Lateral Bracing Covered Patio

[engineea]

Hello all,

I have a quick question regarding lateral bracing for a covered patio. In the past I have typically used knee bracing at simple covered patios or even cantilever steel columns/grade beams for larger patios or decks. Are there any other standard options for lateral bracing of structures of this type?

I have a designer who says he has consulted on projects using a knife plate as 'bracing' but he cannot provide a detail or any other info. Has anyone used any other mode of bracing such as knife plates for this application?

Thanks in advance.

 

[Ron]

You can make your frame rigid and provide moment connections at the beam-to-column and/or column-to-foundation (embedded columns or similar).

 

[engineea]

Thanks for the reply Ron.

To be a little more clear, the intent on this project is to use a wood post and wood beam. I would like to add wood knee bracing for lateral support of the structure but the client does not want that 'look', nor do they want to pay for a steel column/grade beam system. The designer's suggestion is to use a knife plate at the wood post to wood beam but I have never seen that used as an option to knee bracing. Can someone please let me know if that is an acceptable alternative, and if so can you please advise on the appropriate detail/design?

Thanks so much.

 

[woodman88]

I have only used knife plate moment connections for wood posts on small partial shade (no diaphragm) canopies. Where connected to a building there was no diaphragm to transfer lateral/torsional loads to the building. In the case where you have a diaphragm and connected to a building the deflection of the wood post plus knife plate connection will force the diaphragm to apply loads to the building. I would design the diaphragm as cantilevered from the building applying the tension/compression load to a post in the wall and/or to a building diaphragm.
The knife plates I used were 1/2” steel plates with 8x8 wood posts. Two moment bolts through the knife plate and a cross bolt each side and parallel to the knife plate between the moment bolts.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[ishvaaag]

You can use a pyramid-like setup to cover the patio; you may then not even need to use other thing than members in the faces and edges of the pyramid to deliver strength and bracing. Traditional solutions might take some help of at 45º members near the corners linking main sleepers at the base to prevent disengagement of the such main members and reduce their lateral deflection under push, or they would be counting on stiffness coming from the building, that also could be your case. A grillage at the base of the pyramid and whatever up to a more complete 3D pyramidal structure also will be able to deal with significant spans.

 

[Teguci]

Why not use cantilevered columns from grade like a pole barn system? Dig a 3+ feet deep hole, place a small concrete footing, install a wood post (treated at the base) and fill the hole. You'll have to run the numbers but you should be able to configure it.

Unless your client is OK with an ugly knife plate connection or wants to spend the money for upgraded detailing, I'd think your best bet is to hide the structural stuff in the ground.

 

[csd72]

Teguci beat me to it, a cantilevered column system is definately an option.

Another option if you have a raised deck is to brace it below deck level thus achieving the cantilevered column system without the embedment.

 

[engineea]

Thanks for all the responses.

I would definitely go with the cantilevered column system but the issue is that this new patio column is located about 1 foot from an existing pool wall and would add surcharge.

Which leads me to my next question. If I go with the cantilevered system with deepened footing, how can I determine how far to move the post away from the existing pool so that we are not adding surcharge?

 

[Teguci]

From the hip - I'd say for every foot down move it one foot away (4 feet down should be 4 feet from the pool). Ideally the bottom of post should be near the bottom of the pool but I don't think vertical loads are going to add much surcharge (have a look at IBC foundation chapter for illustration). If you need the canopy near the pool then cantilever the canopy over the near side posts and design for uplift on your rear posts.

 

[BAretired]

A plan and section would help.

BA

 

[engineea]

Ok, so I've attached a plan showing the new patio cover, the corner column, and the existing pool. The existing pool wall depth is about 4-1/2 ft. I understand how to avoid surcharge in the case of vertical loads, but how do I avoid it if we have a fixed base supporting the moment of the cantilevered system?

FYI, the client is ok with a deep footing or moving the post away from the pool. I just need to determine how far to move it to see if that is a feasible option.

Thanks again.

 

[woodman88]

If you forces are low enough, you might want to look at pinning the posts at the foundation and using L plates at the beams/post as moment connections.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Teguci]

LOL! Pictures always help!

Laterally brace the canopy to the building and just vertically support the canopy at the column.

 

[engineea]

Thanks Teguci, but I'm not sure what you mean by 'laterally brace the canopy to the building'. Please clarify (and sorry for making you clarify

As for loading, due to the low R value, I'm coming up with a load of about 2.3 kips laterally at the column, so I dont know that a plated moment connection would work.

 

[BAretired]

I think he means...tie the canopy to the building and let the existing building take the lateral load.

However, this may not be easy to achieve depending on the elevation of the two roofs.

Do you have a section or elevation showing the difference in levels?

BA

 

[Teguci]

From your sketch it appears that the support for at least two sides of the canopy is the roof of the house (the canopy is even contoured to the existing house). Whatever framing is used to tie the canopy to the house should be designed to carry the lateral loads from the canopy into the house. If this is accomplished, then the column only has to take axial load.

 

[msquared48]

That was my initial thought too.

Just tie it to the "L" shaped house structure. Don't worry about cantilevered columns as they are not needed here.

Mike McCann
MMC Engineering

 

[engineea]

Thanks,

I now understand what you mean. I can definitely tie the canopy to 2 sides of the existing strucure. What I am unsure about is how the canopy diaphragm is supported laterally on the other sides. In your approach what is going to keep it from moving laterally if there is no resisting element at that corner? Not arguing with the approach, just want to make sure I fully understand so I can justify this through design.

I have attached a link for a section view.

Thanks.

 

[Teguci]

Have a look at the enclosed.

 

[engineea]

Thanks for taking the time for the pdf.
I understand the methodology and had thought you were assuming a rigid diaphragm. My only concern is achieving rigidity for the wood diaphragm. Also not sure if you noticed, but the client is using retractable shades which means there is a 12'x12' hole in the roof diaphragm. Do you think that will be an issue? I dont have much experience with rigid diaphragms.