an existing assembly space that has a soft or spongy floor. During my review it was found that the floor's live load capacity is 50 psf. The building was constructed in 1970 and it has always been an assembly or meeting area. My recommendation to the owner is to either modify/replace the floor framing or change the use of the space from assembly to something else.

My question: Is there any code provision that allows for a space to remain as assembly space with a reduced floor load capacity? Can you limit the occupancy of the space to account for this reduced floor load capacity?

 

[Den32]

The owner can change this to something else, such as a classroom or office space, but the floor will still be 'spongy'. When you say spongy, do you mean bouncy?

What is the floor construction? Slab on deck on steel joists? Wood joists and sheathing?

Can the span be reduced from below (adding a beam or bearing wall)? Even reinforcing the floor to support the 100 psf live load may not eliminate the bounciness.

I am not aware of anything in ASCE that gets around the 100 psf assembly area. Remember that the code are 'recommended minimums' and it is always up to the engineer to determine what the applicable loads are (there are different 'assembly area live loads' - fixed seats, moveable seats, etc.). Posting a sign that says live load capacity is 50 psf will not mean anything to people coming into that room.

 

[H57]

I was thinking of posting something more in the lines of "max. occupancy XXX" which is a reduced occupancy relative to what the space would normally be allowed. This reduced occupancy would limit the number of people and in turn limit the loading for the space.

I'm just trying to see what the code allows as it is a battle to tell a client that even though he has been using a space for assembly for 40 years he has to stop doing it.

 

[H57]

Floor construction is trus-joist open web trusses (steel webs and wood chords). In speaking with the manufacturer they don't feel comfortable modifying the trusses to account for a center support.

 

[steellion]

Check the IBC, as well as the local code. If the room occupancy type is not changing, it is typically not necessary to upgrade the framing to the new code, it can usually be "grandfathered" in. If you have a reason to suspect that there will be changes increasing the floor load, you should look to upgrade.

Think about it like this, if ASCE decided in their next code cycle to change office live loading from 50 PSF to 60 PSF, does that mean that every office building built in the last 20 years is now structurally deficient and needs to be upgraded?

 

[hawkaz]

Steellion,
I'm not sure that would fly- I believe that assembly area live loads have been 100 psf since the 1920s. It sounds like this was either not designed for the correct live load, or the usage changed (perhaps during construction) without anyone realizing the structural ramifications.

This type of joist is not easily modified- I don't envy your position H57.

 

[frv]

It's a bad idea to post a sign limiting occupancy. No one will ever read or follow it. (What if you have a convention of Chinese business people in?).

Second, have you looked into vibration/ Natural frequency issues? I'd bet the sponginess comes more from that than any overloading.

 

[steellion]

Hawkaz, good point, I didn't see before that it was built in 1970. If vibration is the issue, you're going to have to increase the floor stiffness anyway, with more or deeper members. A substandard live load rating just adds fuel to the fire to do structural remediation.

My point still stands in general though: a change in code does not automatically render buildings built properly to previous standards obsolete. It simply means we understand building design just a little bit better. How many concrete buildings built even 30 years ago would meet every single bar detailing requirement in ACI 318-08? And yet they still stand.

 

[H57]

Thank you all for your input.

frv - Yes vibration/frequency was looked at and appears to be the main problem for the sponginess/movement of the floor. Per the manufacturer there is bridging that can be added to help with this, but it doesn't address the strength deficiency that was found during the review of the floor framing. At this point it has been recommended to the owner that the floor system be modified (placement of additional floor members) or replaced. I don't think they'll like those ideas as they've used the space for 40 years but I can't in good conscience just tell them to continue using it as they're performing a building renovation with the intent of bringing in more groups to use this building.

 

[JAE]

According to most vibration studies, bridging doesn't do much to stop vibration (look up articles by Murray). I think they help a little, but not as much as you'd think.

 

[rapt]

In my experience, since 1976 at least, assembly buildings have never been allowed to have a live load reduction used and the design LL is a minimum of 100psf. If this is the case, the floors are underdesigned.

If the building is going to continue to be used for assembly purposes, it has to be strengthened.

 

[larsacious]

I will play the devil's advocate here and say that this floor has been in existence for 40 years. During this time it has likely seen every type of live load imaginable. The guidance given to me by much senior engineers is that as long as the owner doesn't increase the load to the floor then why at this point make them reinforce it?

I would issue a letter stating that it appears the floor was underdesigned, however, there is no evidence of overstressing (confirm) and they should not increase the dead weight of the floor. If they do decide to increase the dead weight then reinforcing will be necessary.

 

[frv]

larsacious..

I've heard that from some engineers as well, and I think it's nonsense.

How do you know it has seen all sorts of live loads imaginable? Perhaps they called it an assembly area and only occasionally had office parties there, where the actual load was very low most of the time. I highly doubt the floor has seen anywhere near 100 psf. Think about how much you weigh and how many people would be required to come anywhere near that load.

Falling watter stood up for decades as well. Does that mean that the owners were foolish to retrofit it?

 

[JAE]

I agree with frv. As an engineer it is your duty to protect the safety of the public. Suggesting that the floor has "likely" seen the type of loading that it will see in the future, and most importantly, provide a level of a safety factor required by the codes is incorrect and not what other engineers would do in a similar situation (standard of care).

 

[larsacious]

frv & JAE,

We do not know the exact circumstances of the OP's assebly area. I am merely suggesting that if they are not changing the occupancy, not increasing dead load, and there are no signs of failure in the existing structure than it seems overly conservative to recommend that the owner replaces or retrofit's the floor. I am only suggesting that there are situations where engineering jugement should also be used in lieu of code absolutes.

 

[Lion06]

I don't think there's any engineering judgment in a code required minimum live load. If it's not good for 100 psf and the code says the floor needs to be good for 100 psf, then you can't say 50 psf is good enough.

What if up to this point it's never seen over 60 psf (not good on paper for a 50 psf live load, but still performing ok because of safety factors), and tomorrow (after you write that letter) they do some kind of drill where everyone in the building has to meet in this room and it exceeds 100 psf.

I don't see any way around the code minimum and I know I would feel comfortable over-riding it.

 

[Lion06]

Sorry, I know I WOULDN'T feel comfortable

 

[BAretired]

According to the Alberta Building Code and the National Building Code of Canada, assembly areas under certain conditions may be designed for a Live Load of 2.4 kPa (50 psf). I attach Table 4.1.5.3 from the ABC-2006 which indicates uniformly distributed live loads on floors of various occupancies.

BA