Transfer Diaphragm

[whyun]

I am looking at building to be designed under 1998 California Building Code Volume 2B (1994 UBC with California amendments).

Building consists of a core tower say 130 feet by 155 feet - 5 stories. At the Main Level (1st floor), the building footprint is larger. Say 30 feet southward and 30 feet eastward from the core. From the main level, we have lower levels called ground, basement, mechanical 2 and mech 1 levels. Mech 1 level being the lowest level. Building sits on a slope - slope goes downwards in the southeasterly direction.

For simplicity, let's say the lateral system is concrete shear walls full height. Diaphragm is composite metal deck.

At the main level, under seismic loading, there will be a shear transfer through the diaphragm extension.

1994 UBC requires the seismic forces be increased by 3Rw/8 if supporting a discontinuous lateral system sych as columns supporting shear wall above or for transfer diaphragm that transfer seismic shear from lateral elements that are offset planwise.

My questions is: Does the diaphragm have to be designed for 3Rw/8 for the main level for my building? Due to the large rigidity of the perimeter walls, significant amount of shear will be transfered thru the 30ft diaphragm extension and forces in the perimeter of the core will "unload". My concrete shear walls at the cores are continuous all the way to the Mech 1 level thus there is no discontinuity but perimeter walls being so rigid, it will behave like a plan irregularity #4.

Anyone who can still remember the 1994 UBC may have some input. My gut feeling is to use 3Rw/8, regardless. Thank you all in advance.

 

[Taro]

1994 UBC Section 1629.7.2 requires only that columns supporting discontinuous lateral load-resisting elements in Seismic Zones 3 & 4 be designed for 3Rw/8 overstrength forces. Diaphragms are not included in this requirement. Section 1631.2.9.2 gives the design force for diaphragms. No 3Rw/8 factor is included for the transfer forces. Of course, the code is only a minimum standard. You may exceed the minimum code requirements if you think it is necessary and the owner is willing to pay for it.

 

[whyun]

Thank you Taro, for your comment. I've re-read the section 1628B.7.2 and found that the requirement is only for the Columns supporting discontinuous lateral systems and/or offset of lateral systems.

In the 1997UBC (Or 1998 CBC Volume 2A), section 1630.8.2.1 talks about a similar situation to be designed under special load combination (omega level forces). This section is not limited to columns. This can also be apply to transfer diaphragms.

Is it correct to conclude that transfer diaphragm design provision in the 97 became more strict compared to 94?

Now, I am inclined to disregard the 3Rw/8. First of all, the condition i have does not have neither vertical nor horizontal irregularity as described in 94 AND 97 codes. However, the magnitude of the force thru this diaphragm is significant and it behaves as if it possessed those irregularities.

Thanks again Taro.

 

[Taro]

I don't think the overstrength factor is required for design of the diaphragm. It is to be applied to the overturning forces only on the vertical members beneath the discontinuous lateral elements. The intent is to prevent local collapse of the vertical load carrying members due to actual forces that may be developed in the lateral elements above. The "column" wording was changed in recognition that transfer girders and other elements may also support discontinuous lateral elements. The SEAOC blue book commentary more explicitly explains that it is the overturning forces that are to be magnified.

 

[whyun]

Thank you so much for your insight. There seem to be some differences in opinion within my company amongst engineers. i would have to read the seaoc blue book and get more clarification on the code's intent. Also, I find that many engineers encounter plancheck corrections where they require the special combination when not necessary. I suppose it is our duty to clearly understand the code's intent rather than being "blindly conservative" when in doubt.

More discussions to follow... hopefully soon. Thanks again Taro

 

[dougantholz]

Whyun,
If your diaphragm is acting to transfer shear from one lateral system to another, I would be tempted to classify that as a collector element. Because it is not acting like a diaphragm to only transfer that story force but also forces from one element to another. It would work the same if you had a horizontally offset shear wall. You would put a drag strut at the end of the wall to drag the force into it from the diaphram.

 

[whyun]

My situation is a bit ambiguous in that technically I can't classify it as a vertical irregularity nor horizontal irreguarity as defined in 97UBC 1630.8.2.1. My shear wall at the edge of the core is relatively short but continue to the foundation, thus no vertical discontinuity. This automatically eliminates the horizontal offset thus no plan irregularity.

Perimeter walls are thinner but pretty much solid all around with some openings so much more rigid than the interior shear walls that are only one bay length. So tremendous amount of shear will be delivered to the perimeter wall via what I call "transfer diaphragm". Then again, this is not a good term because it is just a diaphragm that transfers the shear redistribution only.

I would treat the beams that collect and deliver the forces into the core edge shear wall as being "collectors".

For the project, which is per 98CBC volume 2B (based on 94UBC) the special combo is intended only for columns only and transfer diaphragm need not apply.

Under the assumption that "transfer diaphragms" were intended to be designed for omega forces (which i will verify later on) I was wondering if a non-irregular diaphragm transfering the shear redistribution need to be designed for omega. I am inclined to think not.

This brings about another topic of whether shear stays in a wall in a multilevel ridig diaphragm structure. Traditional hand calcs and "stacking" does not produce accurate distribution of shear. Applying the total story force at accidental eccentricity away from the cummulative mass seems to be the preferred method for static design in many softwares out there...

More to come...