H20 axle load versus Concrete truck axle(s) on residential bridge during construction 3

[Jc67roch]

I am designed a steel beam/concrete slab residential bridge of a fairly short span - 12 to 14 foot span. The bridge is needed to begin construction of a home on the otherwise vacant parcel (on the other side of the parcel). It appears the max load for the beam design (moment) may be one axle of an H20/HS20 truck at mid-span. But in thinking about construction, I know they will have full concrete trucks crossing this bridge during construction of the home. Information on the web suggests the concrete rear axle could be as high as 28 kips on each of the double rear axles. This contracts against the 32 kip rear axle of the H20/HS20 truck I was going to design for. Does the H20 single rear axle load (used on some highways) account for the double axle concrete truck load with the slight spacing? Or do I need to design for the concrete truck axle loads, thus upping my bridge design load? The bridge and driveway would only see these loads during construction or the occasional future large delivery truck.

 

[SlideRuleEra]

A simple approach would be to increase the design criteria to, perhaps, H25/HS25, 25% higher loading than H20/HS20.

Very short-span bridges have a loading quirk, axle spacing is not "slight" compared to span length. I haven't designed a highway bridge, but ran into this issue when designing a private short-span railway bridge for Cooper E80 loading. I decided to use the actual worse case wheel point loads plus impact at those locations.

http://www.SlideRuleEra.net

 

[GC_Hopi]

You probably will not have any luck forcing the contractor to run half loads of concrete so it seems like you need to consider a concrete truck at a minimum. Make sure to post the vehicle design criteria on the drawing. Also depending which state (assuming you are in the US) there may be some guidance from the DOT on what is considered a legal vehicle load. For example see page 45 Link. What you actually end up designing for should be discussed with the owner and check with the AHJ to close the loop?

 

[dhengr]

Jc67roch:
Check with your local municipality or AJH. As a min. for a bldg. permit and occupancy sign-off, you may have to design the bridge for their largest fire and emergency trucks, even though it is supposedly a private driveway bridge.

 

[OldDawgNewTricks]

I always design site structures for the largest vehicle (concrete truck, fire truck ,etc.) that could ever possibly drive over it unless there are physical barriers (bollards, width or height restrictions, etc.) that prevent entry. The driver is not going to stop and check your drawings to see what the design vehicle was and might not even look to see (or care) if there is a posted weight limit sign.

 

[Ron]

All good comments, but do what SlideRuleEra noted...increase loading

 

[MotorCity]

If you design it for HS20 loading, it should support a fully loaded concrete truck. Otherwise, every concrete truck on the road would need a permit to cross a bridge.

 

[Settingsun]

You should use at least the Design Tandem which is 2*25 kip axles at 4' centres. Plus the lane load if you need to comply with AASHTO loading.

 

[JoelTXCive]

Reconciling the AASHTO bridge loading model to the vehicles that are actually on the road has always confused me.

There are many short axle trucks all over Texas (and many other states) that exceed the HL-93 model load. These short axle trucks exceed the HS-20 load and also the shorter 'design tandem' load.

My only explanation is that there are a gazillion AASHTO load factors, along with a bunch of fatigue requirements that keep bridge beam stresses low.

The load factors combined with the statistical lower volume of these heavy trucks makes things work out okay.

For a short span private bridge; I would design the bridge for the true heaviest load the at bridge might see. Do you have to get a permit for the bridge? The local fire department might not respond to the home if they do not feel the bridge is adequate.

 

[jorton]

AASHTO requires design for the tandem as described by steve49. It tends to control on short spans. No need to research actual truck loads imho.