RBstructural
Structural
The language in this provision seems unclear. I want to see how others apply this. In my experience, I see erection engineers factor up the bridge dead load by a factor of 1.3 for qualifying anything under jacking conditions. Load factors for dead load for design per the AASHTO LRFD Bridge Design Spec or the AASHTO Guide Spec for Bridge Temporary works are applied on top of this 1.3 factor. This practice seems conservative, simple and acceptable but not exactly as the provision is written and possibly double dipping on the dead load factors. I view this provision differently.
First, I interpret this provision as only applicable to qualification of permanent bridge members. I view jacks, temporary members and falsework as governed by the AASHTO Guide Spec for Bridge Temporary Works.
Second, I interpret design forces as meaning the factored force used to check against the LRFD Bridge Design Spec member strengths. Say the application is jacking a steel girder directly at its bottom flange. I'd check the girder web crippling / web stiffeners for the typical LRFD Bridge Design Spec load combos from a model analysis. Then I'd check it separately for the load in the web being 1.3 times the dead load in the adjacent bearing plus live load as required. This is different than taking the dead load from the structural analysis at the web and factoring it up by 1.3. Depending on the span type, jack offset from the bearings and overall span length it could be a lot different.
Third, I interpret "jacking in service" as the jacking or temporary support of a bridge that has finished initial construction. I see this provision being applicable regardless if the jacking operation or temporary support condition is subjected to live load. For girder web crippling / web stiffeners checks, I'd apply this provision at the jack location and at any adjacent blocking location for subsequent removal of the jacks. One could argue that if the bridge is transferred to blocking and the jacks are relieved of pressure/removed, that it is no longer a "jacking in service" condition and that this loading provision does not apply. I don't think that is correct.
First, I interpret this provision as only applicable to qualification of permanent bridge members. I view jacks, temporary members and falsework as governed by the AASHTO Guide Spec for Bridge Temporary Works.
Second, I interpret design forces as meaning the factored force used to check against the LRFD Bridge Design Spec member strengths. Say the application is jacking a steel girder directly at its bottom flange. I'd check the girder web crippling / web stiffeners for the typical LRFD Bridge Design Spec load combos from a model analysis. Then I'd check it separately for the load in the web being 1.3 times the dead load in the adjacent bearing plus live load as required. This is different than taking the dead load from the structural analysis at the web and factoring it up by 1.3. Depending on the span type, jack offset from the bearings and overall span length it could be a lot different.
Third, I interpret "jacking in service" as the jacking or temporary support of a bridge that has finished initial construction. I see this provision being applicable regardless if the jacking operation or temporary support condition is subjected to live load. For girder web crippling / web stiffeners checks, I'd apply this provision at the jack location and at any adjacent blocking location for subsequent removal of the jacks. One could argue that if the bridge is transferred to blocking and the jacks are relieved of pressure/removed, that it is no longer a "jacking in service" condition and that this loading provision does not apply. I don't think that is correct.
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. I read this as 1.3 x unfactored dead load. I've reviewed a lot of jacking submittals. I've never come across factoring factored loads. Off the top of my head, I don't recall any submittals based on LRFD; it's always Standard Specs as modified by the project specifications. Typically, jacks have been sized for a minimum of 1.5x the anticipated lifting loads.
