Strong column weak beam - SCWB

[FOX89]

Hello Folks,

For the SMF seismic design requirements, the strong column weak beam approach is required at the connection zone per AISC 341 and 358 which makes me under impression that it is not required for the full length of column to be stronger but only the portion at the connection zone, am i getting it right ? and if so, what extent above and below the connection shall be under this requirement. I have navigated in FEMA but couldn't find any clear requirement.

 

[KootK]

For the SMF seismic design requirements, the strong column weak beam approach is required at the connection zone per AISC 341 and 358 which makes me under impression that it is not required for the full length of column to be stronger but only the portion at the connection zone, am i getting it right ?

Not exactly right. For the spirit of SCWB to remain in tact, plastic hinges must be prevented from forming anywhere along the columns (other than roof level columns in some cases). Column lateral moments typically are at a maximum at the ends and diminish towards mid-height. You could, conceivably, allow your column strength to drop off in the same proportion while still preventing plastic hinges anywhere along their length.

 

[FOX89]

Sure kootk, since the moment diminishes toward the mid-height, plastic hinges will not happed at this location, but only at the maximum moment zone. So can i scale the column moment diagram by Mp(beam)/Mu(beam), reinforce the column till reaching the zone where these moments are equal. Refer to the below diagram.

 

[FOX89]

Any thoughts folks ?

 

[HTURKAK]

AISC Seismic defines the strong column- weak beam concept as the ratio ΣMpc / ΣMpb > 1.0
Where ,
ΣMpc= the sum of the moments in the column above and below the joint at the intersection of the beam and column
centerlines.

ΣMpb = the sum of the moments in the beams at the intersection of the beam and column centerlines.

In a sense , ΣM pc is the summation of the nominal flexural strengths of the columns above and below the joint to the beam centerline.
I could not see your aim . Are you thinking the use of tapered columns?

 

[FOX89]

Yes, either tapered section or heavier section at the connection zone with reasonable extent above and below the beam since i am dealing with double height stories. This will help in optimizing the steel tonnage.

 

[HTURKAK]

This will help in optimizing the steel tonnage.

Not sure this will help to optimize the cost of steel construction imo , for at least two reasons
- In this case ,the manufacturing cost of tapered sections will be higher than the rolled sections . The common practice , tapered sections are used at PEMBs .
- Drift control would be problem and eventually you would need to increase the sizes or use rolled sections.
You are free to study and make a design then analysis the cost for two variants.
This is an interesting topic and i would appreciate if you share your findings .