The world of AASHTO

[Martino8]

We've been asked to design a buried detention vault that is in a right of way along a major city street.
Because of this, the City is requiring that we conform to the relavent transportation codes.
1. AASHTO LFRD Bridge Design Specifications.
2. WSDOT Bridge Design Manual.
3. WSDOT Geotechnical Design Manual.
Since we're primarily a building design office (IBC), the first task is to get familiar with the AASHTO design procedures.

After some early homework, here are some of my understandings so far.
A) All structural design will need to use LRFD design procedures (including soil parameters).
B) The combinations and load factors will vary from IBC/ACI ect, but the design concept is the same.
C) Applied loads x LF < phi x Ultimate Capacity
D) The main difference from our typical projects is that soil bearing capacities will need to be at strength levels.

Q1. Are we headed in the right direction so far?
Q2. Are there potential AASHTO trip ups that I haven't uncovered/considered yet?

 

[DoubleStud]

The biggest difference is understanding the live load and surcharge. You need to understand how the wheel load is distributed through fill (or minimum fill), the impact factor and also the surcharge load when the wheel is next to the wall. How big is this detention vault? If it is not very big, I suggest just specifying a precast box. The precaster will design it.

 

[Martino8]

Vault is a single cell that's approx 100ft x 14ft x 7 feet tall.

We'll have two load configurations.
1. Current Design = 15 offset from road with sloped (up to 8 feet) landscaped cover.
This will probably control for the design of the concrete wall portions of the vault.

2. Future Layout = The City has requested that we design to accomodate a potential roadway widening in the future extending directly over the vault. This will definitely control the design for the top.

To your earlier point, we've been doing all our vault projects with precast hollowcore plank "lids".
As a result, our drawings will need to carefully spell out the condition 2 loads for the precaster.

 

[DoubleStud]

You need to know the condition if it will ever be below the highway. Might as well design the lid to take HL-93 load now. But you need know the condition min and max soil cover. Sometimes less than 2 ft cover controls, sometimes huge soil cover controls.

Detention for what? Storm water? That can easily be done with 3 sided culverts or box culverts with wall at the ends.

 

[Martino8]

The reviewer is also asking for Fire Truck outrigger loads.

Having said that, we're only spanning 14 feet, which is well within the capacity of their standard planks (even for all the different vehicle loads). The one component that isn't understood at the moment is the soil cover depth of a future roadway. But, if we specify a relatively thin assumed cover together with the vehicle loads, they can typically accomodate by grouting more of the cells at the connections to the walls.

 

[BridgeSmith]

I'll second the suggestion to specify a precast box culvert, and let the precaster do the design. You won't have to waste the time familiarizing yourself with all the ins and outs of a new spec, and it will likely be more economical to construct, as well. Precast boxes are the go-to structure type for things like this for a reason.

 

[Martino8]

That seems enormous to be a precast element (9,800 ft3 of storage). Based on preliminary designs, the box would weigh approx 400 kips. Maybe multiple precast boxes, but then they would all have to be tied together etc. Prob a lot easier to dig one giant hole and combine all the storage into one location.

 

[DoubleStud]

That's why I ask what the purpose of this tank. I assume it is for storm water? A little leak at the joint would not really matter. So you make it with multiple precast sections. For example you can do box culvert with 2 walls at the end to cap it.

 

[BridgeSmith]

That seems enormous to be a precast element (9,800 ft3 of storage).

They'll be cast in 6' long sections, and then connected together in-place. We've done culverts that size and larger. You'll lose 2 sq. ft. (2%) of the cross-section for the haunches in the corners, but the lay length would come out to 102', anyway.

 

[JoelTXCive]

There are a lot of pre-cast products out there that might work.

I've done TxDOT projects where the drainage engineers just store water in long parallel lines of regular pre-cast box culverts with equalization pipes every so often.

The pre-cast companies also have specialized products on the market. Below are ones from Contech and OldCastle.

Google "Contech Urban Pond"..

https://www.conteches.com/urbanpond

Or Oldcastle Detention..

https://oldcastleinfrastructure.com/wp-content/uploads/2018/11/OSS_SC-Detention-Insert_102418.pdf