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MikeE55 (Structural) (OP)
26 Apr 05 14:07
I am designing a concrete slab supported by steel beams for live load and a forklift.  Is it acceptable to reduce the live load on a beam when looking at the forklift load?  Also, should the forklift load be increased for impact?  I am trying to design the steel beams by ASD.  AISC ASD Manual A4.2 says live load should be increased to account for impact, but only crane systems are mentioned.  My thinking on reducing the live load is that the full live load can't be applied if there is an aisle for the forklift.  Anyone have any thoughts, or a design guide reference?
whyun (Structural)
26 Apr 05 14:56
Design for the worse of (A) dead load plus full live load without forklift load (B) dead load plus worst case forklift load without live load.

Third option may be, (C) dead load plus forklift at midspan of member plus full live load minus live load at forklift footprint (like you mentioned).

I am not aware of any "code" that has specific requirements.

Also, consider how many forklift may potentially load up a single member (it is not always one, depends on the member length, tributary etc).  Finally, do not design "too close".

Good luck.

jike (Structural)
26 Apr 05 15:09
It may be possible for two fork trucks to be passing in an aisleway over the top of the beam.
JAE (Structural)
26 Apr 05 16:06
Agree with whyun and jike -

Also - No impact for forklifts. - they don't go fast enough to "hit" the bumps and cause a surge in LL.

Be sure to look out for the deflections in your beams relative to each other as one is loaded with the forklift and the other is not - the slab will have to allow for the resulting flex and you could end up with a huge crack problem....this from my own experience on a loading dock slab supported by composite beams and a fairly thin concrete slab.  

With full LL on one beam compared to no LL on the adjacent beams, you get some very odd stress flows depending on where the forklift is located along the span...this initiates cracks in all sorts of directions.

We have tried to drastically limit the deflections and even added a custom combination of poly and steel fibers in the mix to control cracking.

So keep an eye on your tension stresses compared with the cracking stress.
jike (Structural)
26 Apr 05 17:06
The proper design of reinforcing steel in one way structural slabs for concentrated loads (such as fork trucks) is very important.

BOTTOM MAIN STEEL  One needs to look at the truck positioned both parallel and perpendicular to the span and the additional moment caused by overlapping stresses from the adjacent wheel loads.

BOTTOM DISTRIBUTION STEEL (at right angles to the main steel). This is significantly larger than the normal temperature steel.

TOP STEEL   The most critical location is when the wheel load is right next to a support beam. You must also design for the condition when a truck at mid-span causes negative flexure in the adjacent spans.

If the building is designed for a 50 year life, the Owner may also want to design for a larger truck than he currently uses so the building floor slab does not limit his future operation.
jheidt2543 (Civil/Environmental)
26 Apr 05 22:52
One thing you do have to consider is the momentum load from the loaded forktruck starting up and stopping quickly.  That horizontal load can get the structure supporting the floor moving horizontally a lot more than you might think!  Take a good look at lateral bracing.
mrMikee (Structural)
26 Apr 05 23:35
Another thing to remember about forklifts is that when they get overloaded the weight distribution shifts towards the front wheels, depending on the style of the forklift of course.  In other words the rear wheels get lighter and the front wheels take more load.  This could significantly alter your assumptions about the loads as they move across the support beams.  The comment in another post about being careful with loading on a single beam is good advice.  I'd also be concerned about point loads on the concrete slab due to this problem.
SacreBleu (Structural)
27 Apr 05 10:32
As an "aside", this is a classic example of where NOT to design to "R=1.03, say OK".
As Whyun suggested, increase your estimated load liberally.
tincan (Civil/Environmental)
27 Apr 05 19:32
Check whether solid rubber tires or pneumatic.  Have had considerable trouble with the small solid rubber tires because of the smaller footprint.

Best, Tincan
jike (Structural)
28 Apr 05 10:21
Something that has not been mentioned but is quite important....

It is NOT advisable to count on composite metal deck for bottom reinforcing with forktruck traffic. It is my understanding that the deck will eventually loose its bond with the concrete due to fatigue. Also, the slab thickness would most likely be inadequate for lateral distribution.

We typically use minimum slab thickness of 6" for lighter trucks and possibly 7 or 8" for heavier trucks. Metal form deck may be used for forming but this would be added to the required thicknes. We always provide 2 layers of reinforcing steel.
MikeE55 (Structural) (OP)
28 Apr 05 13:56
Thanks, everyone, for your very useful advice and feedback.  I  am proceeding with confidence.  Jike, thanks for the idea to consider two forklifts passing each other.  I had not thought of that one.  I agree with you that the slab must have serious reinforcing.  
AbelGus (Industrial)
28 Apr 05 22:07
In our fabrication shop we often use two fork lifts facing one another to handle a complicated steel shapes.  Also forklifts pass by one another carrying loads.  Not to mention that we also rearrange our storage racks, so aisle configurations change.  You cannot count on a guaranteed path of usage, so go for the worst case.
apsix (Structural)
29 Apr 05 3:01
I have another question on the same subject.
I usually find that slab thickness is governed by shear when dealing with high individual wheel loads, but maybe I'm too conservative.
What effective width is appropriate when calculating the shear capacity? It will obviously be the footprint width (or length) plus some load spread into the adjacent slab.
jike (Structural)
29 Apr 05 11:28
It is my understanding that AASTHO assumes that if the slab, subject to concentrated loads, is adequate for bending then it is "assumed" adequate for shear. Obviously, the tire contact area for buses, trucks and cars on bridges is larger than that for forklifts in buildings.

There has been little research into this shear stresses of concentrated loads. The only reference that I have been able to find is that the effective width (in feet), for a single concentrated load away from a free edge, for shear is equal to 5 times the square root of the thickness of the slab (in feet). Don't forget to add the effects of overlapping stresses and shear stresses due to other dead and live loadings.

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