load rating - loads other than dead and live

[grapefruit23]

I'm working on some bridge load ratings and am not sure how to apply wind load and longitudinal load in the Rating Factor equation.
I'm thinking the longitudinal braking force would be applied to the live load only at Operating level. And the wind load would be added to the dead load since the dead load effect is subtracted from the capacity. Any insight? Thanks.

 

[bridgebuster]

Wind and braking forces are not considered in a load rating. Do you have a unique structure or unusual circumstance that would require these forces?

 

[grapefruit23]

I am looking at many bridges that all can't be done by the state software so there's something "unique" about most of them. Some of them are quite high and we have been asked to include wind forces for those.

MCEB 6.7.6 Longitudinal Loads states that "The rating of bridge members to include the effects of longitudinal loads in combination with dead and live load effects should be done at the Operating Level.... In addition, longitudinal loads should be used in the evaluation of the adequacy of the substructure elements."

So I am interpreting this as the braking force is the longitudinal load and it should always been included at Operating Level. For substructure elements I'm unsure if that means longitudinal loads should be included in both inventory and operating. I'm thinking wind may also be only at Operating Level?

Also I'm unsure how to include the wind and braking forces into the rating factor equation and if the same A1/A2 factors should apply to them if they were be to be grouped into the dead/live loads. If I use Table 3.22.1A, the wind and longitudinal force the factor would be 1.3 which is the same for A1 and A2 at the operating level.

My thought with subtracting the wind load from the capacity is that it's not related to the truck loading and the purpose of the load rating is to determine the allowable truck load under the worst conditions of section loss and dead/wind loading. Then the braking load would be included in the bottom of the equation with live load since worst live loading would be the truck braking on the bridge....

 

[Lomarandil]

I also have never seen lateral forces (wind, braking, seismic, etc) included in load ratings.

Load rating (in the US) isn't purely an engineering task. It's a quasi-legal process, defined by the state (who sometimes defer to AASHTO MBE). So if a regulatory agency is asking you to include wind forces, and presuming that it's not already in their load rating manual, they probably need to tell you how they want wind included.

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just call me Lo.

 

[bridgebuster]

The MCEB has been replaced by MBE. It doesn't require including wind or braking force. However, if you're going to include braking force, you could consider it as an axial stress over several girders and add it to the live load bending stress. IIMHO, including wind is overkill but if you're going to include I agree, treat it as dead load.

 

[BridgeSmith]

As bridgebuster alluded to, if you do include wind or braking force, you would do so combining the stresses in the girders, braking force added to the live load and wind load added in with the dead load.

The only applicable strength load combination for a rating that includes wind would be Strength V, where the HL93 live load is factored at 1.35 and wind at 0.4, which is doubtful to control over the Strength I Combination, where the Live Load is factored at 1.75.

Also, for almost all bridges, the net stress on the girder from wind or braking would be zero, anyway, since wind is a lateral load (unless the uplift case controls the rating, which would be unlikely) and braking force is an axial load, while the live load is a vertical bending load. Wind load applied to a flange produces compression in one edge of the flange and tension in the other, so the net change in stress for the flange is zero. A similar effect happens with the girder as a whole for an axial force, such as braking. The neutral axis may shift, but the overall capacity of the girder doesn't change.

All that said, I agree with bridgebuster, wind and braking forces are applicable for load ratings in all but the rarest of circumstances. Forces due to curvature would be included, however.

 

[grapefruit23]

Thanks for the responses. I am looking at columns only. I'll checkout the new MBE.

 

[BridgeSmith]

In the hundreds of load ratings I've done, I've never load rated columns, or any other substructure element. My understanding is load rating is required only for superstructures. Unless the columns you're referring to are part of the superstructure, you might want to doublecheck with whoever is requiring the rating to be completed, and be sure that load rating the substructures is required.

 

[Lomarandil]

HotRod, just to clarify a few things:

All that said, I agree with bridgebuster, wind and braking forces are applicable for load ratings in all but the rarest of circumstances. Forces due to curvature would be included, however.

I believe you might have meant to include a negative, that wind and braking forces are not applicable in all but the rarest circumstances. Is that correct?

Also, for almost all bridges, the net stress on the girder from wind or braking would be zero, anyway, since wind is a lateral load (unless the uplift case controls the rating, which would be unlikely) and braking force is an axial load, while the live load is a vertical bending load. Wind load applied to a flange produces compression in one edge of the flange and tension in the other, so the net change in stress for the flange is zero.

We may go off-topic with this discussion, but I'm not sure I follow this logic. Regardless of the net stress across the entire flange (and separate from the discussion including lateral loads in load rating), don't we either need to consider additive stresses (in an stress-based/ASR/WSR approach) or interaction equations (in a capacity-based/LFR/LRFR approach)? Sure, sometimes there's redistribution with plasticity, but at some point a flange tip in too much compression or tension can be a problem, regardless of the rest of the flange.


Separate from all that, I also have only seen substructures considered in cases of extreme damage (e.g. earthquake or vehicle impact) and condition/inspection rating, not for load rating.

----
just call me Lo.

 

[BridgeSmith]

Yes Lo, I did mean to say that including wind and braking in a rating would be a very rare case. In the throes of rewording the statement a couple of times, I ended up with it backwards.

With regard to the net stress, there is a reason why the LRFD limit state for girder flange stress includes only 1/3 of the stress due to lateral load when combined with vertical load stress. If the lateral load produces, say 5 ksi at the flange tips, and the vertical load stress is, say 40 ksi, one flange tip will be at 45 ksi and the other will be at 35 ksi. Unless the loads are high enough to produce yielding of a substantial portion of the flange, the capacity of the flange to carry vertical load is not significantly affected by the lateral loading.

Wind load stresses are typically small relative to the live load, especially if the deck acts as a diaphragm for lateral loads, which it nearly always does. This generally results in the Strength I load combination(1.25DL + 1.75LL) being the governing load case by a large margin over the other load cases (Strength V is 1.25DL + 0.4Wind + 1.35LL).

 

[Lomarandil]

Absolutely. I initially read your statement to imply that lateral loads should be neglected entirely. I don't agree with that, but in most practical cases, lateral loads either do not control or have a relatively minor effect.

----
just call me Lo.

 

[BridgeSmith]

I haven't verified bridgebuster's statement that wind and braking are not included in a load rating, per the MBE. I've read most of it and I haven't seen anything to contradict that statement.

The vertical loading effects of curvature should be included, where applicable, though.

 

[pikku14]

Not sure on what your contract or typical load rating set-up is in the US, but something I've done in the past to enable planning (for e.g. is there need to strengthen/replace) is to simply define a utilisation of elements such as the substructure where lateral effects have a significant effect and the traditional load rating factor don't really fit all that well

 

[davidbeach]

Admittedly a total tangent, but repeatedly reading the title to this thread I have to toss out the possibility of zombie loads if you're wanting loads other than dead and live. ;-)