Min length of precast panel next to opening? 3

[7788_011]

Is there any limit for the length of the precast panel legs next to opening? I have seen some people specify 600mm while some make it 300mm despite that their loads are quite close. I would argue that as long as the N-M satisfies (ligs provided if required) and there is no problem for construction, fire etc it shouldn't be limited to a certain value. Maybe I miss something?

 

[Trenno]

As there any return walls? Is the wall only a single level height as shown?

As a start I'd keep the min length as 4x the wall thickness but detail/design it as a column.

Also think about the formwork, with closed multi-level core boxes, I usually get rid of small little door nibs (say <200mm) that architects usually show as concrete. They can form the door jamb in something non-structural.

 

[JStructsteel]

I would say 6" min, you need to get reinforcing in the leg, and the proper cover. More than likely the controlling case will be shipping and handling. Overall, the panel would probably check as a cantilever over the door way, depending how large it is. Another think though is the anchorage of the panel to the foundation, what sort of dimension is your anchor?

 

[7788_011]

@Trenno, some are single story warehouses some are multi-storey buildings. Is there any reason behind 4x? I know the only thing for 4x is fire but do I need to design this leg to fire as column? And even if I design it as column to fire, in many case it will still pass when the axial force is small. For strength, I did design it as columns when the length is small. Just don't know where this min length comes from.

@JStructsteel, 6" will be really small for construction I think? I have seen people using strongback to deal with the cantilever issue you mention. For anchorage I just use 20mm Dia dowel bars so edge distance is not a concern in my opinion.

So the architect wants 300mm length for this project and the axial force is just 150kN. I design it as column (panel thickness is 200mm) and it passes easily. So not sure if I miss anything else or do I need to ask the architect to extend it.

 

[KootK]

I've gone with a 12" minimum width myself in these cases. There isn't anything scientific about that though. Shipping and handling is the limiting case but my precasters will install strongbacks on that outstanding leg to maintain it's integrity during handling. Some things that factor in:

1) In an exterior panel, the leg is usually a wind column as well.

2) There will be in plane shrinkage stresses tending to crack the outstanding leg at the top of the opening.

3) If there's an adjacent panel and panel to panel connectors will be used, some out of plane force will be exerted on the panel in installing those connectors.

4) Sometimes I have prestressing in that outstanding leg. When that's the case, I don't want the P/A getting crazy.

 

[JStructsteel]

6" is half the weight of 12", so will 12" need strong back?

 

[KootK]

6" is half the weight of 12", so will 12" need strong back?

For me, the weight of the outstanding leg would have nothing to do with the potential need for a strongback. It's a fragile bit at most of the widths being discussed and the strongback would be more of a qualitative precaution in my mind. I imagine that the "design" load case is probably some kind of accidental impact.

 

[JStructsteel]

Sometimes precaster will put a angle or something on the back for shipping. once erected, its removed. Impact loading is likely due to it being a door.

Architect wants 12" so OP should have no issues.

 

[KootK]

Sometimes precaster will put a angle or something on the back for shipping. once erected, its removed.

That's precisely what I meant by "strongback".

Impact loading is likely due to it being a door.

For me it's just the thing bumping into other panels on the truck and, perhaps, the foundations when it's set in place.

 

[JStructsteel]

I have seen strongbacks also to be a permanent fix. Usually a HSS tube welded or fastened to the leg.

 

[Retrograde]

The Concrete Institute of Australia guide CIAZ10 recommends min 600mm unless the panel is structurally adequate if the leg were removed.

[Edit for typo]

 

[KootK]

Nice. One of the most useful things I've gotten from Eng-Tips over the years is that there's great stuff floating around outside of North America that we really do not have comparable versions of inside North America. Here's a link to Retrograde's source document for any interested parties: Link

 

[Trippelsewe]

For applications where it isn't load-bearing I've done as small as 125mm thick x 200mm wide, installed with strongback no problem.

If there is load and thickness is sufficient to fit in ligs you can design it as a column.

 

[7788_011]

Thanks everyone.

I just got the Guide that Retrograde mentioned where it says "Typically, 600 mm is a comfortable minimum width for legs and mullions, while 900 mm is a comfortable minimum depth for spandrels". But I failed to find the reasons behind 600mm & 900mm. I would argue that 600mm as min. for legs and 900mm for headers seems a bit too much? Maybe it is based on 150mm panel where you don't put ligs in? Otherwise I don't why 200mm x 400mm will not work if the strength passes?

 

[human909]

I would argue that 600mm as min. for legs and 900mm for headers seems a bit too much?

Remember this is just general guidance and advice. So it likely would be on the conservative side.

Maybe it is based on 150mm panel where you don't put ligs. Otherwise I don't why 200mm x 400mm will not work if the strength passes?

That would most likely be correct, 150mm precast panels are a pretty common size here. This is guidance only, naturally 200x400 can likely be made to work.

The modern document with the same image can be found here:

https://www.ccaa.com.au/documents/INDUSTRY_GUIDE_T55_Guide_to_Tilt-Up _Design_and_Construction.pdf

Nice. One of the most useful things I've gotten from Eng-Tips over the years is that there's great stuff floating around outside of North America that we really do not have comparable versions of inside North America. Here's a link to Retrograde's source document for any interested parties:

CCAA (Cement Concrete & Aggregates Australia) really does have some EXCELLENT free guides. A list can be found here:

https://www.ccaa.com.au/CCAA/Public.../Guides/CCAA/Public_Content/LISTS/Guides.aspx

I'd be keen to hear how useful and informative you find them. Obviously they are to different codes but there might still be some good gems in them. Overall I can't comment too much on them as I have spent most of my shortish career in steel. But I'm now trying to expand my skillset to be suitably well rounded. I just wish the Australian Steel Institute guides were free and as useful!

I just had a junior engineer run the calcs on post load on concrete slabs. The [Guide to Industrial Floors and Pavements - CCAA] makes things very easy. Though I'd expect it to be fairly conservative. I got him to run the calcs using an alternative approach and he could squeeze a fair bit more out of the slab.

 

[Tomfh]

300mm is common, and 200 (or even less) not that uncommon.

 

[Retrograde]

The Guide does state that the recommendations are "rule-of-thumb".

Otherwise I don't why 200mm x 400mm will not work if the strength passes?

To my mind that is dependent on how you are assessing if the "strength passes". If you are doing a simple tributary width calculation to find an axial load in the leg, I would be sticking with the 600mm. If you are doing a plane stress analysis, then I would be more comfortable with a smaller dimension.