ASCE 7-16 WIND LOADS

[joelperez]

Hi guys, I have the following question, I"m designing a steel portal frame structure that its going to be closed
by masonry walls. This walls arent going to be attached to the steel structure so Im asking myself if the columns are going to be receiving
any wind loads since they dont have any element, like purlins, that its going to transmit the load to the columns. I should considerer
this system a enclosed building but only apply the wind loads of the roof?

 

[phamENG]

I would be surprised if there's no load path at all. How tall is this portal frame? Are you planning on cantilevering the masonry wall that high with zero connection to the building behind it? Seems like a stretch, even at 10 or 12 feet. Even if it's just locked in at a spandrel beam, the load will still find its way back to the portal frame. If you are really 100% independent, then you need to look at the deflection of the CMU wall and make sure the gap is large enough to prevent the wall from impacting (and thereby imparting load to) to the steel frame.

 

[joelperez]

The height of the ridge is 15ft and the eave height is 10ft

 

[phamENG]

What kind of deflections are you getting at the top of your masonry wall?

 

[joelperez]

I would have to do a calculation by hand to know the deflection because I aint modeling the walls

 

[phamENG]

Well yes...you would need to do that calculation. Were you just going to tell the contractor to cantilever masonry walls 10 feet and hope for the best?

 

[bones206]

As the designer, you are responsible for providing a load path for all loads and understanding all of those load paths. Just spend some time examining the load path from the wall cladding all the way to the foundation and see where that takes your design.

In terms of detailing "hard wall" cladding alongside a steel portal frame, Alexander Newman's book is a good resource:

https://www.amazon.com/Metal-Buildi...1828966/ref=pd_lpo_1?pd_rd_i=0071828966&psc=1

 

[Ron247]

Verifying the load path you are designing is what is required. If the item you are designing is not at the start, end or anywhere in between the load path the wind or seismic is taking, it should not need to be designed for those loads. What is the portal frame for, if not lateral loads? The load path most of us are describing is your design load path with any distortions to your load path considered. If the masonry wall deflects until it touched the portal frame, the portal frame is in your load path.

We spend hours designing and checking load paths while Mother Nature loads, designs and can break them, almost simultaneously and in that order.

 

[southard2]

Well you could certainly design the building this way. If we can design cantilevered walls to resist 10 foot earth banks you should be able to do this. You will end up with a huge footing and yes you will have to check the connections. You will also probably end up with 12" CMU walls which is real over kill for a building of that height.

The roof portion of the building will definitely still catch some wind and will have to have a load path provided to the ground. You can do this with X-cables, braces, or even tie the frame off to the CMU walls.

My personal opinion and preference is that if you are going to have CMU Walls cladding the building you might as well use them. You can't avoid a roof diaphragm situation anyways and like I said the roof will catch lateral wind load unless it is a flat roof. But since you have a gable roof the minimum lateral wind load on the vertical projection of the roof I believe is 10 psf (ASD) and it could be higher than that once you calculate the MRWRS pressures....

John Southard, M.S., P.E.

https://www.pdhlibrary.com/

 

[Aesur]

I agree with others here, if I have CMU walls and there isn't a specific reason to not used them I would use them as shear walls to stiffen up the building, else the joints and flashing will be annoying to deal with. Depending on your seismic region, you may find that with a cantilever CMU wall and a portal frame you may need to provide expansion around the columns as the two will move independently of each other. This is typically at headache even in low seismic regions when dealing with PEMB with CMU wainscots, the detailing can get messy fast.

It sounds as if you may be doing a pre-engineered metal building with someone else designing the foundations and in this case the exterior walls, is this the case?

To answer your original question, I agree that if the CMU walls are cantilever and a completely separate system, then your portal frames would only see wind from the portion of roof and width of beams/columns. Make sure whom ever is designing the CMU walls isn't intending to tie into your portal frames before going this route though.