Moving, rack, and equipment loads on elevated floors 1

[jmy61]

I have been asked by a client to verify the adequacy of the second level floor in their manufacturing facility to support equipment, storage rack, and fork truck loads. The facility was initially constructed in the 1950's and multiple additions were added through the 1980's.

The original structural design drawings are available for most of the floor areas I am to analyze. According to those drawings, the second level floor structure varies throughout the facility, and different areas were designed for various uniformly distributed live loads. The original structural design drawings make no mention of the floors being designed for fork trucks or other moving loads, and given the relatively light live loads noted on the drawings for some second floor areas, I am certain fork truck loads were not intended for those areas. With that said, Client has been driving fork trucks on most of the second floor for many years.

Generally speaking the second level floor structure consists of a formed (i.e. no steel deck)concrete slab spanning between composite steel beams spanning between steel girders. In some areas of the facility the steel girders are composite while in others the girders are not composite. The girders are supported by steel columns.

Client wants the freedom to place the equipment and racks anywhere on the floor, and the fork trucks must be able to access the equipment and racks. Consequently, I am faced with what seems like countless possible combinations of equipment, rack, and fork truck locations in a given floor area.

Does anyone have a recommendation as to an engineering software package that can accommodate the moving loads of the fork trucks as well as the stationary loads of the racks and equipment? In lieu of or in addition to software, are any design guides available?

Thanks in advance for any input you can provide.

 

[BAretired]

What a nasty problem! Can your client at least give you the wheel loads of the forklift and the maximum concentrated load of the racks and spacing of the legs?

You will probably have to assess each area in turn with a few trial arrangements. You can hopefully discuss with the client the most likely arrangement of racks and equipment.

In the end, you may be able to make some recommendations but you will not be able to anticipate all combinations of load patterns. Unlimited freedom to move things around wherever your client wants is an unrealistic expectation unless the floors are considerably over-designed.


BA

 

[dhengr]

Jmy61:
I agree with BA, that is one hell of a messy problem. It seems that the client has been doing something stupid for years and getting away with it, by the grace of God. And, now he wants you to bless it, sign off on it and be responsible/liable if anything goes wrong. I too, would look at it an area at a time, literally marked off as ‘area A,’ ‘area B,’ etc. on a key plan, which is part of your report and recommendations, and insist on loading signage, etc. Even with some of the original plans in hand, I would want to verify some of the steel strengths, conc. strengths, rebar locations and existence, etc. etc. Maybe not all, but some statistical amount, to see that they followed the plans, and you have something to hang your hat on. And, the client better be willing to pay for this and your investigative time, or all bets are off. Tell him to get the local bishop to bless the place, if that’s what he wants, they are in the blessing business; engineers can’t work that, they can be held liable in court, if something they can’t see causes things to go south.

What is the vert. movement/ matr’l. handling system btwn. the two levels? Can you have several of these, at different locations, at different fork lift and sq.ft. fl. cap’y. levels so that an area which can take a larger fl. load which relates to a heavier racking system and handling equip. can be directly accessed. While a lighter area has a lighter limit on its elevator access. I don’t believe you can offer an infinite racking arrangement on a relatively light cap’y. existing structure, and ever finish the project. Your going to have to look at each framed area and pick a couple optimal rack locations and travel pattterns and finally assign them a cap’y. based on what the structure will take. Then if they want to change that, they have to come back for a re-strudy.

 

[wannabeSE]

Countless iterations of loads shouldn't be required. Just think about the perfect storm of loading. That seems to be what the owner wants.

If the equipment and rack weights are known, it should be pretty easy to estimate a conservative uniform load required. Just assume the area is overcrowded with the heaviest racks or equipment. Then assume the fork lift sits in the worst location on each span (this might be converted to an equivalent uniform load). Use this loading to check the structure with pattern (skip) loading like other live loads. If the equivalent load is less than 80 or 100 psf, exiting or other occupant live loads may need to be considered. If it is a high seismic area and a lateral analysis must be performed, some engineering judgment is needed to determine what percentage should be include in the seismic mass.

The report can lists the areas that meet the owners requirements (of placing the storage or equipment anywhere), and the uniform live (superimposed, collateral) load capacity (reduced to account for the fork lift).

 

[jike]

You will first have to identify areas with similar spans, continuity and reinforcement. Then evaluate them for concentrated loads from fork trucks. Don't forget to look at overlapping stresses from adjacent wheel loads. Also passing trucks (one loaded and one unloaded)in two way aisles. Evaluate main reinforcement as well as distribution steel. I recommend using the Westergaard Method from his report to the Bureau of Public Roads in 1930's. I know of no software for this. I use generic trucks unless the Owner wants you to evaluate for a specific truck manufacturer. Include 25% impact.

Racks and equipment are much more difficult because of the wide range of sizes, arrangements and spacings. Overlapping stresses of concentrated loads need to be a consideration. Heavier equipment loads can be accommodated over smaller areas because of the tendency to spread out to the unloaded adjacent areas. Include a minimum of 50 psf in unloaded areas for personnel. Asked for more details.......

 

[Brad805]

Have you confirmed the drawings are representative of what you are analyzing? Old building drawings are notorious for lacking information and over the years old buildings are typically renovated countless times. This aspect tends to be a big challenge in such an investigation.

Assuming the drawings are reasonably close to what you have I suggest you do some rudimentary hand calculations based on some assumptions on possible load cases. I would concentrate on a few of the typical slab and beam spans that peak my interest. After that, you need to sit the owners down and try to bring them into reality. Most of the time once forced to consider the future building use owners will figure out a reasonable work flow, and use for the space. This will allow you to complete the evaluation in a methodical manner. Right now it seems you have the classic owner that does not want to do any of the work and figures that wonderful engineer guy can figure it all out without any guidance from them. We are not mind readers, they have to do some work. They need to give you some guidance as to the potential forklifts they might use, the racking options they see practical for their business, and some idea what type of equipment they may envision in the future. From what you describe your scope of works is very poorly defined, and that leads to scope creep that your bosses might not like.

 

[jmy61]

Thanks to each of you for your reply. Because my initial post was getting so long, I minimized the extent of the details I provided. I do have some of the information you mentioned in your replies.

In our initial meeting I told the Owner that at a minimum I will have to visually asses the existing conditions and compare the current structure to the requirements of the drawings. I also told the Owner I must have the fork truck make and model in order to calculate the actual wheel loads. Additionally I told the Owner he would have to supply me with the necessary rack information and equipment information in order for me to calculate those respective point, line, and uniformly distributed floor loads.
Owner says he will provide me the needed information. Assuming he does, I should have be able to develop reasonably accurate floor loads. I always apply at least 50 psf live load across the remaining floor area.

To determine an allowable load rating for each floor area, my plan has been to determine what loads placed in what locations have the worst affect on a floor area; however, even knowing the equipment, fork truck, and rack loads, it seems a large number of combinations will have to be checked to determine "worst case", and the worst case combination for flexure will likely be different from that for shear.
This process will have to be repeated for each of the different floor structures so I am looking a quite an undertaking.

I was hopeful there may be software out there that can be used for such an analysis, but sounds like I am out of luck. On a similar, albeit much smaller, project several years ago I used the Westergaard Method, and it looks like I will be doing so again.

Thanks again for your input. If you have any more ideas or suggestions, I am interested!

 

[BAretired]

The client may not know the magnitude of load on the legs of the rack. If he is storing metal parts, it is very easy to overload the rack over time. Even paper storage can be very heavy. There should be a maximum weight of material to be placed in any compartment of the storage rack, but even that can be abused over time.

As far as "worst case" arrangement is concerned, there are some arrangements which would seem to make sense. For example, racks and aisles between racks will likely be placed parallel to walls, so it doesn't seem necessary to consider racks placed at odd angles to walls.

BA