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Waqas Haider

Structural Systems according to UBC-97

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Assalam o alaikum.

My building has special moment frames and 3 to 4 shear walls. The bay where shear walls are placed, no column is in the bay and beams are coming directly at top of shear wall and for the very specific bay, slab is also resting directly at shear wall at that location. How to decide in which category, exactly, my structure comes? UBC structural systems are

1629.6 Structural Systems.
1629.6.1 General
. Structural systems shall be classified as one of the types listed in Table 16-N and defined in this section.
1629.6.2 Bearing wall system. A structural system without a complete vertical load-carrying space frame. Bearing walls or bracing systems provide support for all or most gravity loads. Resistance to lateral load is provided by shear walls or braced frames.

I think my structure does not lie in this category because majority of slab and walls are resting at beams connected with columns. Am i right?
 

1629.6.3 Building frame system. A structural system with an essentially complete space frame providing support for gravity loads. Resistance to lateral load is provided by shear walls or
braced frames.

Is this structure is where no lateral load is resisted by frame and shear walls resist lateral loads? How is it possible that no lateral load is resisted by frames?


1629.6.4 Moment-resisting frame system. A structural system with an essentially complete space frame providing support for gravity loads. Moment-resisting frames provide resistance to lateral load primarily by flexural action of members.

If I have 7 bays in a direction at a grid and I have 5 to 6 grids in building. An only end grids have shear walls in one or two bays, Will it be considered as building MRF or Dual system? What is exact differentiation between them?


1629.6.5 Dual system. A structural system with the following features:
1. An essentially complete space frame that provides support for gravity loads.
2. Resistance to lateral load is provided by shear walls or braced frames and moment-resisting frames (SMRF, IMRF, MMRWF or
steel OMRF). The moment-resisting frames shall be designed to independently resist at least 25 percent of the design base shear.
3. The two systems shall be designed to resist the total design base shear in proportion to their relative rigidities considering the
interaction of the dual system at all levels.

Is there any limit that at least this much of base shear must be resisted by shear wall to consider it as Dual system otherwise it is MRF?

Thanks.

 

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Your system most probably gonna fall under 'building frame system' = lateral from walls + gravity from frame

Remember that, mere presence of frame with walls does not make the system as dual system or whatever...inertia of columns is usually so small to that of walls. A building could have very few walls and many columns but still the columns resist 0% of lateral loads. It depends on relative stiffness.

Run the analysis as is wit any R factor, check the horizontal reactions for walls and columns. See how much percent they resist. You would notice in such type of structures that lateral resistance by walls is around 70% and rest is from columns. Check these numbers. If it is walls=100% and cols=0% for lateral and opposite for gravity, its an ideal building frame system. If however columns also resist 'few percentage' (see SEAOC) of lateral and walls resist 'few percentage' of gravity then there is an additional task to perform to make sure it is still a building frame system.

Conceptually you have to make sure that, during an earthquake if walls are resisting portion of gravity load, they should possess this capacity (do not lose load carrying capacity during an earthquake). If so the framing is still a building frame system.

 

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It should be dubbed as building frame system.

The relative stiffness of vertical members of lateral load system and their strength and ductility decides the category of structural system.

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On 1/5/2018 at 11:44 AM, asadishaq said:

If the frame is located in zone 3 or 4 it should be SMRF, and if it is in zone 2 it should be IMRF.

Really?

Moment frame type has nothing to do with location except some limitations in codes. Can't you have special moment frame in Zone 2? Yes, you can!

Type depends on the ductility and framing of building system. Do you think it has enough ductility? If yes then it could be special even in Zone 0. But obviously it will cost more to provide that ductility and detailing for zone 0.

But generally speaking, you could design special frames in zone 2B or ordinary in higher zones, depends on the framing.

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On 25/01/2018 at 2:31 PM, wakas said:

And how we decide seismic design category for zone 2B?

There is no seismic design category in UBC. Its simply zone 2B in UBC.

What you probably meant is the equivalent seismic design category of ASCE 7 related to zone 2B of UBC. If so, then you have to convert your zone 2b spectra to ASCE 7 spectra and to find out S1 and Ss. After that you can find out the sdc from ASCE tables based on this S1, Ss and Fa and Fv.

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Adding my cent, Basically R ductility factor and detailing of the structure matters, due to greater ductility demand in higher seismic zones 3,4, R value is high also detailing requirement is high to make the structure capable to dissipate energy. UBC  defines some restrictions as per different zones, in 2B IMRF minimum, and zone 3,4 SMRF. in 2B SMRF can be designed but in 3,4 it will be too uneconomical. we can violate code but for that greater effort for design is required, have to go for performance based design but it is better to hide behind the code.

Regards

 

 

 

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