Averaging Loads 2

[DaveAtkins]

My client wants to use the second floor of a former jail facility (which was designed for a 40 psf live load) for storage of Banker's Boxes, stacked seven high in a rack system (live load = 146 psf at the boxes).

I produced a report which shows how to lay out the boxes, with wide aisles in between, so as not to exceed the 40 psf live load. The Building Inspector in the city where the facility is located will not accept this, stating that our State's Building Code requires 125 psf minimum uniform live load for storage.

Any ideas? The client does not want to upgrade the existing floor.

DaveAtkins

 

[jechols]

There is no such thing as over-designed.

 

[miecz]

Others have already asked but I'm curious as well: What live load are you using between the boxes?

 

[DaveAtkins]

I used zero live load between the boxes. This room is not a path of egress, and no other load is intended between the boxes. Only one or two people will be in this room at any given time.

DaveAtkins

 

[kslee1000]

You need to put up physical boundaries/barriers in all three dimensions to achieve your goal, unless the celing is the limit for height.

A light-duty rack system could be the choice.

 

[RareBugTX]

The Building Inspector is right, in my opinion. The current or future owner, will locate the furniture around or boxes for the matter, in the most critical arrangement. I think the floor system should be upgraded for the new code required floor live load.

 

[apsix]

"I used zero live load between the boxes."
Sorry, I couldn't endorse that approach.

 

[hokie66]

Just one more vote for the Building Inspector. As csd72 said earlier, I think you would have to justify the floor by calculation. If it was a jail, it is quite likely that the floor is actually stronger than the published live load. The ones I did are. Load testing is another possibility.

 

[UcfSE]

concretemasonry gave some very sound advice. I think it's more than worth listening to.

 

[DonPhillips]

One more vote for the building inspector. A change in use is a change in use and you have to either upgrade the structure to meet code or adjudicate it for a fixed layout.

The fire marshall usually will keep an eye on the space doing annual inspections and if the layout changes, can let the building inspector know, who can order the owner to correct the violation.

I would ask the building official for an adjudication order. Typically, this means submitted for plan review so I would discuss this plan before applying.

Don Phillips

http://worthingtonengineering.com

 

[youngstructural]

Quite the conversation; I have to say that I'm on the side of the Building Official, to a point. I do think that the floor could be re-purposed for a specific storage use, however you would have to somehow ensure that it would retain that use and no other. I know that specific load situations are often permitted back home (Ontario, Canada) with the posting of a "permanent plaque in both official languages". Basically a sign that stipulates the permitted use, and proscribes any deviation without the specific written permission of a licensed engineer.

Also I think that zero live load is a bad idea, should never be permissible in any situation, and would not be at all wise. Technically this would mean that will all your boxes in place, you can never retreive them or go in to look at them. We cannot simply eat our live load and material resistance factors: That is not what they are intended for, and they are already serving their purpose by being factored into the solution. Bear in mind that our loadings are purely static simplifications of a much more complicated reality. The fact that they are higher than normally needed is partly due to our "hiding" dynamic effects within the static specified load. I would encourage you to consider a 15psf live load between the boxes, and factor it up with a dynamic load factor of 1.25 as per designing a crane rail. That would be realistic, in my opinion, for a light duty load involving a couple of people accessing, adding to, and working with the stored files. After all, are you really sure no one will ever tip over one of your lines of seven high bankers boxes? Or chase their child around them?

One last point: Two very eminent and respectable engineers have both, independantly, in two seperate countries, given me the following advice (paraphrasing, of course):

"The client has a problem. We have specialist knowledge which allows us to understand and potentially help aleviate the client's problem. We do not have a problem, and must not act as if we did."

to which the Canadian engineer added:

"We have a duty of care first and foremost to the public, not our client. We have a duty to discharge our services with loyalty towards the client, but life safety must govern our recommendations to the client".

and the Kiwi engineer added:

"It's difficult to say No. Particularly for professional problem solvers like us, itching to not let the problem beat us. Don't be affraid to say No when it's the right answer."

What the client is asking might not be possible. That's life, and no matter how you cut it, its not your problem.

Good luck, and keep us up to date!
Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[BEFORT]

You guys are all way too optomistic. My experience says there,s no way your going to change the building officials mind. Plus, his arguements is about has good as yours.

Someone mentioned, if you have enough information, checking the capacity of the system. I've found that some structures will surprisingly exceed what is expected. If you do not have the drawings them maybe you could determine the specifics with proper testing.

 

[DonPhillips]

It is not changing the building official's mind, the code is quite clear and the building official does not have the authority to grant variances. That right is given to the appeal board. You have to adjudicate it to the board to deviate from the code and if you have the fire marshall and the building official not in opposition to a variance, most boards would grant the variance.

This kind of variance happens a lot when a warehouse is converted to an assembly use, and you have to adjudicate occupancy to 299 to avoid a whole lot of systems and requirements that cost money, and the tenant will only have 50 to 100 people. Occupancy reductions like this must be adjudicated and the fire marshall enforces the posting of that occupancy. The exception is toilet rooms, where the building official can accept a lower occupancy to reduce fixture counts. When houses are converted to some sort of commerical use, occupancy reductions are a lot cheaper than structural modifications of a floor system covered in finishes.

This is coming from a former building official.

Don Phillips

http://worthingtonengineering.com

 

[Tomfh]

What sort of floor is it? What is the actual psf capacity?

 

[swivel63]

the official is right. we have to design for what we don't know, because it'll be our behinds that sign and seal the drawings knowing the intent behind the change. who are the lawyers going to go after if something happens?


don't existing structure (higher phi values for concrete design, coupled with exactly strength and not design strength) apply?

 

[oldrunner]

It's not the building official call to change the code. He's absolutely correct. The UBC also requires a minimum of 25 percent of the storage live load will be included in the seismic calculations.

 

[civilperson]

A zero calculation for the aisle is a future lawsuit. Dollies and pallets will be placed there to load and unload the shelves. Try adding some columns under the floor to share the load with the lower level or take the new columns to the basement and build footings.

 

[DaveAtkins]

Hi, all,

I talked to a representative from our State Department of Commerce (they have responsibility for Plan Reviews), who said pretty much what you are all saying (but he was more conservative). He said he could not accept zero live load between the boxes, but could accept 50 psf. When he was told that this is more than what the floor was designed for, he sympathized, but said a floor designed for 40 psf should not be used for storage.

As for finding the existing structure's actual capacity, I was not given enough fee to do that (it is a post tensioned structure, and I do not have shop drawings). I told the Owner that I doubt it was overdesigned to the point that it could carry 146 psf at the boxes, and 50 psf between the boxes.

DaveAtkins

 

[csd72]

Dave,

Youngstructural makes some very good points but I must say that I make a point of never saying no to a client.

In our profession almost anything is possible it is usually the cost that is prohibitive.

I would usually phrase my answer as "The only way that I see this to work is by X,Y or Z." and to cover yourself mention that you expect it to be expensive.

I also think that the building official is, in a way, doing you a favor by giving you an out. If anything went wrong with this floor lawyers would have a field day with the grey areas.

 

[NerdSE]

PT slabs are usually designed for serviceability. It is likely if you can perform a Yield line or some other upper bound analysis, you will discover a significant amount of reserve capacity.

However, what you are missing is the significant amount of creep and long term deflection that will accompany this load. Your floor system will crack and deform to the point where the doors will start sticking. Post tensioned concrete is highly susceptible to creep due to the higher compressive stress and thin sections and with the higher sustained load you will have big problems. If this is a direct applied texture ceiling such as hotel construction, the lower floor tenant will not be happy with you at all.

You can bet that your buddy that is trying to talk you into this has all kinds of contractual protections and will be handing your business card to the lawyers when they arrive. He/She may even hire them! They will save on the initial construction because of your recommendations and then ask you to pay to have it done correctly. Free upgrade, what a deal!

 

[youngstructural]

Watch out for upper bound analysis on slabs: Johanssen's Yield Line Theory, etc, all miss out on even the most rudimentary SLS considerations. In fairness Johanssen did propose adaptions to Yield Line that allow for a GROSS estimate of the simple deflections, however I think that NerdSE has a great point: This is a bad situation, and I think that your final advise to the client was perfect.

Let us know what the client decides to do,
I'm very curious!

Thanks,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...