Driveway Bridge Engineering Review

jhpf · Nov 10, 2003

Our firm provides consulting engineering services to a several municipalities in our state. Sometimes, we are asked by local land use commissions to review the structural, hydraulic, and foundation aspects of private driveway bridge applications. These bridges typically span watercourses and can span as much as 50'. Often times, steel stringers are used with (non-composite) timber deck.

Unfortunately, the engineering work in support of these driveway bridges is minimal at best. For instance, bridge rail systems might be relatively custom and without supporting computations, never mind be of a crash-tested design. Stringer design computations do not often include contributing deadload. Concrete abutments and wingwalls are not typically designed to be stable on a per unit of wall length basis. Reinforcing steel is not developed properly. Proper hydraulic analysis using such models as HEC-2 or HEC-RAS are not typically included. Scour analyses are never included. Sometimes, an applicant will submit HY-8 or try and compare his proposed waterway area to the waterway areas of the closest up and donwstream structures.

While it may seem excessive to some applicants, we typically suggest to the municipal land use commission that we are working for, to see to it that proper engineering standards of practice go into the design of even private driveway bridges.

Does this seem excessive to anyone? Has anyone had similar experience, and if so, what have you required of the applicant?

koodi · Nov 10, 2003

jhpf,

by definition "Proper engineering standards" should always be practiced. However, it seems improper that a driveway bridge should meet the same standards (at least structurally) as a highway bridge.

For instance, a 10 ft simply supported crossing to a private residence, being required to support a 32 kip axle. The maximum legal load is a 20 kip single axle and 34 kips for a set of tandems.

The 32 kip axle is causing a bigger moment than the bridge will ever see. Hence, right off the bat, there is a safety factor of 1.5. This factor of 1.5 is then magnified (via impact, load factors, strength reduction, etc...)

Any thoughts...?

jhpf · Nov 11, 2003

koodi,

Thanks for the input. I agree that the HS20 design vehicle may be excessive for a driveway bridge. The heaviest vehicle types that these bridges may see during their life time could be a fire truck, concrete truck and/or a tractor trailer (furniture) moving truck. But these loading instances would be few and far between. I have yet to see an applicant's engineer design a bridge for these more specific lower load cases. The bridges are typically always designed for HS20 live load, though for shorter spans the entire truck may not fall on the bridge.

Could you direct me to where to find the information on the 20 kip axle vs. 34 kip tandem axles you mentioned?

Rich2001 · Nov 11, 2003

I agree that the bridges are typically designed for an HS20 load.

It has been my experience that this is required by the landowner's insurance carrier.

I have also seen in some states if the driveway connects to a state route any bridges in the driveway must meet the state's minimum load requirements, in case a school bus accidently turns down the driveway.

curvbridger · Nov 11, 2003

I have always been interested in driveway crossings, and hope to one day build a few as a "retirement" job.

Due to the possibility of fire engines, concrete mixers, fuel oil delivery trucks, etc., I have always held that H15 or H20 should be used, as a minimum.

As for traffic railings, you cannot just apply existing highway standards. This is bureaucrateering, not engineering. Considerations of frequency of demand, traffic speed, and consequences of failure demand a different det of design criteria. That being said, The use of timber curbing alone, or a simple pipe handrail, both of which I have seen in some locations, seems inadequate. I lean toward a combination curb/rail which impacts the fender and the wheel, and is capable of withstanding 50 to 100 plf.

Hydraulics - It is excessive to require $2000 of hydraulics engineering for a $10,000 bridge. If existing crossings exist, then a comparison to upstream and downstream openings and a high water elevation history would seem to be due diligence. Probably, there are waterway permits required, which set the legal requirements for analysis. I would hate to see much added to these for purposes of bridge safety/adequacy.

curvbridger

jhpf · Nov 11, 2003

curvbridger.

Seems that we agree on the live load.

Bridge rails are a tough one to decide. To me, they are the most likely part of a driveway bridge to be damaged or to fail, simply because enough attention is not paid to them during design because it is a driveway bridge.

I have to disagree with you on the money issue though. A part of me understands where you are coming from, but by my way of thinking money can't trump safety. So even if the bridge doesn't cost anything to build, it may still cost something to get designed.

If the bridge crosses a stream that was studied in detail by the FEMA in a Town's Flood Insurance Study (FIS), then I think it may be reasonable to simply see that the proposed abutments are not in the regulatory floodway and that the low chord is above the "with floodway elevation" in the Town's FIS (with some freeboard). In this case, I think it is reasonable not to make an applicant perform a bridge hydraulic analysis.

But if the abutments are proposed in the regulatory floodway, or if the stream was not studied by the FEMA using detailed methods, then I think it is reasonable to request a bridge hydraulic analysis.

In our engineering travels, we have found out that even along an unstudied FEMA reach, a Town has an obligation to manage their floodplain activities consistent with the NFIP, to maintain the Town's overall insurance standing with the NFIP.

curvbridger · Nov 11, 2003

Jhpf,

I think that the primary area in which we disagree is in the area of consequences of failure.

I agree that railings fail alot, but the life and economical damages are relatively minor, compared to highway structure failures. Even if the railing is struck and fails, the vehicle will often just drop a wheel over the edge of the bridge and not even leave the cartway, if the speed is 10 mph.

Hydraulic calculations are appropriate for some structures, but I have seen some ridiculous applications. Two years ago I performed an analysis for a stream crossing on a waterway which was a seasonal wash in rural West Virginia. The stream grade was over 8 percent, and there was NOTHING in the 100 acre contributary drainage area except an abandoned and condemned cabin. Here, by observation, backwater elevations are meaningless, but I had to calculate them to the nearesat 0.1 foot! There were essentially no consequences that could be traced to an inadequate pipe size. To top it off, this was for a temporary construction condition.
On another project, our firm submitted a proposal for over $30,000 in hydraulic calculations and permit preparation fees for a $2000 repair to an existing culvert. The hydraulic flow characteristics of the repaired culvert would have been essentially unchanged.

We should spend design and construction time and dollars to reduce the damages which have the highest probability of occuring.

koodi · Nov 11, 2003

Here's a document about legal loads, the tires themselves are usually maxed out at 6 kips:

http://www.wsdot.wa.gov/FASC/EngineeringPublications/manuals/Vehicleguide.pdf

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