Seismic Force Percentage for Dual System, Building Frame System and Shear Wall Frame Interaction

[MAB70]

Hi all,

I have three question with regards the lateral resisting systems in the ASCE7;

1. When you use dual system of moment frames, it says to be capable of resisting 25% does that mean it doesn't necessarily need to have enough stiffness to attract 25% of the load and only as long as its designed for 25% ? or shall i increase the stiffness of these frames so naturally attracts 25% of the load?

2. When you use building frame system using the shear wall option, in any building naturally the shear wall cannot represent 100% of the lateral stiffness of the building so what is the minimum acceptable percentage of lateral force that it needs to attract.

3. Point 2 leads to this one as what will be the difference between building frame system (shear wall) VS Shear wall frame interactive system, since you will have to have other components in the building taking load anyway.

Regards,

 

[r13]

Isn't this clear enough - you shall ensure the moment frame consists of at least 25% of the total rigidity of the seismic resisting system. The rigidity of the moment frame can be more (to share more force), but no less than the thread hold.

Dual System:
For a dual system, the moment frame shall be capable of resisting at least 25 percent of the design forces. The total seismic force resistance is to be provided by the combination of the moment frame and the shear walls or braced frames in proportion to their rigidities.

Combinations of Framing Systems:
Different seismic-force-resisting systems are permitted along the two orthogonal axes of the structure.

 

[KootK]

1) The stiffness does not need to be adjusted to attract 25% of the load. It would be pretty difficult to make a moment frame system that stiff in most cases.

2) I don't know that there is a hard %stiffness target in ASCE7 or ACI. In high seismic zones you don't have to worry about this because your gravity framing must be designed to accommodate the drift of your designated lateral system. Fact check me on this because it's been a while but I think that older versions of either ACI or the PCA handbook used to suggest that the shear wall story stiffness ought to be at least six times the sum of all the column story stiffnesses within a story in order to ignore the lateral effects on the gravity system.

3) I believe that the dual system gives you a break with respect to some of the seismic load calculation factors owing to that system's greater redundancy relative to single VLRS systmems.

 

[r13]

I think the issue here is that, if the 25% criterion is not met, you are forced to pick another seismic response factor, then distribute the force according to structure rigidity as usual.

 

[HTURKAK]

Mr KootK explained case..I just want to add the general criterias to classify the SFRS. ASCE 7-16 Table 12.2-1 shows the Seismic Force-Resisting Systems and relevant Design Coefficients and Factors. Regarding ur querries;

1= In order to classify the SFRS as Dual system, the moment resisting frames shall carry at least 25% of the seismic force. Notice that , the structural walls are the primary LRFS. If a SFRS composed of structural walls and MRFs and MRFs carry less than 25% , The system shall be assigned as Bearing wall system.

2= The answer is , at least % 75 . The structural walls shall be primary SFRS and frames are primarily for gravity loads.

3= The same building can have different SFRS along the two orthogonal axes of the structure. The relevant R ,Ω o , and
C d will be different at two directions. If interaction exists , that members and connections shall be designed for the maximum force and effects.

 

[MAB70]

@KootK, thanks here are my replies;

1. If that is the case then theoretically if you have only one frame in a big plan that is not stiff but designed for 25% of the force shall be considered as dual system? that doesn't seem right to me. Why would the code ask you to design for 25% if the frame itself in the event of the seismic is not stiff enough to attract these loads.. in some cases it might even attract 1/tenth of that value (theoretically). Actually designers might take advantage to increase the response modification factor and only add one local frame and design it for 25% as if the whole system will work as dual.

2. Makes sense if that would be the case which represent almost 85% of the force.

3. If we follow the logic in item 2 then yes i agree with you.

 

[MAB70]

@retired13 ,

your quote ''you shall ensure the moment frame consists of at least 25% of the total rigidity of the seismic resisting system'' is not what is mentioned in the ASCE. It says clearly that it should be capable of resisting ( The word capable is what confused me) is it refereed to the stiffness or design force?

Kootk replied that is refereed to design forces which contradicts your understanding also.

 

[MAB70]

@HTURKAK,

1. I agree that withouth th 25% you shouldt consider it dual but i disagree that you have to go for bearing wall system, you still have the building frame system which defined in ASCE as following: ''A structural system with an essentially complete space frame providing support for vertical loads Seismic force resistance is provided by shear walls or braced frames'' so you can use only a shear wall system for lateral resistance under building frames for vertical support.

2. Where does it states the 75% for using only shear walls under building frame system ? the only one mentioned is when you combine it with the original moment resisting frames (Shear wall interactive System).

3. My question is not related to orthogonal seismic system directions, interaction can occur in each single direction.

 

[r13]

I don't understand your point of view. The source of the quote below is linked for your information. Link

In mid-to-high rise structures, dual systems (DS) enable a structural designer to satisfy the stringent drift limitations of current codes without compromising ductility. Currently, ASCE 7-05 permits a variety of structural systems to be used in combination as a dual system yet the design requirements are limited to the following: Moment frames must be capable of resisting 25% of the seismic forces while the moment frames and braced frames or shear walls must be capable of resisting the entire seismic forces in proportion to their relative rigidities. This paper assesses the significance of the 25% design requirement for the secondary moment frames (SMF) in dual systems with consideration of current structural engineering practice. Three 20-story buckling-restrained braced frame (BRBF) dual system structures have been designed with varying relative strengths between the braced and special moment frame systems. The SMF system has been designed for 15%, 25%, and 40% of seismic demands and the BRBF system design has been adjusted accordingly based on its relative stiffness with respect to the moment frame. These structures have been examined with nonlinear static and nonlinear dynamic procedures with guidance from ASCE 41-06. The drift, displacement and ductility demands, and the base shear distribution results of this study show similar responses of the three prototype structures. These results indicate a secondary moment frame designed to less than 25% of seismic demands may be adequate for consideration as a dual system regardless of the 25% rule.

 

[HTURKAK]

Dear Mahmoud Belal (Structural)(OP); your querry no. 2 defines dual system with shear walls with MRF 's. If this is a real question, pls share the structural plan and details ( RC with shear walls or Steel str with braced frames) so we can discuss.

%75 percent is implicitly stated. In order to define dual system, MRF's shall carry at least 25% of the seismic force. That is, if the MRFs carrying less than 25% , the SFRS shall be classified Structural WALL system...ASCE definition is BEARING WALL SYSTEMS or SHEAR WALL-FRAME INTERACTIVE SYSTEM.

 

[KootK]

1. If that is the case then theoretically if you have only one frame in a big plan that is not stiff but designed for 25% of the force shall be considered as dual system? that doesn't seem right to me. Why would the code ask you to design for 25% if the frame itself in the event of the seismic is not stiff enough to attract these loads.

The key to this is to have an accurate understanding of the philosophy behind the dual system approach. It works like this:

1) Firstly, the shear walls and moment frames must resist the regular design earthquake acting together, with forces apportioned to the two systems in accordance with stiffness principles. There is no requirement for how much force goes to either system other than it be consistent with stiffness and deformation compatibility.

2) After the design earthquake is over, it is expected that:

a) The moment frame system will be in pretty good shape because it was probably shielded from serious damage by the stiffness of the shear walls.

b) The moment frame system will be capable of independently stabilizing the building under the action of gravity loads (not seismic loads). The 25% value is largely rooted in judgment, rather than theory, and is intended to provide enough stability that the building won't flop over just under gravity weight while giving some allowance to the fact that, when the shaking has stopped, the building may no longer be plumb.

In summary, relative stiffness is irrelevant for the event for which the 25% force is intended because, when that event occurs, the moment frame is assumed to be acting alone and without the benefit of the shear walls contributing.

 

[r13]

Here is another paper discussing "dual system", Link

1. Introduction
A dual system is a structural system in which an essentially complete frame provides support for gravity loads, and
resistance to lateral loads is provided by a specially detailed moment-resisting frame and shear walls or braced frames.
Both shear walls and frames participate in resisting the lateral loads resulting from earthquakes or wind or storms, and the
portion of the forces resisted by each one depends on its rigidity, modulus of elasticity and its ductility, and the
possibility to develop plastic hinges in its parts. The moment-resisting frame may be either steel or concrete, but
concrete intermediate frames cannot be used in seismic zones 3 or 4. The moment-resisting frame must be capable of
resisting at least 25 percent of the base shear, and the two systems must be designed to resist the total lateral load in
proportion to their relative rigidities.

In the dual system, both frames and shear walls contribute in resisting the lateral loads....

 

[KootK]

Actually designers might take advantage to increase the response modification factor and only add one local frame and design it for 25% as if the whole system will work as dual.

And I would encourage them to do so with one caveat:

I'm not sure if this has been clarified in recent editions of ASCE7 but, at one time, there was debate about whether or not this provision should be applied to the entire building or to each braced frame line independently. In my opinion, it should be the later in order to preserve building torsional stability. One might be able to achieve the same thing with fewer moment frames than braced framing lines if they deliberately distributed the moment frames about the building plan with an eye towards torsional stability but, as far a I know, that's not how the code is written.