Aci 21.1.1 Energy Dissipation Confusion

[Waqas Haider]

Assalam o alaikum Dear friends,

I have two questions. I am to detail SMRF for zone 3. I was reading ACI 318-11 chapter 21. Now i have some confusion.

1) 21.1.1 says, ........................................For which, design forces , related to earth quack forces, have been determined on the bases of ENERGY DISSIPATION IN NONLINEAR RANGE OF RESPONSE.

My question is What is meant by energy dissipation in non linear range of response??

What i have get through my research and understanding is,

When Earth quack comes, due to inertia of building, it produces lateral forces in building. Due to these forces, structure deflect. If members of structure have big amount of reinforcement and cross sections, the force (or in other words energy) exerted by earth quack to the building will not yield the members and structure and members will restore to initial positions. In this case, members will not yield, and will remain in ELASTIC RANGE. But this makes structure too costly. On the other hand, if members have ability to yield but not fail, this energy will be used to yield the members and hence much of this energy will be dissipated in non linear range of members response. And we design structures for this because this is does not make structures too costly. So we design structure in the 2nd way. But due to this energy dissipation , structure is no longer in use at service stage and needs some or much requirement. We compromise over serviceability stage upto some extent and does not compromise over limit stage.. Is my concept right???? Is this the mean of energy dissipation in non linear range of response??

2) Commentary of R 21.1.1 says,

The integrity of the structure in the inelastic range of response should be maintained because the design earth quack forces, defined in documents such as ASCE/SEI 7, the IBC, the UBC and NEHRP provisions are considered less than those corresponding to linear response at the anticipated earthquack intensity.

What does it mean??

Jazak Allah...

[Syed Umair Haider]

W salam,

 

These concepts need some fundamental theory of development of seismic analysis procedures (static procedures) in codes as briefed below,

 

When engineers decided to go for an earth quake resistant design,then they initially proposed to assign a horizontal load of "0.1 x Weight of structure"  to cater for seismic forces.With the passage of time several geo-technical and site specific response characteristics were included in analysis for evaluation of seismic forces and structural members were designed to resist these forces in their elastic range i.e to not yield under these forces.

 

Structures designed accordingly surprised engineers, as they were observed to show little tolerable non structural damages in seismic events considerably greater then those considered in evaluation of seismic forces.This leads to the development of concept of energy dissipation and over strength factor i.e under cyclic seismic loading structures have the ability of resistance beyond the elastic range of stresses in members (after yield), in proportion to their ductility.Since then started consideration of this structural over strength characteristics that consists in reduction of design seismic forces in accordance with their energy dissipation characteristics or mathematically reduction of base shear by division with over strength factor.

 

For ex in accordance with UBC97, if on a structure the actual seismic force i.e Cv.I/W=550 & over strength or ductility factor is R=5.5 then Seismic shear will be 550/5.5 = 100, then structural members will be designed to remain elastic or not yield under the lateral force of 100, whereas they will dissipate the remaining 450 in inelastic range or in terms of energy it can be said that this structure is able to dissipate 450/550x100 = 81% seismic forces through its ductility and is required to design elastic only for 19% of actual seismic forces.

 

In the lights of above these clauses could be defined as follows,

 

") 21.1.1 says, ........................................For which, design forces , related to earth quack forces, have been determined on the bases of ENERGY DISSIPATION IN NONLINEAR RANGE OF RESPONSE".

 

 For every structure,seismic forces are evaluated in accordance with corresponding over strength factor that indicates the extent of probable energy dissipation.

 

2) Commentary of R 21.1.1 says, 

The integrity of the structure in the inelastic range of response should be maintained because the design earth quack forces, defined in documents such as ASCE/SEI 7, the IBC,  the UBC and NEHRP provisions are considered less than those corresponding to linear response at the anticipated earthquack intensity. 

 

As defined above, structures are designed for seismic forces that are reduced by over strength factor however actual fores are times greater than that, therefore code requires that when structure is subjected to actual seismic forces(plastic state),then although structural damages in members are tolerable but integrity of structural members should necessarily be maintained so that structure will not collapse.This condition is another form of philosophy of safety in code under seismic events that says "Under major earth quake,structure should be designed to have structural & non structural damages but should not collapse".

[Waqas Haider]

Really thanx a lot brother. You have presented wonderful explanation. Last day i have searched a lot over it and found a very useful document which challenges practice of reduction of forces due to this over strength. i m unable to attach the document at the moment. i am unable to find option of attaching. i will attach later. After reading your explanation, now i m clear completely. Jazak Allah.

[Waqas Haider]

This is a critical analysis about approach of codes to reduce seismic forces in terms of over strength factor. 

overstrength theory.PDF

[Muneeb Badar]

Dear  Mr. Waqas,

 

Your study about Non Linear Analysis is quite impressive. I just want to add some more details.

 

There are three types of design.

1- Service Level Earthquake (SLE)

2- Design Basis Earthquake (DBE)

3- Maximum Considered Earthquake (MCE)

 

SLE is at service level. Which means that in EQ there should be no problem for non structural members.

DBE: In code based design, we use DBE level earthquake. This earthquake is frwquent earthquake with return period of 475 years( I think). During this code uses actualy 100% forces and reduce these forces by reduction factor R. Means we design our structures for reduced forces as explained before. Actually in this case code allows the structure to yield at certain locations for certainm members. But code do not specify these locations and members as R is the overall division factor. So during an earthquake it is not necessary that the members will yield which we assumed to be yield. For example if beam at any location yielded then it will not transfer forces to the column. But we are not sure that column will yield first or beam will yield first. Code tried to take this by incorporating the stiffness modifiers but still it is not 100% reliable. So thats why nobody can gurrantee the building to perform 100% in earthquake when designed by Code.

 

MCE: This is the third and extreme level of earthquake.Return period is 2475 (I think). This can only be incorporated by using Non Linear Time History Analysis. In this we can check the reinforcement designed on DBE level for capicity check by considering the actual yielding and energy disspation. We can set a limit for structure for example Life Safety, Immediate occupancy or collapse prevention. We allow the memebrs to yield upto acertain level and beyond that level we ll not allow the structure to yield. We can actually see that which member is yielding and the after yeidling what is the force and design for that force.

 

The difference is that in code based design we aare not sure how much energy is dissipated in the structure. Code just provide 2 or 3 types or R factors depending upon the building type. So ist ios not 100 % accurate. Secondly code reduces the forces for all modes of structure but actually it can not happen. We can not reduce all modes because we can only reduce the modes which are yielding.

 

 

So thats why 21.1.1 says that integrity should be maintained. But code do not give any gide line how to maintain that integrity. By using that R factor??? how can we do that????? So code just wrote that thing.

Actually that integrity can only be assured by using the Non Linear Time Histoiry Analysis and stops the stucture to deform beyond which we do not require to deform.

 

Sorry if I write something out of topic.

 

Thanks

 

Muneeb

[WR1]

Very well explained dear Muneeb!

[Waqas Haider]

Thank you Muneeb for sharing your knowledge. It really helped me a lot in clearing my confusion. 

[UmarMakhzumi]

 

 

The integrity of the structure in the inelastic range of response should be maintained because the design earth quack forces, defined in documents such as ASCE/SEI 7, the IBC, the UBC and NEHRP provisions are considered less than those corresponding to linear response at the anticipated earthquack intensity. 

 

 

There are different ways in which your question can be answered. I will try to explain how existing code provisions team up to satisfy statement posted above or how the analysis steps taken by a Structural Engineer work to meet the 21.1.1 Clause. Here is the response in-terms of code clauses enforcing integrity:

 

1) The seismic-load-resisting system must have sufficient strength to limit ductility demand. R implies level of accepted ductility but code also imposes maximum period to be used for strength check T< C_uT_a.  The reason: Over-estimate of period results in lower required strength. This indirectly puts a requirement for minimum strength. So code is enforcing minimum strength.

 

2) Similarly, the seismic-load-resisting system must have sufficient stiffness to control drift. Drift control  indirectly puts a requirement for minimum stiffness. Code wants you to have minimum stiffness.

 

3) Critical elements are required to be designed for amplified load. (Use of Omega for collectors, elements supporting discontinuous frames etc. See AISC 341 for more detail). This is damage control.

 

4) The seismic-load-resisting system must have sufficient integrity to prevent separation of elements and components. This is satisfied by ensuring deformation compatibility and designing all elements for tie/ anchorage forces.

 

5) Stability is satisfied by requiring the seismic-load-resisting system to have sufficient strength and stiffness to ensure that second order effects are not significant. Second order check is performed. See equation below.

 

 

This ensures that even under large seismic displacements, we have sufficient ability to resist seismic load.

 

When doing your structural analysis to code/ ASCE 7, you are indirectly striving to achieve integrity that would satisfy much higher forces than what you have designed for. The above points just provide summary of some of the checks built in the code to ensure that.

 

Thanks.