Bridge loading factor

[youngstructural]

Hello All;

Here is my question: In absence of code guidance, I am looking for opinions (perhaps based on ASSHTO or CAN/CSA S6 or another international code) on what live load factor is appropriate for a "known" loading onto an existing bridge for evaluation purposes?

If you want to know why I'm asking, please read on...

I have been involved in the accessment of an existing bridge in rural New Zealand. The bridge is expected to start taking 100 coal mine transport trucks daily. The truck loads are to be well controlled, having been given a design load of approx 67 tonnes and these trucks will be weighed when they leave the mine.

Given that the bridge concerned is 80+ years old and shows some signs of deterioration (autogenous healing, surface pitting accross the deck surface, some cracking, etc) I have applied a 0.9 material strength factor in addition to the concrete code material strength to account for the deterioration. This factor comes directly from the Transit New Zealand Bridge Manual SP/M/022. I have also applied the 1.9 live load factor required of conforming loads, since I believe the 1.49 factor for overload cases is not applicable; This is not an overload case, but will be a routine load on this bridge, thus the "regular" 1.9 live load factor should apply.

The client is very happy with our report, however the issue has come from external reviewing engineers. They insist that the bridge should not have a 0.9 deterioration factor and that the 1.9 live load factor is entirely inappropriate for a known load. In this case it makes all the difference, and determines whether or not the bridge requires posting, or perhaps even replacement.

I look forward to your thoughts,
Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[gwynn]

The reduced live load factors on overloads are there to account for the accuracy of load knowledge on overloads. I would look at the reduced factors allowed for overloads when the actual load is known. CAN/CSA allows for an annual permit class for known frequent overloads. I would take this permit class as appropriate.

The other factor which can be controlled is to reduce allowable speed over the bridge. CAN/CSA S6 allows for reduced dynamic impact loads based on the allowed speed. This is common on old bridges used for mines or logging. This does need to be monitored.

If this was my project I would be looking at reduced speeds over the bridge and, if needed, the permit class allowing for this type of load rated vehicle to cross this bridge for the next year. The permit class would require full inspections annually if it is to be renewed, but depending on the expected life of the mine this may be cheaper than replacement or posting.

The deterioration factor should be based on your judgement if you are the one who inspected the bridge.

My copy of CAN/CSA S6-06 is at the office so I cannot give you the exact reductions in load, but they are significant. They also depend on a number of other things, such as how well the bridge was inspected, failure modes and load sharing.

 

[youngstructural]

I'm not familiar with S6-06, since it came out after I moved down here to New Zealand, however the previous edition (2000 if memory serves me) had similar provisions. I'm just questioning how rational it would be when looking at a case where the load is going to be routine. Although I appreciate that's what you're talking about with mine vehicles, etc.

And the NZ provision have reduced dynamic load effects for reduced speeds as well, however we're talking about a bridge in the middle of nowhere with straight approaches. While I was on site inspecting I clocked a number of vehicles doing 130km/hr accross the bridge. And I would certainly want to see routine inspections if we were to consider a reduced live load... I'm still nervous given that the loads are going to be so routine.

Thanks for your help gwynn!

Any other opinions?

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[asixth]

Did you do the calculations based on the 67 tonne live load, the Australia code has a Heavy Load Platform where 400 tonnes must be able to cross the bridge under controlled conditions.

 

[miecz]

Based on AASHTO's old Standard Specification, for our normal design truck, which is a fictional entity, we factor the weight by 1.3 and again by 1.67 for a total of 2.17. To that, we add an impact factor that is typically around 1.25.

For an extra heavy vehicle, a permit is required, so the weight is well known. Sounds similar to your coal mine trucks in that regard. For the design of a permit vehicle, we factor the weight by 1.3, and then add an impact factor, again, typically around 1.25.

What's different about our permit loading, is that we don't expect 100 of these a day. For one thing, that would increase the number of cycles for fatigue considerations, and would increase the likelihood of more than one vehicle crossing the bridge simultaneously.

That being said, I think that a load factor of 1.49, together with an impact factor of 1.25 would be appropriate for your condition. A load factor of 1.49, with no impact factor seems risky to me.

 

[youngstructural]

Thanks everyone... We'll see what comes out in the wash.

And for the record, I did apply a reduced live load factor, however I allowed for a UDL to represent any other possible vehicles and unexpected events. The reviewer insists that the truck should load the bridge alone (ie: No other vehicles) because the length of the trucks exceeds the length of the bridge. I'm just not comfortable with both reducing the applied live load and removing the UDL.

Also, the deterioration factor is a matter of judgement, and as the engineer who actually inspected the bridge, I'm not at all comfortable with removing it. Otherwise we're saying that an 80 year old bridge with pitting, some localized softness, actively showing signs of autogenous healing, etc, is as good as the day it was poured.

Thanks again!
Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[VoyageofDiscovery]

According to the CSA S6, if the traffic on the bridge is controlled, then the actual axle loading can be considered as the alternative normal live loading as long as lane load is also considered with the actual axle loadings.

If this is not a two-girder system, and you inspected the bridge yourself incorporating it into the design, then the Live Load Factor is 1.42, however if the bridge is required for emergency vehicles or essential to the local economy the corresponding factor is 1.49, for short spans the factors are 1.9 and 2.0 rspectively.

HTH

 

[youngstructural]

Thanks HTH, I wonder why the increased factor for short spans? Am I missing something obvious? I could see some sence in a higher live load factor for short continuous spans, buy why for all short spans?

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[VoyageofDiscovery]

The Code is not clear as to why, however "short spans" only applies when using actual axle loads. Beyond this it explains "short span load factors are to be used for shear effects in beams up to 6m span and for moment effects in beams up to 10m span" as well as "shear and moment effects in floorbeams where the tributary spans are up to 6m."

HTH (hope this helps)

VOD

 

[youngstructural]

Too funny! Thanks VOD... Too bad the commentary isn't clear on the issue. I'm contemplated getting a copy of the new S6 code, despite the fact that I'm all the way down here; It's a very well respected code the world over, and I find the Kiwi code lacking in guidance. I'll probably order it sometime...

And I've appreciated the input. I'm still not convinced dropping the live load factor is the right thing to do, and I'm certainly going to argue against loosing the deterioration factor. I just wish I had more substantial information to back me up.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[apsix]

YS
Extracts from the Aus Standard;
AS5100.7; Bridge design Part 7: Rating of existing bridges