Penetrations through a Box Girder

[efFeb]

Good Morning,
I am working on a steel building which includes box girders (on the order of 18-24" deep)
I have been getting requests from the contractor for penetrations through a couple of these beams. I've worked to get these openings shifted to generally-preferred locations (centred in the span and vertically on the beam).
I expect to be able to check the web openings (through both webs) in essentially the same way as I would for a W section with a single web, but just want to know if anyone who had come across this before might have any advice or thoughts on additional considerations that need to be made for penetrations in a hollow rectangular section like this.
Thanks!

 

[BAretired]

Your question is too general. Why are you choosing box girders? A framing plan might be helpful.

BA

 

[KootK]

I don't know of anything specific to box girders and, like you, would be seeking to adapt the AISC design guide provisions for wide flange beams to this situation. If box beam specific guidance exists, I'd expect to find it the bridge design universe.

 

[r13]

Are these steel box girders? If so, IMO, the stress changes could be quite difficult to evaluate, because of losing continuity in the plates, and the increased potential of stress concentration.

 

[Agent666]

I'd say you're on the right track to check an equivalent welded wide flanged member, provided you're assessing the load accurately on each web of the box girder (you may have more load on one face with longer slab spans or something.

Regarding penetration location I would say the sweet spot for penetration location along a beam is usually around the 1/5-1/3 span location for a simply supported span, but really depends on how hard you're working the penetration length/depth parameters compared the box girders web depth. At this sweet spot location you have a more favourable balance of shear and moment which will allow for larger allowable penetration depths/lengths if required. Also don't forget about the additional deflection due to the penetrations, as a composite member will usually be governed by serviceability requirements.

While I'm not a bridge guy, so don't have any specific bridge related advice for box girders. As far as beams go you can also look into SCI P355, this advice is far more modern when compared to the AISC advice in design guide 2.

What are we talking about here in terms of penetration size and box girder size/plate thicknesses? You'd be surprised how much you can take out of a member if your loading isn't maxing out the full sections capacity.

 

[Agent666]

One other thing to the OP, typically round penetrations perform much better and are less likely to require stiffening and/or can be located at closer spacings relative to each other than square or rectangular penetrations (even if they are slightly deeper than an rectangular opening.

If so, IMO, the stress changes could be quite difficult to evaluate, because of losing continuity in the plates, and the increased potential of stress concentration.

This is not really an issue if you follow the commonly recognised design advice for penetrations through beam webs. Stress concentration isn't a concern in plastic steel design/methods, which will apply in most cases for the assessment of the tee or stiffened tee above and below the penetrations in a I-section member. Additionally, following advice on detailing internal corners with correctly radiused transitions eliminates any potential tearing of plates (typically 2 times web thickness in the SCI advice if my memory serves me correctly).

 

[efFeb]

Thanks everyone for your advice!
The openings are not too large in comparison to the section depth, and are located in areas with low shear, so it is not surprising to me when I am finding that no additional reinforcing is required when checking in a similar way to a plate girder. I was mostly interested to know if anyone who had come across a similar situation might have any thoughts about additional considerations or resources to use.
Thanks again everyone!

 

[r13]

Obviously I was overthinking. What the box girders support, if not crane rail?