Operating Rating Level - interpretation 1

[dgidwani]

Can anyone suggest a reference which will provide clear guidance on circumstances in which one may use the Operating Rating Level?

The AASHTO Manual is not quite clear. It seems to be saying that the O.R.L. should be used for an extreme circumstance. Can construction loads from heavy rigging equipment be considered one such circumstance? Or is it meant only for emergencies or life-threatening situations etc?

Thanks for your time.

 

[Qshake]

In most cases, the bridge is designed for the HS20 or similar design truck.

Following the bridge design, a rating is performed to determine the operating and invetory rating. Those trucks have special requirments that must be factored into the design. For example, one gives the maximum safe load that may traverse the bridge. However, it may do so only as a sinlge lane load and moving at a speed of 20 mph or less. Those two restrictions have far reaching impacts.

First, the single lane load serves to decrease distribution factor for a truck loading. This in-turn decreases the load to a single girder. Second, the speed restriction means that impact need not be considered. This can increase the load by 30%. Overall, it means that more load can traverse the bridge.

Once again, design and rating will not always coincide.

 

[dgidwani]

Thanks for responding once again, Qshake. However, you don't seem to be addressing the questions asked.

In evaluating an existing bridge to determine if a certain construction equipment may operate on it, can one use the Operating Rating Level? This would allow a live load factor of 1.3, as against 1.3x1.67 (AASHTO).

 

[Qshake]

With all due respect, I have answer the question. The bridge is designed for the basic load and and rated for other truck configurations.

Most bridges are then rated for the following trucks -

HS Truck (Should yield some percentage of the actual design weight in tons)

H Truck (should yield some higher percentage of the actual design weight in tons) This is easy to visualize since the H truck is lighter than the original design vehicle.

3S2 Truck - Typical 18 Wheeler.

Other trucks that may be involved in the rating are:

Lowboy, Construction equipment hauler gross weight of 73,280#.

Once the inventory and operating ratings are known the permitting agency will be able to assess whether or not a vehicle can traverse that structure.

We don't design bridges for specific trucks.

 

[dgidwani]

Lets say we have established the Inventory and Operating Level Ratings for HS20 trucks. For example, lets say they are 1.1 and 1.8.
Now, there is a construction project which needs a heavy crane to be placed on the bridge. This crane causes effects equal to 1.7 times those caused by HS20 trucks. The crane will operate slowly, and only a few times.
Would this crane be allowable, in your opinion?
The AASHTO manual states that allowing Operating Rating Level loading would require more frequent inspections.
Your comments are most valuable, and greatly appreciated. Thanks.

 

[manual90]

We do not design bridges for specific trucks, but you can rate them for specific trucks.

To answer your question (I hope), It will be up to the owner of the bridge if they will allow you to use the operating loading.

If calculated correctly, the bridge can safely handle the operating load, however if it were exposed to the operating load on a regular basis, the life of the bridge could be reduced and/or maintenance costs could increase.

It is not uncommon to allow "overloads" greater than the inventory but less than the operating rating, but it's also not uncommon for owners not to allow it if they do not have a vested interest in allowing the overload.

 

[CanyonRat]

Operating Rating: Represents the absolute maximum permissible load level to which the structure may be subjected to.

To find out the different operating level requirements you have to talk to the owner; usually the state dot.

This can be adjusted. If you slow the vehicle down to 5mph (not 20mph imo) then you can eliminate impact. If you designate where transversely the vehicle can travel then you can eliminate wheel fraction.

In order to pass an overload vehicle across a dot bridge the company submits the truck train configuration, an analysis is run, and the decision is made by the state.

Track cranes shouldn't be a problem but tire cranes can be especially if counterweights are on which they probably will be because it takes a few days to break it down and a truck is needed to transport the boom, etc.

If the load rating is 70% and if I'm doing my math right you get a rating of 1.87 times the HS20 then you should easily be able to pass it with the modifications mentioned above.

I have been involved with few ratings where the rating exceeded the Overload Rating with the wheel fraction and impact taken out. Mostly off of gut feel the vehicle was allowed to pass with no problems but the decision was the state's.

Hope this helps,
MikeD

 

[AndyZ]

Getting back to the original post...

"The AASHTO Manual is not quite clear. It seems to be saying that the O.R.L. should be used for an extreme circumstance. Can construction loads from heavy rigging equipment be considered one such circumstance? Or is it meant only for emergencies or life-threatening situations etc?"

Each owner agency has different rules, I have designed bridges for operating levels during construction after notifying the agency. In addition, some owners will post bridges for operating level in order to allow trucks from a nearby manufacturer, industry, shipper or school to pass over the bridge. Operating level posting require inspections on at least an annual (every year) instead of a biennial (every other year) frequency. Most owners will issue permits for special vehicles that exceed actual design loads on bridges up to the operating level (this is helpful for delivering bridge girders to the next bridge).

The way I understand operating level is that it can be used for a long time, but over time may allow for minor structural damage ( for instance: excessive cracking in the deck, or wear on the bearings) Inventory level should allow for bridge use for an indefinite period of time, Operating best used for permit vehicles, until repairs can be completed or other short term (compared to the life of the bridge) use.

In all cases, the owner should be made aware and give approval to using the operating stress level for determining load capacity, especially during construction conditions. (Construction conditions may in fact be controlled by other temporary structural conditions, like buckling and not by an operating stress condition!!!!)

Some states, for instance Pennsylvania, actually design bridges for an overload vehicle at an operating level in addition to a more normal truck design at an inventory level.