A lateral resisting system with knee brace?

[dcceecy]

we are designing a government office building in an area of low seismic and wind loads. so we picked a lateral resisting system with shear only connection between beam and column, and a diagonal brace between them as shown in attached pdf.
I have some questions:

1. is this a right computer model for the framing (in pdf file)?
should I use fixed connection for column to footing?
Does H/400 drift limit for wind sounds right?

2. The wind load will control the design but I still have to add those seismic information in structural notes. so

What is seismic force resisting system per ASCE 7 Table 12-2-1 for such framing?

Building frame system:
steel eccentrically braced frame, non-moment resisting,
connections at columns away from links,
or Ordinary steel concentrically braced frame,
or Steel System not specifically detailed for seismic resistance...?

Thanks.

 

[JoshPlumSE]

If you use R = 3 (or less), then the AISC 341 seismic detailing provisions don't apply. In ASCE 7 this could be referred to as item H. Steel Systems not specifically Detailed for Seismic Resistance, Ecluding Cantilever Column Systems.

Keep in mind that your connections should still be designed to Omega*Eh. Since Omega is the same as your R value, that essentially means you are designing your connections as if your structure remains perfectly elastic during the seismic event.

Therefore, seismic will actually control your connection design whenever your wind loads are less than 3 times your calculated seismic force.... A lot of people miss that.

Josh

 

[asixth]

In reality the base is going to be somewhere between a rigid and pinned connection. You can designed as fixed, but the slightest of rotations which will occur somewhere from the baseplate to the footing will redistribute moments and drastically increase drifts. If checking the storey drifts, I would definitely allow some rotation at the support. Possibly set the spring rotational stiffness to 0.9*4*E*Ic/Lc.

 

[ash060]

You have a knee-braced frame which can only be used with R=3 check the AISC 341 Commentary and they are listed in section C-11

They are considered OMF. Design guidelines are also given

 

[rowingengineer]

dcceecy,
I can't really expand on the answers given above for most of the questions expect to say H/400 sounds a little conservative.

I have done a few frames like this in the past, low seismic region however. However I would not use a HSS or steel angle, reasons, HSS walls are limited in bending and steel angle in compression are not effective.


Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[Ron]

If you have a knee-braced frame, column fixity is not necessary.

 

[frv]

Ron is correct, but you will likely have large drift values if you do not fix the base.

 

[abusementpark]

If you fix the base, you will have to design your foundation for that reaction. Depending on the situation, it could be more economical to use heavy steel sections to lower the drift, rather than use fixed bases. It might cost more to design the foundation for fixity.

 

[haynewp]

Where does it say you have to use omega for the connections when using R=3?

 

[jgeng]

I have a very similar situation. The structure is designed to be free standing inside another structure so no wind load. Although the code requires no seismic design I was going to use the SDC A criteria of Wx*0.01 anyways to provide some lateral stability of the structure(drift will likely be the controlling criteria?). I have a grid of bents just like this supporting a storage area and have the option of putting the beams on top and splicing a short distance from the column or coming into the side of the HSS column as shown on the attachment, any comments on pro/cons of the two approaches? I need to maintain clearance under the structure so no X or chevron bracing can be done. If the beams are siting on top could I design bolted connections to the columun top plate as moment connections? AISC360 talks about this connection but does not have design example? How else could I most cost efficiently provide a moment frame connections? I like the knee braces too but how far done do they need to come to be effective? And as mentioned above since angle don't take compression well what should be used. Would all agree most design assuming steel colums anchored with a baseplate are pinned connections? I am looking to keep costs down as much as possible. I know pipe columns would probably be cheaper but how significant is the difference?

 

[ash060]

jgeng

I have done the moment connection with the beam sitting on top of the column.

I use the procedure in AISC Design guide 16 for flush moment-end plate connections, just change the forces around to meet my conditions. But don't be surprised if you get thick plates and/or large bolts during your design.

I typically only do it when the lateral loads are small otherwise the numbers get out of end

 

[JoshPlumSE]

Haynewp writes:
"Where does it say you have to use omega for the connections when using R=3?"

That's just the basic theory behind seismic design. Though reviewing the AISC Seismic Design handbook, I found the quote (in Part 2: systems utilizing R = 3):

"Members and connections of braced-frame systems with R=3 need only comply with the Specification. The Seismic Provisions are not required for use in the design of braced-frame systems when R is take as equal to 3."

Therefore, it appears that the connection design does not necessarily need to be design for that overstrength force. I'm not sure that I would personally be comfortable with that if it were my project. But, I'm not sure there is a code provision which explicitly prohibits it.

 

[haynewp]

I don't see where it is required either. If that is the theory for seismic design, then there is an omega listed for a lot of systems other than steel for SDC A,B, and C where I don't apply the omega on those connections either. Or do you mean just for steel design?

Do you also apply omega on the connections for R=3 in SDC A and B? I have never heard of this before.

 

[WillisV]

I agree with haynewp - never ever heard of using omega factors for connection design on R=3 systems. It is not required and, in my experience, certainly not done in practice.

 

[bdruehl]

a little off point, but i had a similar design problem although with wood and in a seismic zone for a storage mezzanine in a commercial building. trying to do a timber frame with kneed braces was close to impossible due to having to i think it was 25% of the live storage load (125#) in the design. any success with "timber frames" with braces? actually i think that i found out that a timber frame wasnt even allowed in siesmic E? any experience to share?

 

[DHKpeWI]

See attached file from AISC Design Guide 3 on "Servicabilty." The deflection limits are based on thetype of cladding. Note that for wind most of the deflections limits are based on the 10 Year wind, which is 70%-75% of 50 year wind.

DHKpeWI

 

[prsconsultant]

While using R=3, it is not required to design the connections for overstrength factor. The connections are designed for forces resulting from the governing member designs.

 

[jike]

Check drift for structure under 10 year wind in accordance with the materials of construction. ASCE DG3 recommends H/500 for racking. We would use H/500 for brick or glass walls.

Check drift under strength level seismic loads using amplication factor Cd and compare to ASCE 7-05,
Table 12.12-1.