Which seismic base shear V is larger? 1

[StructLin]

One is a steel eccentrically braced frames, non-moment-resisting, connections at columns away from links, the other is special steel concentrically braced frames, if they have same effective seismic weight, which one will have larger seismic base shear?

 

[MiketheEngineer]

I believe the base shear will be the same. It is how the shear is handled by the structure that is different.

 

[Lion06]

I don't have a code in front of me, but just compare R values. If the weight is the same the only difference in seismic force will be the response modification factor.

 

[OldPaperMaker]

Structlin,
It looks like the "R" value for the first Building Frame System is 8 and the value for the second system is 6. Since Cs=(Sds)(I)/R that means that the second system (special steel concentrically braced frame) will have the higher Base Shear.

 

[whyun]

R is 8 for EBFs without moment connections and 7 for SCBFs (ASCE 7-05), thus seismic base shear for the SCBF is larger. The fact that the effective weight are equal does not affect the seismic base shear.

The response modification factor is a function of global ductility of the seismic system selected and the inherent overstrength.

 

[prsconsultant]

Stiffness of the frames would also play role in the magnitude of base shear. Stiffness directly influences the building period which in turn will play a role on base shear.

 

[StructLin]

If you look at ASCE7-05 Table 12.2-1, steel eccentrically braced frames, non-moment-resisting, connections at columns away from links has a value R=7, while special steel concentrically braced frames has a value R=6. If you only consider this, you will get a conclusion that special steel concentrically braced frames has a larger base shear. But if you look at "ASCE7-05, Chapter 12.3.3.4 Increase in Force Due to Irregularity for Seismic Design.....", it requires a 25% design force increase for connections and collectors. My understanding is eccentrically braced and irregularity means same thing, thus it will lead to an opposite conclusion. So which conclusion is correct?

 

[whyun]

Structure Irregularity and Eccentrically Braced are independent and unrelated. An EBF structure can be either Regular or Irregular.

The definition of an Irregular Structure is any structure that is determined to meet one or more irregularity types in Tables 12.3-1 and 12.3-2.

 

[StructLin]

whyun:

Thank you very much for your information. It does helped me a lot and solved my puzzle. But can you please give me some visual example of eccentrically braced frame and still be considered as regularity structure, because I still have hard time to imagine how can "eccentrically" braced be considered as "not irregularity".

 

[whyun]

If you have the AISC Seismic Design Manual (2006), refer to page 3-112 for an elevation view of an eccentrically braced frame. Eccentrically braced just means that the braces do not meet at the center of the beam (in which case we would call it Concentrically Braced). Eccentrically braced frame relies on the ends of the "link" to hinge at the limit state, rather than the brace members to buckle.

You can have a building consisting of eccentrically braced frames (previous codes did not make a distiction whether beam connection at columns away from the link is moment connected or not) laid out such that none of the listed horizontal irregularities exist. This means that your frames are distributed relatively equal on each side of the center of mass, no offsets exist and building is more or less square to rectangular in plan view.

I won't ge into much detail about vertical irregularities but you can control that by making sure mass at each level is lower than the ones below, stiffness and strength of a particular level is lower than the one below, and all the frame bays are full height.

The term "eccentrically" in Eccentrically Braced Frame refers to the eccentricities in the detailing and not in the context of eccentricities for checking the torsion of the overall floor.

 

[StructLin]

Thanks a lot, whyun.

Now I understand "eccentrically braced" is talking about an individual member (column or beam), while irregularity is talking about the whole building. This solved my puzzle completely. Thanks for your help.