Slab on grade design 2

[cl220]

I am looking to design a slab on grade as a standard (non site specific) foundation. Does anyone have a good rule of thumb to use when little info is known?

 

[MiketheEngineer]

Just pour a bunch of concrete on the ground about 4'' thick and level it.

NO good answer to your question without better info esp concerning soils.

Around here we use perimeter footings with SOG. In parts of Texas I know they use post tensioned slabs.

Need more info

 

[cl220]

Thanks Mike I need the slab to withstand a large aircraft rolling over it. Normally I design for worst case soils such as clay.

 

[rowingengineer]

designing a slab for a large aircraft is not something you want to do by rule of thumb. Get a geo tech to do some bore hole, use an approprate design referance to design the slab on ground. Depending on how big the aircraft and what part of the airport this slab is going, you will need to get hte loadings for the wheels either at landing or full of people.

http://www.nceng.com.au/

"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected” Petroski 1992

 

[spats]

If you're doing non site specific design, I hope your company has good liability insurance... and be prepared to lose it.

 

[cl220]

I have the design loadings, but the foundation is meant to be used anywhere. Do you have a good reference in mind?

 

[MiketheEngineer]

Typically a 747 requires a minimum of 12''concrete but often going to something like 18'' but it depends on the soils.

Get some help on this one. 747's cost around $200 million PLUS!!!
There are worse things than clay!!

 

[cl220]

It is a foundation that has a 2% chance of a plane going onto it. It was previously sized up with a 6" slab. I was going to take a look at increasing the depth of the slab. Looking at previous designs we had from 30 years ago they just designed everything accounting for clay.

 

[SLTA]

wow - that made me laugh really hard! rule of thumb and oh, guess what, an airplane on it!

2% chance = if you don't design for it, it will DEFINITELY go there. Take Mike's advice and get some help, using super-thick slabs.

 

[MiketheEngineer]

They use more than 6'' for simple interstate highways. You really do need to get somebody who has experience in this area and a soils test.

Also - is the plane just going to taxi on it OR land on it. TWO different loading conditions.

As slta said - if there is a 2% chance - I can almost guarantee you it will happen next week.!!!

 

[hokie66]

Mike,
"Just taxiing" is actually a more severe loading condition then landing. Ends of runways and taxiways see more load than the part where there is uplift on the wings.

 

[cl220]

Thank you guys for your responses. This foundation would be located off of the runway not on a surface that sees traffic. I have been preaching soil tests. Problem is I work with mainly mechanical engineers.

 

[hokie66]

Is it a slab on grade or a foundation? You have called it both. If a foundation, what does it support? If those mechanical engineers won't listen to you, let them do it themselves.

 

[mijowe]

just curious, what wheel load are you designing for?

 

[Ron]

First of all, it sounds like you want to design a pavement, not just a slab on grade. Further, it sounds like it might be an apron in front of a hangar or perhaps even a hanger floor slab.

If it is a pavement, design it as such. You will need to know wheel loadings and load frequency. For concrete you generally design so that the tensile stress at the bottom of the slab does not exceed 50% of the modulus of rupture of the concrete...then you don't have to worry too much about the number of load repetitions.


If it is a hangar slab, design everything but the integral footings as a pavement and then design the integral footings for the building loads...then tie the two together, considering lateral thrust resistance as well.

The US Army Corps of Engineers has some pretty good info on concrete pavement design for aircraft.

 

[MiketheEngineer]

hokie66 - I tend to agree with you BUT I have been on a few very hard "long" landings where the pilot simply "planted" the plane about half way down the runway and went to full engine reverse, spoilers and lots of brakes!! I could almost hear the runway groan a few times.

As a matter of fact. Hitler had the Autobahn built with very thick roadways that often were two or more miles dead straight. Why - so they could use them as runways if necessary,

The USAF even explored that possibility after WW2. My Dad was there and flying F-86s.

You could surely land a lot of planes on I-70 in Kansas!!

 

[Ron]

You can land a 747 on most any highway in the US....you just can't do it repeatedly.

 

[msquared48]

I-10 is specifically for Warthog and Area 51 traffic.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[keenaseng]

Hokie66 - You describe "just taxiing" as a more severe case of loading than 'landing'... Is that official or just hearsay?

I am not familiar with the design of pavement slabs for aircraft wheel loadings, so I am just thinking it through... I agree that air speed over the wing generates uplift making the plane lighter - by this I can see how a plane 'taking off' would be less load than taxiing.
However considering a plane landing we are talking about initial dynamic impact, then under full brake with aileron + elevators creating down force to bring the plane to rest in short distance - Hard to imagine this loading condition is going to be less that a near stationary plane?
Thinking about a mass opposing gravity vs working with gravity, I would have thought that takeoff<taxi<landing?

 

[Ron]

In general, hokie66 is correct about the taxiing loads being higher. The initial impact of an aircraft on landing is often less than its taxiing load. Further, the taxi loading often achieves an almost resonant quality...depending on the wheel pattern and the joint spacing.

Due to the elastic response of a rigid pavement, the forward speed of the aircraft lessens the load that the bottom of the pavement feels.

The next time you fly out, look at the pavement and you'll probably see more evidence of maintenance on the taxiways than the runways.