MOMENT FRAME 2007 CALIFORNIA BUILDING CODE 1

[nrguades]

Hi to all..

It seems like LADBS has not posted recent information bulletins that are based on the new codes. Is the Steel Moment Frame Connection Design will be based on FEMA 350 or with the latest AISC 341,358 & 360.

thanks..
Noel

 

[whyun]

2007 CBC Section 2205A.1 requires that general steel design shall be in accordance with AISC 360.

2007 CBC Section 2205A.2.2 requires that seismic requirements for seismic design category D and higher shall be in accordance with AISC 341.

AISC 341 Section 9.2b allows the use of AISC 358 as one of the options for beam-to-column connections.

FEMA 350 is obsolete. I do not have the latest COLA building code yet but it will adopt the above with their modifications, if any.

 

[nrguades]

Thanks whyun for the info...

We need to update our moment frame connection design spreadsheets then.

Noel

 

[nrguades]

Hi Again..

I have additional questions regarding the new code.

LADBS Interim Bulletin, "Method for distribution of lateral forces in wood frame buildings assuming flexible diaphragm", it says that diaphragm can be assumed flexible by complying some listed conditions, one of which is the shear panel and holdowns shall be designed to exceed the design load by at least 20%.This bulleting is referenced to the older code.

There are no latest interim reports published by LADBS yet on their website regarding this issue. But on ASCE 7-05 section 12.3.1.1, Diaphragms of wood structural panels or untopped steel decks in one-and two family residential buildings of light frame construction shall also be permitted to be idealized as flexible. Section 1613.6 of IBC 2006 also added conditions for assuming flexible diaphragms. No mention of 20% increase.

Can we can disregard 20% increase on two storey residential building for assuming flexible diaphragm on light frame construction and also for buildings having rigid analysis calculations?

Thanks,
Noel

 

[whyun]

The intent of the 20% increase is to account for the high likelyhood that a wood diaphragms in residential structures will behave like a rigid diaphragm due to the fact that the overall diaphragm dimensions are relatively small and diaphragm spans are short. It is a "dumb" shortcut to avoid doing both flexible diaphragm analysis and rigid diaphragm analysis and designing for the greater effect of the two.

When you take the envelope approach, there is no need for additional 20%. When you can justify that the diaphragm is flexible (per code definition), there is no need for additional 20%. When you have only the flexible diaphragm analysis results without justifying that the diaphragm is flexible, you need to apply the additional 20% to account for the uncertainty that it may be more rigid than assumed.

I have not seen the latest LADBS IB's on this matter so we'll just have to wait and see.

 

[nrguades]

Thanks Whyun...

It looks like if we will not check the redundancy per ASCE 7 section 12.3.4.2 where (p) can be taken as 1.0, then we need to multiply our horizontal seismic forces by 1.5 (from 20% flexible diaphragm assumption & 1.3 redundancy factor) to get the wood shearwall sheathing/nailing type. Is this correct?

Regarding AISC 358-05, there are only two Pre-qualified Connections covered at present for Special and Intermediate Moment Frame Connection. One is Reduce Beam Section and the End Plate Connection. The Welded Flange Plate is not yet included. Is Welded Flange Plate is still allowed to be used for intermediate moment frame connection considering that on page 6.2-73 of AISC 358 on the References and All Connections, FEMA 350 is listed as a reference in consideration to Appendix P of AISC 341-05.

Thanks...

Respectfully,
Noel

 

[whyun]

Redundancy factor and the issues discussed in my previous posting are independent and unrelated.

On the issue of moment connections, FEMA 350 is one of the documents reviewed by AISC as a basis for writing the Standards contained in AISC 358-05. Building officials will enforce the Standards adopted by the building code. Any obsolete standards, technical papers or articles are not enforceable.