I am performing a load rating of a steel multi-girder bridge with a concrete deck - a fairly routine analysis. We have five girders, and the curb-curb width is 14 feet. To determine the LL distribution for the interior girders, we used Table 3. 23.1 which gave us S/7 for a single lane. For the fascia, it appears that we are limited by the provision of Section 3.23.2.3.1.5 giving us a minimum of S/5.5. There is no provision for fascia girder distribution under a single lane. If I go with a direct interpretation, my fascia girder would be subject to more load than the interior, even though a simple beam analysis would result in a significantly less load. Any thoughts?
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AASHTO 17th Ed. Interpretation
- Thread starter cazkoop
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It would help if you gave the girder spacing...I'm guessing it's around 3.5 feet?
Your interpretation of Section 3.23 is technically correct but I think what is missing is the intent of the specification.
It is my opinion the S/5.5 qualifier is intended for multi-lane bridges since it would be a rare occurance where you had 4 or more stringers and the bridge was designed for only one lane.
For your bridge with a concrete deck slab (very stiff) and closely spaced girders, the girders should be checked with the same DF.
The lower limit is #wheels/#girders which = 0.4 for your bridge. This is the absolute lower limit allowed by AASHTO and would be used for LL deflection check and possibly design if a more precise method of analysis were used.
Assuming a 3.5 ft spacing, a one-lane DF = 3.5/7.0 = 0.5
For multiple lanes, DF = 3.5/5.5 = 0.64
Using the lever rule (simple span) and assuming a wheel could be located directly over the exterior stringer, the DF = 0.5.
It is my opionion the S/5.5 is too conservative here, and a DF = 0.5 should be used for the load rating regardless of the lever rule results (i.e. DF = S/7).
Your interpretation of Section 3.23 is technically correct but I think what is missing is the intent of the specification.
It is my opinion the S/5.5 qualifier is intended for multi-lane bridges since it would be a rare occurance where you had 4 or more stringers and the bridge was designed for only one lane.
For your bridge with a concrete deck slab (very stiff) and closely spaced girders, the girders should be checked with the same DF.
The lower limit is #wheels/#girders which = 0.4 for your bridge. This is the absolute lower limit allowed by AASHTO and would be used for LL deflection check and possibly design if a more precise method of analysis were used.
Assuming a 3.5 ft spacing, a one-lane DF = 3.5/7.0 = 0.5
For multiple lanes, DF = 3.5/5.5 = 0.64
Using the lever rule (simple span) and assuming a wheel could be located directly over the exterior stringer, the DF = 0.5.
It is my opionion the S/5.5 is too conservative here, and a DF = 0.5 should be used for the load rating regardless of the lever rule results (i.e. DF = S/7).
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