Proper Impact Factor for Composite Floor 3

[olsont]

I'm currently designing an elevated composite concrete slab with metal deck floor system. The composite slab is supported with composite steel beams and columns. The floor system is accessed using a ramp from an existing floor. I've been told that the proper impact factor for the overall floor system should be 1.3 and the proper impact factor for where the ramp and the new floor come together should be 2.0. Are these impact factors correct? if not what should they be, or where can I find this information?

 

[jike]

Tells us more about the use of this floor. Is it subject to fork trucks? cars? What is causing the impact?

Floors with composite deck are generally not recommended for use with moving wheel loads.

 

[cntw1953]

1.3 impact factor is per AASHTO if it hasn't changed. Check with the person for reference on 2.0 factor and let us know the source.

 

[JAE]

I don't think AASHTO has any bearing on a "floor". Maybe a good reference but certainly not an applicable code document.

 

[cntw1953]

For vehicular traffic on a suspended floor system, unless there is specific data for the impact, the factor/provisions from AASHTO wouldn't be too far off the point, although the floor design still have to satisfy the governing building code. I would consider the floor is of mixed use, and consult both (AASHTO & Building) code.

 

[cntw1953]

Correction:

AAHTO max impact factor was 1.25 (?), 1.3 was specified by AISC for crane load.

 

[olsont]

The floor is being used for storage and there will be fork truck loads on it.

 

[cntw1953]

Then you won't need AASHTO. I think someone can point out the direction to obtain a copy of design loads for the forklift, try talk to the warehouse owner first.

 

[jike]

We have always used an impact of 1.25 for fork trucks on structural floors. The critical load cases are often two trucks passing. I suggest using Westergaard procedure for combining wheel loads and for calculating distribution steel.

I would caution you in using a composite metal deck for a slab with fork trucks. Research has shown that the composite deck will debond from the concrete under repetitive concentrated wheel loads.

You should be using a slab with 2 layers of reinforcing steel. You can use metal deck or plywood to form the bottom of the slab.

 

[olsont]

I'm only using the metal deck as a form. I contacted a metal deck supplier and they recommended using the SDI Composite Deck Design Handbook (1997). This manual has a forktruck loading example in it....that's what I've based my design on.

 

[jike]

I have looked at the SDI procedure and it leads one to believe that you can use composite metal deck as the replacement for bottom steel for fork trucks and wire mesh for distribution steel. The procedure seriously underestimates the required amount of distribution steel.

Further, it is based upon one fork truck, assumed to not be repetitive loading. How many fork trucks are not repetitive loading? How many facilities have more than one fork truck operating on the same floor, either now or in the future?

Be cautious of the following statements in their procedure: "Because the very nature of these types of problems can require the designer to make individual decisions as to how some aspects of the problem will be treated, it is not intended to negate the experience of the designer. The designer may have a method which has worked well in the past and as long as the minimums are met there is no effort by the SDI to make a change. This example problem covers different loading cases in an effort to cover most of the combinations that can occur. There may be cases that are more severe. In any case the designer must review the job’s particular set of circumstances and make the judgments necessary for the design."

"If the designer considers the truck loading to be repetitive so the slab will be subjected to many cycles of loading, it is suggested that studs be required and that a thicker slab be investigated so the resistance exceeds the demand by a greater margin.

 

[JLNJ]

+1 to what jike said.

Put some real rebar in for fork truck loads.

 

[olsont]

I'm planning on using a 3" x 18GA. metal deck with 8" total slab thickness for the slab. The slab has #5 bars spaced 12" O.C. top and bottom, and these bars run in the same direction as the metal deck. Also I have #4 bars spaced 12" O.C. for the transverse steel.

 

[JAE]

jike, I wonder if there are composite metal deck-slab fatigue studies out there? The primary failure mode of these decks is shear-bond loss between deck and concrete (horizontal shear) and with repetitive loading perhaps the dimples in the deck just aren't enough to ensure long term shear bond.

 

[jike]

JAE:

Yes, shear bond loss is the problem and there was research done over 30 years ago on this very problem where it was observed that the deck will debond under repetitive wheel loading (fatigue). I have read a couple of these reports! If I recall correctly, the composite decks with wire welded to them and the keystone deck performed better.

SDI seems to give the impression thruout its example that you can replace bottom rebar with composite deck and use wire reinforcing for distribution steel for fork truck loading but then includes a statement at the end recommending studs, thicker slab and a greater margin of safety when you have repetitive wheel loads.

olsont:

I am glad you are using 2 layers of rebar! That is what you have to do if you want the slab to perform properly over the long haul!

 

[olsont]

jike:
It sounds like you have alot of experience with elevated slab design. Have you ever designed one using composite metal deck and concrete for the floor system?

 

[jike]

olsont:

Yes, I have designed many, many buildings since 1971.