Canopy Post Design 2

[Johnathan_MEng]

Hello everyone, I am a recent mechanical engineering graduate assigned as a project manager, but my boss wants me to help him design a free standing mono-sloping canopy (18'x80') with a 1:12 slope (14'6" eave height). He will have my work checked by a senior structural engineer. I am following the ASCE 7 MWFRS for open buildings, which luckily, everything is relatively straight forward.

My only issue now is determining how a 4"x4" galvanized post will compare to a 6"x6" galvanized post. Could anyone help me with this? The only calculations I have are the lateral forces acting on the beams, which obviously vary with difference surface areas.

Also, the freestanding canopy will be going flush against an existing building, but only to a certain extent. About 50' of the length is against an existing building that's about 20' in height, the remaining 30' of the canopy will be completely open. I have classified everything as an open building, although I believe it's directionally dependent. Please let me know if this is correct, thanks.

 

[JLNJ]

The lateral force resisting system plays a huge role. What is your system?

 

[Johnathan_MEng]

The lateral force resisting system will vary, the 80' span consists of 5 posts on each elongated side; one side will have the 3 middle posts on plates that are bolted into the concrete, while the other 2 cornered posts will be cemented 3' deep into the ground. On the other elongated side of the 80' canopy, all posts will be cemented 3' into the ground. Hopefully this will be enough.

I am also unsure if I should be using the C&C system as described in ASCE 7, I've read a few threads about that on here and that's clearly a whole different controversial conversation. Since this canopy is free standing and unattached to the existing building, I figured MWFRS is appropriate.

 

[STrctPono]

1:12 slope is pretty gradual so your lateral loads should end up being pretty light. I do not have ASCE7 in front of me at the moment but there is a specific section for calculating the positive and negative pressures on the monoslope roof. It's important to understand this because it varies depending on whether the wind is "Obstructed" or "Unobstructed". In the areas that you have the canopy butting up to the adjacent building this should actually increase your uplift pressures on the roof. For the areas not adjacent to the building your downward pressures should be higher. Make your life easier and run both scenarios. Take the worst case of each one and design the whole canopy for that. Don't try and optimize the canopy design based on where the canopy is relative to the existing building. I hope what I said makes se

A transverse section cut at each post location will show you your lateral force resisting system. A moment frame. The moment frame is your MWFRS. You have 5 of them total. The 3 interior ones will have the same loads. The 2 at the end will have different loads. Again, make your life easier and design for the worst case scenario.

Draw a moment frame on your paper. 2 columns with a beam at the top that links them together. The beam has moment continuity with the columns. The columns are pinned at the base not fixed. Apply your vertical loads and your lateral loads and solve for your moments and shear as well as your base reactions. Design your members per AISC. Don't forget to check deflection limits since it many times governs with steel.

I don't know why you are embedding any of your posts 3ft into the concrete. This makes your life much harder and offers little benefits. As I previously stated. Assume all bases are pinned not fixed. Put a base plate with anchor bolts on the bottom designed to handle the maximum amount of uplift that will develop.

 

[Johnathan_MEng]

Thank you, STrctPono for the insight you've provided me, I really do appreciate that. I will start on creating a moment frame for each post.

My boss assumed the 3' concrete depth would help, unfortunately the posts not going atop the concrete land in an asphalted parking area. I believe we'll be digging this up to place the posts.

I've calculated the velocity pressure as per ASCE 7, and got q_z=74.8156 psf at 16ft maximum height with unobstructed flow, but this seems slightly low with 155 mph (interpolated the basic wind speed (V)). Does this number seem correct?

 

[dik]

Are you looking at a rigid frame with supports approx 14' tall on one end and 20' tall on the other? If so, HSS 4x4 are pretty slender... you might be looking at HSS 6x6 or even HSS 6x8... Do you have a floor plan... what is the roof construction? snow accumulation loading? wind loading? seismic loading? Don't know about your wind loading... seems high for these areas...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Ron]

@Johnathan....
First, a terminology correction. The "posts" that will be "cemented" in the ground are actually columns embedded in concrete (not cement). Depending on the size of your footing or pier, these may be considered fixed connections and can be used for lateral load resistance (moment frame).

The embedded columns are common for canopy construction. I've done hundreds of them. The columns are embedded in a footing or pier and become fixed connections, assuming your foundation satisfies the overturning moment requirements. They should be analyzed as C&C, not MWFRS. Use the monoslope factors in ASCE 7 for open buildings.