HS-20 load distribution and design loads on driveway deck

[Robcat71]

Please forgive me, I do not have a lot of experience with AASHTO...

I have a project that involves an exterior elevated driveway over the first story of a building (HS-20 loading). The driveway consists of a radiant heated concrete deck (7-9" depth) on top of high density rigid insulation (4" min), waterproofing membrane, supported by a structural reinforced concrete deck on 1-1/2" inverted composite decking. The driveway is supported by wide flange structural steel beams spaced 5 ft. on center, parallel with the "traffic" lanes. Girders are spaced every 30 ft. perpendicular to the drive lanes.

I believe I need to consider two loading conditions: 1. (Max tire load) 16 kip point load (10"x20" contact area), and 2. (Lane loading) A 640 lb/ft uniform load plus 18 kip and 26 kip point loads for maximum moment and shear distributed over a 10 ft. length. I do not think the lane loading will control in this case. I found a reference that the effective width "B" of slab used for design can be taken as B = 4 + 0.06*L (ft.) where L is the center to center spacing of supports. In my case, L is 5 ft., so B is equal to 4.3 ft. Seems reasonable, but that is quite a bit wider than the tire contact, so I'm not sure if it is conservative. I also assume you need to follow the AASHTO load factors for DL and LL with impact considered instead of ACI load factors in this case.

This is a bit tricky as its a building structure, with an elevated exterior drive lane and drop off area above the first floor. Not something you see everyday, or want to! I appreciate any input/expertise to see if I am approaching this correctly.

 

[Lomarandil]

The lane loading as you describe it is a slightly older approach (meant to simplify moving load analysis). Nowadays, we either design for a nominal truck/tandem, or HL-93 includes that truck/tandem plus a uniform loading. This includes local consideration of that 16k wheel point load, but also potentially the effect of multiple wheels.

It's worth mentioning that HS-20 is something a lot of engineers talk about, but may not actually be sufficient for your design. Fire trucks, emergency vehicles, garbage trucks can exceed HS-20 loading. Might want to double check what you need in your jurisdiction.

For equivalent width of deck strip under a wheel load, you want AASHTO Table 4.6.2.1.3-1. Slightly different than the equation you're showing (but in your favor).

Presuming the rest of your structure is per ACI, I wouldn't adapt AASHTO load factors. Those are also connected to the target reliability for a bridge.

 

[Robcat71]

Thank you Lomarandil, I appreciate all of your input. Good point regarding exceeding the HS-20 loading, as that was a big concern. Luckily we have ruled out emergency vehicles, as their access would be from ground level. I will take a look at the AASHTO table you provided for the effective deck strip under a wheel load.