Strengthening existing steel warehouse

[Pervispasco]

Hello everyone,

After years of following this forum, I've consistently found it to be an invaluable resource for technical discussion. Despite reviewing numerous threads on shear flow, I haven't yet found a solution to my specific problem.

I'm currently working on a project involving substantial strengthening of an existing multi-bay steel portal frame warehouse, primarily due to added roof loading.
Our current approach involves the addition of an extra I beam to the underside of the rafter to meet the required inertia and strength capacity (refer to the attached diagram).

In situations involving simply supported beams, I would typically rely on the VQ/I formula to determine shear flow and design the bolts accordingly. However, given that we're dealing with a portal frame rafter where bending moment, shear, and axial loads reach their maximum at a single section, I'm not sure whether I should consider different formulas and methods or if VQ/I remains applicable in this context with no extra forces to consider.

Your help and valuable insight would be much appreciated.

Thanks

 

[SWComposites]

suggest you show a side view of the existing portal frame and where exactly you want to add the I beam.

 

[CDLD]

VQ/I is still valid.
I assume you need the reinforcing to extend to the columns, in which case you will have to develop the moment also.

 

[CDLD]

Also slip critical bolts are required here in my opinion

 

[CuriousBearcat]

@Pervispasco, This built-up beam is a tall section, and in my judgment, it has high likelihood of "buckling" in the middle - meaning in this instance, the beams will try to "break" putting one row of the bolts in the localized region in high tension. Hence, in addition to VQ/I, I also recommend checking the beam and bolts for such action.

-Just a curious engineer

 

[Eng16080]

Assuming the new I-beam section will be attached to the columns at the ends to resist moment:
[ol 1]
[li]Design the bolts connecting the two I-beam sections to resist shear flow per VQ/I[/li]
[li]If the axial force is to be shared between the two sections, ensure that the bolts connecting the two I-beam sections can resist half the axial force (or whatever proportion the new piece should resist) in addition to the shear force noted above[/li]
[li]Check lateral torsional buckling of the new built-up section, as mentioned by CuriousBearcat[/li]
[li]Check that the modified moment connection to the column is adequate and the column can resist the increased moment[/li]
[/ol]

This built-up beam is a tall section, and in my judgment, it has high likelihood of "buckling" in the middle - meaning in this instance, the beams will try to "break" putting one row of the bolts in the localized region in high tension.

I don't understand why bolts would be in tension in this case.

 

[CuriousBearcat]

@Eng16080, the bolts will be in tension because:
[ul]
[li]This is a tall beam section without any transverse stiffeners and the web buckling effects will be considerable in my opinion. [/li]
[li]Due to this web-buckling action, the upper beam section would try to rotate in one direction (say, clockwise). The lower beam section will try to rotate in the other direction (anti-clockwise).[/li]
[li]Due to this effect, the pivot for rotation for the beams will be the bottom-flange end (for top beam) and top flange end (for the lower beam).[/li]
[li]As such, the bolt row away from this pivot will experience highest tension - to counteract this rotatory effect.[/li]
[/ul]

-Just a curious engineer

 

[Eng16080]

CuriousBearcat, Ok I see what you're getting at. The built-up section is trying to fold/hinge at the interface.

I would typically provide the normal buckling check and if the built-up beam is found to be adequate in that regard, I don't think I would check the bolts for the tension force you mention. Actually, I've never considered this before. How would you even go about determining this tension force? I suppose you could determine the moment that would cause the web to bend under a rolling action and then design for that?

 

[CuriousBearcat]

Eng16080, yes the folding of the built-up section is what I meant - that's a much concise and better way to describe what I was trying to say - thanks . To determine this tension, would be a difficult ordeal, knowing that the "web" is not exactly a plate and any numerical method will need a lot of approximations and assumptions and will not be that easy. But based on Pervispasco's description of this beam/strut as a :substantial strengthening of an existing multi-bay steel portal frame warehouse", I recommend creating a finite element model in any of the software that is available, and check for the tension created. This would be the most precise and reliable methods to check this section. If I have the specifics of the load and project and software available, I can certainly guide in this.

-Just a curious engineer

 

[CDLD]

I assume the top flange would be held laterally by the purlins/diaphragm, I wouldn't be overly concerned with tension in the bolts.

Another area of concern is that the bottom flange of the top beam would be in compression.
If this section is designed to plastic strength, the bottom flange of the top beam would be fully yielded without lateral support (assuming purlins sit on top of section).

With built up sections I usually try to avoid lowering the neutral axis below the bottom flange.

 

[canwesteng]

The contribution of the bottom flange to the moment resistance is negligible in any case, so you could just ignore it, but I don't think the destabilizing effect from the bottom flange being in compression would be notable in any case, with the centre so close to the shear center