Loading Pad Design 1

[kxa]

Does anybody know of a good reference material for the deisn of a concrete loading pad that can handle the weight of a fully loaded tractor trailer. What would the design load be?

I am considering an 8" thick or perhaps a 9" thick concrete slab and about 60' to 70' long but need to design the reinforcements. Also, what kind of joint material to use in between the slabs.

Thanks,

 

[SlideRuleEra]

US Army "Pavement Design for Roads Streets, and Open Storage Areas, Elastic Layered Method"

http://www.army.mil/usapa/eng/DR_pubs/dr_a/pdf/tm5_822_13.pdf

The soil supporting the concrete is more important than the concrete itself. For good soil, properly prepared, 6" of concrete with no reinforcement is more than enough. For poor soils, your proposal, "filled" with rebar is most likely inadequate.

http://www.SlideRuleEra.net

 

[Ron]

kxa...If you are designing for a loading area, you need to consider several aspects of the design. First, as SRE mentioned, the supporting soil is important. It will affect the performance of the pavement moreso than the thickness design. Secondly, you need to consider the joint type and configuration, as you will certainly need joints, but they will need to be able to transfer load from one side of the joint to the other without significant differential movement so that pumping is minimized. The spacing and configuration of the joints is very important so that cracking is reduced.

Then you need to consider the concrete mix design for strength and durability. This is a fairly high abrasion application with lots of loaded twisting and turning. The surface durability must be good.

For thickness design, the PCA Pavement design method is good or you may use the AASHTO Rigid Pavement design procedures. The PCA method is a bit more rigorous and accounts for a few more parameters (radius of loading, edge considerations, etc.).

The layered elastic approach is also good as long as you know and understand the elastic properties of the concrete and the subgrade, and apply them properly in the analysis.

In general, semi-trailer loading areas will have concrete thicknesses in the range of 7 to 9 inches. Be sure to put strong controls on the flatness of the subgrade and thickness control of the concrete as these contribute significantly to crack potential.

All joints should provide load transfer, whether through a key design or by dowels. Be careful with doweling in two directions to intersect at joints....causes cracking. For these applications I would dowel the joints perpendicular to the traffic and key the joints parallel to the traffic.

Use the largest aggregate practicable as this will reduce shrinkage and provide better load transfer across saw-cut control joints. I would not use concrete with a nominal coarse aggregate size smaller than No. 57 stone.

 

[civilperson]

Design for 16 kips/axle with 88 kips total for 48 feet in length x 8.5' width. estimate tire pressure as 95 psi.

 

[dik]

Is that 32K for two rear axles and 56K for something else? or 64K for two rear axles and 24K for something else?

Dik

 

[Ron]

The distribution I use is 18k per axle on the trailer tandem, 16k per axle on the tractor tandem, and 12k on the tractor front axle for a total of 80k, since many states limit total to 80k.

 

[kxa]

Thank you all for great responses. There is going to be a ramp to this loading area which I am planning on using an 8% slope for. The ramp will be about 16' wide by 50' long and I probably use joints at 25' spacing with 1" dia.x 2' dowels at 18" spacing for the transverse joints. One side of the dowels will need to be greased to allow for some movements.

As far as the soil goes, the client claims he has good sand and may not go for a full fledge boring. I said that I'll base my work on 2 tsf bearing and it will be his responsibility to verify. Is there a quick way of sizing the pavement and the required rebars?

 

[Ron]

kxa...rebar shouldn't be structurally necessary. I would use either WWF or rebar for crack mitigation. If you use WWF, use the sheet type, not the rolled type. I would put the joints at 16 feet o.c. instead of 25 feet....this will help control cracking.

Find out the use frequency of the ramp and then design according to either the AASHTO method or the PCA method. Either will yield a pavement thickness that will likely be in the 6.5 to 8 inch range.

Again, make sure you put a lot of quality criteria in the concrete mix. Don't just let them drop some ready-mix in a hole.

 

[SlideRuleEra]

kxa - Be sure to put dowel caps on the greased ends. When properly installed, they provide a small, hollow space at the end of the dowels. Then, if the slabs expand on hot days (the joints get smaller) there is a place for the dowels to go. Otherwise the dowels will be put into compression, which can cause concrete cracking. Using these caps is a common practice. Here is a link to typical products

http://www.masco.net/catalogs2/concrete_forming_accessories/slab_horizontal/doweling.htm

http://www.SlideRuleEra.net

 

[civilperson]

A transition from the 8% slope to the loading pad is MANDATORY. Use a vertical curve with the tangent point beyond the pad. Check the under clearance for your specific trailers to determine the length of the vertical curve.