Load rating of existing bridges

[FSS]

Does anyone know a source for info on how to load rate an existing bridge (relatively small two-lane, one center span and two end spans)when there is no design information or drawings available? I assume field inspections and some sort of testing are in order, but to what standard?

 

[Qshake]

Usually field measurements are taken and everything is measured. Everything! I assume you already have the necessary documentation for load rating. In many of states I work in the DOT provides information for load rating regarding bridge types and information of the material dominant for certain periods construction.

Also, it wouldn't be a bad idea to contact the DOT's bridge management staff to see if any information is available from "Off-System" bridge inspections. All states are required to have this type of information and is collected or the bridge revisited every other year.

Much of the information necessary for determining the loading is easily identified, i.e., roadway width, curb and parapet dimensions, slab thickness, wearing surface limits and thickness, beam depth and cross-section (older "S" beams are readily identified over "I" beams), and haunch thicknesses. Also, diaphragms are readily visible for measurement.

You can see the FAQ here in this section for how to find steel properties for the beams.

Load testing is seldom used. Testing coupons taken from the structure is not so common but many agencies are reluctant to pay for that service if it can be avoided.

One last comment, if you're dealing with steel and a possible construction date prior to 1960 the construction is probably not composite.

Good Luck

 

[FSS]

Thanks for reply. Unfortunately, bridge is on "private property" and is not in the DOT system. And it consists of prestressed concrete members (no info on prestressing).

I would appreciate if anyone can point me to a document or reference that applies.

 

[mikeh]

If the concrete girders were pre-cast, there will be a manufacturers stamp on each girder along with the year of manufacture. With luck, the manufacturer is still around. If not, it may be possible to compare the section properties with those sections manufactured around that date.

Most pre-cast suppliers conformed to a the basic girder layout and reinforcment design in order to get easier approvals for the transportation department. Furthermore, very few private bridges are designed from scratch, rather they are typically bought "off the shelf". Your local pre-caster can help you with the specifics.

Once you have a good idea of the original design, a good source of info for evaluating and rating existing bridges is the Ontario Bridge Design Code and the Commentary.

 

[Kazzio]

I used ot work in the Michagan Dept of Trans bridge management program. Previous posters are quite correct. Initially you need to create a drawing of the bridge. Any damage or failure must be noted. Since the structure is presstressed, you will need manufactures shop drawings to assess the capacity. Software is usually used to assess the structural capacity of each and every member. You just tossayour dead load, wind loads, and seismic loads on it, then see what kind of a stress range it can handle and identify the corresponding safety factors. Unfortunatly we had the worst time with reliable ware for this at mdot. Some of the stuff out there didnt even get simply supported reactions correct.


As far as private sector brisge rating goes, I have seen alot of crap out there. Some reports I have seen didnt even do a rudimentary analysis. The attitude most regulators have is that should you with to put your seal on it, its your decision how it gets done.

 

[breaks]

PennDOT has a prestressed girder rating/analysis program PS3 that it relatively easy to use. I'm not sure how to get this program unless you're a consultant to PennDOT, but you might try their web site. Given load and geometry inputs, it outputs easy-to-read inventory and operating ratings.

As far as references go, try checking out the AASHTO Manual for Condition Evaluation for Bridges (1994 with 1995 interim) if you haven't already.

 

[carlorom]

Hi
Can't answer question directly, but am interested in how and what you measure as my field is Geomatics deformation measurements. Often find that engineers can't tell me exactly what and where measurements are needed. If I can help let me know, otherwise will follow this discussion with interest

 

[asdf]

In addition to gathering your field measurments, you may want to rent a reinforcement locating instrument. This may give you a more accurate idea of what is in the concrete. You may luck out, and the heel of the beam at the abutment may be broken off, exposing the strand ends, or rebar. Check out the DOT websites for a program called BARS - Bridge Rating Analysis System. It is used in everywhere, I know it is on the Ohio DOT site. For your simple span, you are going to need the span lengths, deck composition for dead load calculations, beam spacing, and beam properties. Do not forget to measure any section loss, concrete or steel. If you have trouble, post back, and I will walk you through the process.