Do I need roof bracing for this one?

[J189]

There is a two storey structure. First floor is concrete slab and the roof is steel.
Below is the layout of steel roof. red rectangular section means 300x1000 concrete columns from first floor while cyan lines mean steel roof beam on top of concrete columns. purlins run in horiztonal direction.
With a structure like this, do I still need roof bracing? My understanding is I don't have to because with concrete columns under, both ways can be treated as portal frame. Am I wrong?

 

[jayrod12]

I probably wouldn't consider them a portal frame without the appropriate attention to detail paid to the connections of the steel to the columns. You would also need beams in the other direction.

You said 2 storey structure, perhaps you could consider the columns cantilevering past the floor to pick up the roof.

 

[KootK]

Your closely spaced lateral load resisting systems will mean that your diaphragms will be small and, thus, perhaps suited to a diaphragm construction less substantial than horizontal bracing. You'd still got diaphragms, however, and they deserve some manner of attention.

 

[J189]

Thanks jayrod12 and KootK.
May I ask why you dont consider them portal frames? The columns do run from first floor to roof. I have added the connection of steel beam to column. (no moment into columns though). Any comments on the connection?

I forgot to draw the fascia truss. please see the new picture below. green lines means fascia truss. with 200 PFC chord I would think they are okay for lateral load without internal strut and bracing?

 

[jayrod12]

You can't have a portal frame if there's no moment connection between beams and columns. What you are describing is a cantilever column system. A portal frame utilizes moment transfer between beams and columns to transfer lateral loads.

 

[J189]

Hi jayrod12. I get what you mean. You are right it is not portal frame ( I can have a moment connection to the concrete column but I don't think it is required here). What I was trying to ask was, as all the internal concrete columns below can provide lateral resistance in both direction, do we still need roof bracing?

 

[human909]

Hi jayrod12. I get what you mean. You are right it is not portal frame ( I can have a moment connection to the concrete column but I don't think it is required here). What I was trying to ask was, as all the internal concrete columns below can provide lateral resistance in both direction, do we still need roof bracing?

You are the engineer. Your need to answer that with rational analysis.

In the y direction the answer might be that you don't need bracing if you have sufficient moment capacity in the columns and foundations.
In the x direction the answer is likely that you do need bracing but again you need to check the loads and capacity yourself.


To be explicit. How are wind loads in each direction being transfered to the ground?