applicability of live load factors on retail layout

[structineer]

I have been working on the structural capacity assessment of an existing two way flat slab with drop caps. They are looking at bringing in a retail tenant that sales alcohol. The rack loads for the displays are very high. They disclosed the exact rack layout for the tenant. I took the rack layout and equated it with an area load that produces similar moments and shears in the slabs. The equivalent area load is 180 psf. From testing, we found that the slab has 150 psf capacity for flexure, but only 100 psf capacity for punching shear. I have done research and it appears that the best route to take for increasing the punching shear capacity is installing steel collars around the columns.

To my question - live load factors being 1.6 are much higher than dead load factors of 1.2 due to dead loads being estimated much more accurately than live loads. Being that we are looking at a single client and a specific rack layout, would it be acceptable to use a load factor less than 1.6? This is a tricky one and I do not think I will go this route, but wanted to get others opinions.

 

[OHIOMatt]

Personally I would not do it. You mention that it is retail, so you cannot predict what will happen. Also, you seem to adress the load of the display cases, but I do not see what you are doing to adress occupant load. What happens when they fill the ailes with people for a wine tasting?

 

[structineer]

OHIOMatt,

I did consider the occupant loading in addition to the rack loading. If I were to reduce the live load factor on the racks, I would have to back out the occupant load and hit it with the 1.6 live load factor. You are right that I cannot predict what will happen, but if I provide a design based on a specific rack layout and qualify my design specifically for that layout instead of giving an allowable psf, would it be feasible to reduce the factor down to 1.2? I'm thinking no. It just leaves loose ends.

 

[OHIOMatt]

Okay, I must have mis-understood. the equivelant uniform load of 180 psf is applied just in the rack areas only. The remaining floor slab will be loaded to say 100 psf for first floor retail (or what ever the code requires in this case).

I would not use a lower factor for the rack areas, I just cannot see any code justification for it.

 

[JAE]

The only thing you might look at (and it probably wouldn't help all that much) is treat the actual rack framing as a dead load (assuming it is fixed/bolted to the floor) and
then treat the bottles of booze on those racks as live load.

I don't see the rack framing itself as being variable like a live load - while the change in bottles and product on those racks having a higher variability.

 

[dhengr]

Structineer:
And another issue to consider... Those wine bottles, etc. vibrate and clink like hell, and make everyone very nervous and annoyed, when foot traffic causes too much floor vibration, so check that out also. I got involved in this exact problem some years ago after a building had been converted to a liquor store. When you walked, heavily, down an isle you could see the bottles moving and hear them clinking.

Why don’t you talk to your local AHJ, and see if they wouldn’t give you some allowance for this type of loading. Fully loaded, and fixed racks would seem fairly exactly defined and worthy of some consideration along your line of thinking, for this exact (only this) load arrangement. Of course, flat pint bottles pack tighter than 1.75l bottles, etc., and one self fully loaded and another empty could give you some special bending moment problems, but not likely the worst shear problems. The displays at the end of isles are the most variable, two or three cases wide and four or five high, and changed regularly.

 

[msquared48]

Just state in the drawings that with the racks fully loaded, the customers and employees cannot be.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[structineer]

MM - this is a large liquor/beer/wine store, so if the racks were unloaded everyone in the town would be "loaded" lol

 

[structineer]

dhengr,

Good point on the vibration - I will look into it. I believe the dead load and dampening of the conc floor will minimize the vibration. We have 9" thick slabs. It definitely would be the first thing to pop in my mind if this were a slab on bar joist floor. They provided rack cut sheets, end cap cut sheets, etc., so I am using the maximum loads that the racks could hold. It does seem to me that with accurate knowledge of rack layout and capacities that one could reason to use a smaller safety factor.

 

[hokie66]

Don't reduce any live load factors, but the design would have been for uniform loading, not strip loading. In my experience, 100 PSF has been adequate for supermarkets and liquor stores, except in the "back of house" high storage areas. That said, if punching shear is marginal, you are right to want to reinforce. Curious to know how you determined the floor capacity by "testing".

 

[structineer]

hokie 66

We performed destructive testing to determine negative and positive, column and middle strip reinforcement sizes and depths. We also did ground penetrating radar to determine the amount of bars in the respective strips. Of course we also took cores for compressive strength tests. There was enough reinforcement present to obtain a capacity of 150 psf, however, the negative reinforcement was up to 5" below the top of the slab in certain areas, limiting the capacity to 100 psf. We are going to add flexural reinforcement to the top of the slab in the form of frp rods.

 

[hokie66]

Thanks for the explanation. Top steel being not high enough is unfortunately a common thing. The only way to ensure it is chaired properly and stays there is to provide full time inspection, which unfortunately does not / did not alway occur.