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Averaging Loads
2

Averaging Loads

Averaging Loads

(OP)
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

RE: Averaging Loads

I think he's clearly wrong, but now that he's judged it that way, I don't think you have a choice but either upgrade the floor or tell the client that it can't be done.

Change gravitational acceleration from 386 in/sec^2 to 386*40/125, LOL?

RE: Averaging Loads

The only other option is to justify it by calcualtions.

Either:

1. If you have enough information check that the existing structure works under the loads.

or

2. Do a calculation justifying that the moment is no more than 5% above the design moment (There is a 5% increase allowed somewhere but I cant remember).

RE: Averaging Loads

I don't have a solution for you, Dave, but this is why architects will go to great lengths to label the room anything but "storage".

RE: Averaging Loads

I agree with the Building Inspector.  The design load is based on the occupancy classification of the floor.  If the code requires 125 p.s.f. for storage then that is what the floor should be designed for.

The problem the Building Official is trying to avoid is future change in what is stored and how it is stored.  The current owner, if he recieved approval to follow your shelving plan, at some time in the future, he could narrow up the asiles and add more storage.

After all structural engineers and building inspectors are overly conservative.  We always determine the load and double it don't we.

A more important consideration is a future owner who decides that since the room is a storage area that he can load it up to 125 psf because that is what is required by the state code.

An additional comment about engineers;  I have been told that after hurricane Katrina, that there was a new respect for the engineers that had over designed all the buildings built before the hurricane.

RE: Averaging Loads

along the lines of jike's post ... maybe define the room as "filing".  for my 2c worth, it looks like a reasonable analysis has been completed, creating a reasonable floor plan for comply with the building's limitations

RE: Averaging Loads

The files are only temporary pieces of furniture, but you know it is going to be used as a storage room, so you are responsible for the future use as a storage room. Since your plan was submitted and rejected by the city, there is a record that you were aware of the new proposed use.

I suspect that your client knew of the possible problem from the change of use and wanted to find someone to sign off and minimize or eliminate his liablity.

Dick

RE: Averaging Loads

You didn't state the building code here - but in the IBC, section 1607.2 it states: "For occupancies or uses not designated in Table 1607.1, the live load shall be determined in accordance with a method approved by the bulding official."

You could probably successfully convince the building official that your bankers boxes, stacked as required by you (and the layout permanently posted on the wall) is NOT a use designated by the table - i.e. the 125 psf light storage isn't what you are technically doing.

However, you would then need to convince the official that your layout, and the resulting 40 psf average live load, is OK.

RE: Averaging Loads

I think the building inspector is absolutely spot on.  Neither you nor the inspector can control how this 40 psf (2 kPa) floor is used by this owner and, perhaps more importantly, future owners.

It is not an impropbable sceanrio that a future tennant or owner will increase the amount of storage on that floor 'assumming'  it it fit for that type of loading, i.e. 125 psf (6 kPa).

RE: Averaging Loads

How did you get the 40psf average load? If your bankers boxes are more than 40psf and you apply the 40psf between them then you technically get more than 40psf!

RE: Averaging Loads

I think you guys are missing what DaveAtkins is doing here.

I would agree that the building official is interpreting the code the right way for this application, but the previous posts are assuming the space is being designated as "storage".  

Consider a typical office building where the original design for the second floor is 50psf (office space).  A future client wants to use a portion of the room for storing boxes.  Does this mean that the original design should have been for 125psf?  We can't assume what future tenants/clients will use the space for.  DaveAtkin's client is using this room/space for storage, but may not be reclassifying the space.  A future tenant could use it for anything.

For this application, the current tenant should be able to use this space to store the boxes with an approved plan from an engineer while satisfying the building inspector.  If a new tenant moves in, he/she has a responsibility to verify the space has the capacity to be used as storage or whatever use he/she intends.

I think DaveAtkins is on the right track as long as the client understands that deviation from his layout plan could spell disaster for the floor framing.  With that said DaveAtkins needs to weigh his liability if the plan isn't followed to the T.

RE: Averaging Loads

I think you can load with the owner request load ammount. I can bring 2 examples. In europe, I designed s concrete slab for normal residential use, and the owner loaded it with skids od cement sacks, 3-4 times the normal load.  Nothing happend, just a small streach of bottom slab. So this means that the reinforcemt has a lot of reserve.
Second case was in Canada, where the engineering firm loaded a 100 years old 5 store structure, at one of the floors with 15 rows of concrete blocks, ( equal to approx 5 times a normal live load), and nothing happend, just a small diflection of 5 milimeters.
I suggest that you go and test the slab with the required loads, keep it loaded for few days, and come to a solution.
This is my best advice.
Thanks

RE: Averaging Loads

i think the building owner is right.

RE: Averaging Loads

official, i mean.

LOL

RE: Averaging Loads

swivel63, I was with you, before your second post that is, LOL.  

I think it's too much to expect engineers to design for "what somebody might do to the structure in the future."  

DaveAtkins was asked to check out a well defined current situation.  If the owner wants to add a bunch of other stuff 10 years down the road, it's his responsibility to get it checked out at that time.  Like rgerk typed, an owner could always change the loading without asking anybody.

RE: Averaging Loads

Maybe this will work out - design, or ask the owner to provide, permanent (tied down to the floor) storage containers/shelves that can limit the quantity of the boxes to be stored in the designate areas according to your layout. The owner will then be the sole responsible party if the plan is changed, which can be checked easily. Do post the layout plan with load limits clearly spelled out on the walls.

RE: Averaging Loads

(OP)
Thanks for all the responses!

I can understand both sides of the argument.  I kind of like what kslee1000 is suggesting, except...what if I made the Owner paint lines on the floor, showing where the aisles MUST be?  I wonder if the Building Official would accept that.

DaveAtkins

RE: Averaging Loads

Back to the original post, what live load is used on the wide aisles between the stacks of boxes?  Is this undercutting any code-prescribed amount?

RE: Averaging Loads

"...what if I made the Owner paint lines on the floor, showing where the aisles MUST be?  I wonder if the Building Official would accept that."

Can't hurt to ask!  That seems to be a sensible compromise to me.

RE: Averaging Loads

Ugh! Sticky situation

Sounds like the Building Official is being a stickler.  If it was called out as "Storage" you got lucky w/ only 125psf for light storage in lieu of 250psf for "Storage"

Stay away from calling out aisles.
Aisle = Corridor = 80psf  

I'd go for laying out where the racks go and label it as "Files" on the dwg.  Show the room as a "File Room".
Per IBC, "Files and Computer rooms shall be designed for heavier loads based on the anticipated occupancy."

Which it seems you've done.


RE: Averaging Loads

There is no such thing as over-designed. smile

RE: Averaging Loads

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

RE: Averaging Loads

(OP)
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

RE: Averaging Loads

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.

RE: Averaging Loads

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.

RE: Averaging Loads

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

RE: Averaging Loads

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.  

RE: Averaging Loads

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

RE: Averaging Loads

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

RE: Averaging Loads

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...

RE: Averaging Loads

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.

RE: Averaging Loads

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

RE: Averaging Loads

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

RE: Averaging Loads

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?  

RE: Averaging Loads

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.   

RE: Averaging Loads

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.

RE: Averaging Loads

(OP)
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

RE: Averaging Loads

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.

RE: Averaging Loads

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!

RE: Averaging Loads

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...

RE: Averaging Loads


The most critical issue with increasing load on a PT floor is typically concern of punching shear, which could determine an upper bound for the increase in the slab capacity fairly quickly. Look at that value and work backwards.

Regards,

AUCE98

RE: Averaging Loads

Dave,
I designed a similar situation in a telecommunications facility and heavy battery loads.  I was able to locate the loads along a bearing wall and develop a bracket system that attached the wall and cantilevered out at the base to pick up the batteries.  I am not sure if you can live with this type of layout, but we were able to make it work for our project.  Best of luck.

p.s.  The building official has a valid point.  It is difficult enough to argue with a building official even when they are completely wrong.  

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