Concrete box culverts are one of those items that very few engineers have to tackle regularly.
I will assume you have already performed the hydrology and therefore know the design flow and channel tailwater, as this is not specific to concrete box design and not part of the scope of this article. Once you know those two values, you’re on your way:
STEP 1: Pick a Size. If you’re using precast, they only come in certain sizes so you should pick a size from manufacturer’s size lists in your area (or ASTM C1433, from which they are based). Keep in mind you can have the boxes standing upright or lying flat, and you can put as many as you want side by side. If you’re using cast-in-place you can, of course, choose any size you like, but as I will outline later, precast has the added advantage of allowing the engineer to offload alot of the dirty work to the supplier.
STEP 2: The Hydraulics. If it’s a stream channel, use CulvertPro or HY-8 to find the headwater and inlet and outlet velocities. Go back to the previous step and adjust sizes until the headwater and velocities meet the owner’s criteria. If there is no criteria laid out for you, a good guide would be a headwater that’s below the top of culvert (top of road is another popular one) and local inlet and outlet velocities that are less than 2.5x the channel.
STEP 3: Structural Design or Not. For precast units, most specifiers make the Contractor do the structural design. It’s not that they want to offload the work (although some probably do) but all the suppliers I know have more experience than me in doing the structural design, and offer it as a free service for buying the precast units from them. It’s really just a matter of calculating the amount of steel in the cross-section because the concrete forms are the same. A quick note on the drawings or specifications like “Structural Design of Precast Concrete Box Units shall be according to the AASHTO LRFD Bridge Design Specifications, version xxx” and you’re done. If you want to familarize yourself with the calculations, or check someone else’s, continue with step 4:
STEP 4: The calculations. They are based on the determination of the cross-sectional area of steel required (As), and you have several ways to tackle this:
- The software of choice is called BOXCAR, produced and distributed for free by the American Concrete Pipe Association. (UPDATE: BOXCAR is $220 from the ACPA) I don’t know of any other.
- The AASHTO LRFD Bridge Design Specifications are the bible. The applicable section is 12.11, “Reinforced Concrete Cast-in-Place and Precast Box Culverts and Reinforced Cast-in-Place Arches.” This section is only six pages long and will lay out the specifics for concrete box culverts, but the bulk of the calculations are based on section 5, “Concrete Structures.” The structure will require analysis for live loads, dead loads, and earth loads as a beam supported on columns. I can’t go through it here because it would be too long.
- Alot of the work is done for you in ASTM Specification C1577, “Standard Speciﬁcation for Precast Reinforced Concrete Monolithic Box Sections for Culverts, Storm Drains, and Sewers Designed According to AASHTO LRFD.“ This specification identifies the areas of steel required in both directions, at various heights of cover, for all of the standard box sizes. These are the “design tables” that every engineer looks for!
We also have a Design Example.
Please feel free to comment and share your thoughts. Do you do anything differently? Let us know in the comments section below.