Fire Truck Surcharge on Cantilevered Retaining Walls

[dengebre]

We are designing several tall cantilevered retaining walls that range from 8’-0” to 16’-0” of retained soil. The walls are subject to a traffic surcharge load as there is a parking lot within several feet of the walls. In the past we used a surcharge of 250 psf and believed this adequately accounted for any traffic loads. However, we are concerned if a large fire truck should park next to the wall and if the 250 psf surcharge is adequate to account for this load (especially if the truck has outriggers).

Is there a uniform surcharge load that can be used to properly account for the operating weight and reactions from a fire truck? This may be pretty straight forward to those of you who do DOT work on a regular basis, but our firm designs buildings and we usually do not design walls subject to these kind of loads. Thank you in advance for your expertise.

 

[dcarr82775]

In our area the fire truck outrigger load is almost 40 kips unfactored. It depends on how close you think the outrigger could get, and if the worst case outrigger load can occur right up against the wall. Assuming it is somewhat close to the wall, I wouldn't assume the 250psf accounts for it, especially on the short walls. You can use a boussinesq distribution on the wall for the point load. Many publications provide the distribution (vertical and horizontal) equations. That would be conservative

 

[dengebre]

Many thanks Dcarr82775. Unfortunately there is a potential for the outrigger to be right up against the wall. As building designers what few walls with vehicular surcharges we have designed were car parking garages with no fire truck access. What useful publications could you recommend for the point load distribution on the wall? My apologies for the newbie question but this is the first we have ever had to deal with a high point load such as this. Thanks again.

 

[dcarr82775]

Most foundation texts also have Boussinesq distributions in their sections about retaining walls.

You could also try NAVFAC DM 7.2 (google that). It stands for Naval Facilities Design Manual 7.2. I think that one has a pretty straight forward chart with equations. It is a free download, an oldie but a goodie.

 

[SlideRuleEra]

See Chapter 5 of the CalTrans Bridge Design Specifications. In particular check Paragraph 5.5.5.10.3 on Page 5-40. Here is the link:

CalTrans Bridge Design Specifications - Chapter 5 - Retaining Walls

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Doctormo]

Almost every retaining wall has the possibility of a fire truck behind it and most are designed for uniform surcharges such as 250 psf. If the wall is a reinforced concrete wall, you can crank up the uniform load a little to be safe if you want and be close enough (375 psf or 500 psf, 3 or 4 ft of soil equiv. like mentioned in AASHTO). A concrete wall will distribute a concentrated load over a significant width such that it approaches a uniform load condition from a stability standpoint. Other wall types may behave differently.

There is a case to be made that the firetruck situation is an extreme event as well and the load would not be factored (or lower FS used). Of course, the fire truck could be there putting out a fire due to an earthquake and additional tremors are occurring as the truck arrives. Many walls have loaded concrete trucks right behind them as the pavement goes down and they are designed for 250 psf. It is easy to get carried away with these What If scenarios in my opinion but sometimes major problems occur when an excessive loading condition like a large crane picks from behind a wall.

 

[yakpol]

Here's a free spreadsheet that may help you to calculuate bussinesq pressure on the wall due to combination of area loads.

http://www.yakpol.net/surcharge_down.html

Yakpol