Stiffness Modifier

[rummaan17]

Dear Members,
I would like to have your feedback on the use of stiffness modifier. We generally use the value dictated by ACI committee as to model the cracked behavior, which are 
Compression Member : Wall, Column, Pier = 0.7
Flexure Member : Beam, Spandrels = 0.35
Slab = 0.25

What if i want to use 0.5 for columns or 0.2 for beams. Are there any restrictions. I have played around with it on software like ETABS and noticed change in flow of forces as it transfer towards more stiff element.  Your kind comments would be highly appreciated. 

Thanks

[Badar (BAZ)]

Use these stiffness modifiers to satisfy the drift requirement of the code. You shouldn't use the values lesser than the ones provided by the code; they are already on the conservative side.

Do not use these modifiers to check the strength demand of individual members. Make another model, without applying any modifier, to check the strength-demand. These stiffness modifiers will result in a design in which you would under-estimate the demand in beams, and over-estimate in columns.

The value of stiffness modifier for the wall depends on weather the wall is in tension, or not. If the edge of the wall is in tension, then the modifier is 0.35.

[UmarMakhzumi]

15 hours ago, baz said:

The value of stiffness modifier for the wall depends on weather the wall is in tension, or not. If the edge of the wall is in tension, then the modifier is 0.35.

Please shed more light on this

[Badar (BAZ)]

This is from the commentary of code: " If the factored moments and shears from an analysis based on the moment of inertia of a wall, taken equal to 0.70Ig, indicate that the wall will crack in flexure, based on the modulus of rupture, the analysis should be repeated with I =0.35Ig in those stories where cracking is predicted using factored loads."
 

[Sohaib Iqbal]

how we can determine in etabs either wall is in compression or tension?

[EngrUzair]

2 hours ago, baz said:

This is from the commentary of code: " If the factored moments and shears from an analysis based on the moment of inertia of a wall, taken equal to 0.70Ig, indicate that the wall will crack in flexure, based on the modulus of rupture, the analysis should be repeated with I =0.35Ig in those stories where cracking is predicted using factored loads."

The referred code clause is Section R10.10.4.1 in Commentary portions of both ACI 318-08  as well as ACI 318-11.

[Badar (BAZ)]

8 hours ago, Sohaib Iqbal said:

how we can determine in etabs either wall is in compression or tension?

Use the principle of superposition to calculate the combined effect of moment and axial force (P/A +- My/I).

[UmarMakhzumi]

8 hours ago, baz said:

Use the principle of superposition to calculate the combined effect of moment and axial force (P/A +- My/I).

@Sohaib Iqbal,

You need to compare this number with modulus of rupture.

18 hours ago, baz said:

This is from the commentary of code: " If the factored moments and shears from an analysis based on the moment of inertia of a wall, taken equal to 0.70Ig, indicate that the wall will crack in flexure, based on the modulus of rupture, the analysis should be repeated with I =0.35Ig in those stories where cracking is predicted using factored loads."
 

@baz, thanks for the refresher.

 

[rummaan17]

Dear baz, UmarMakhzumi,
 

Actually, to be precise while designing Shear walls in ETABS alot of times we happen to meet un-usual behavior as some part of Pier and spandrel failing in shear near opening. We usually use 0.7 modifier for wall.  I have seen if I lower down this stifness modifier value in the vicinity of opening, force get trasnfered and design become safe. Also. i have seen that this things work to certain extent only as after certain points we cannot transfer forces to other places.

is this a correct approach to make some element crack before that other. 
 

[Badar (BAZ)]

2 hours ago, rummaan17 said:

I have seen if I lower down this stifness modifier value in the vicinity of opening, force get trasnfered and design become safe.

 

 As I have already written above, we don't need to apply stiffness modifiers in order to get design forces.

2 hours ago, rummaan17 said:

.

is this a correct approach to make some element crack before that other. 
 

Crack is a general term. In RCC structures, we avoid shear and anchorage failure and intend that structure fail in flexure if loads exceed the design limit.

As far as seismic design is concerned, we follow capacity design approach. In this approach we avoid brittle modes of failure (such as shear and anchorage) and make sure that element is designed for shear, or anchorage forces, that will result from inelastic deformations in ductile range. We also want that the flexural failure of beams occur before the flexural failure of column (if the structural system has moment resisting frame) in case of major seismic event. We also want that the diaphragm and foundation behaves elastically.

[rummaan17]

Dear Baz,

applying modifier takes into account the second order effect which Is also described by code. Building drift limit is mostly checked at serviceability stage with modifier 1 as it would represent un-cracked state.  
When a member crack in flexure or shear, most of the energy is dissipated. I guess its not correct to design the building for foces without consider rupture moment as it would be very conservative.

The thing I am trying to have member input on is the flow of forces when stifness modifiers are used.

Any addition would be appreciated. Thanks. 

 

[Ayesha]

3 hours ago, rummaan17 said:

Building drift limit is mostly checked at serviceability stage with modifier 1 as it would represent un-cracked state.  

Where does it say in code to do this. Please provide a reference?

[Badar (BAZ)]

11 hours ago, rummaan17 said:

 Building drift limit is mostly checked at serviceability stage with modifier 1 as it would represent un-cracked state.  

 

You have got it wrong. One needs to satisfy drift limits by considering the loss of stiffness due to cracking. The code has specifically directed us to do that. I will give you the reference of UBC 97. For the determination of design level displacement, the code says, "The mathematical model shall comply with Section 1630.1.2." The section says : Stiffness properties of reinforced concrete and masonry elements shall consider the effects of cracked sections".

11 hours ago, rummaan17 said:

The thing I am trying to have member input on is the flow of forces when stifness modifiers are used.
 

Can't comment anymore on that.

[rummaan17]

Dear Baz and Ayesha, Thanks for responding and keep thread alive.

ACI 318-14 Table 6.6.3.1.1 (a) dictates the stifness modifier used in the analysis. ACI 318-14 6.6.3.2.2 indicates that for service-lad analysis these modifiers can be multiplied with factor 1.4 .
Column/ Wall (Uncracked) =  0.7 x 1.4 = 0.98 = 1 Ig
 Beams / Wall(cracked ) = 0.35 x 1.4 = 0.49 Ig

The picture I am attaching below Is from a design pdf of a renowed International consultant that I usually follow. I am open for further comments in this regards. 

[Badar (BAZ)]

You have added to my pool of knowledge. I had not gone through the 2014 edition of the ACI 318 code yet. 

The picture that you attached should concern with loadings in which pre-yield behavior is taken into the account to calculate deformations. My earlier comments were related to deformations related to design level seismic forces.

Read the following excerpt form the commentary of the section 6.6.3.1.2 of 318-14.

The type of lateral load analysis affects the selection of appropriate effective stiffness values. For analyses with wind loading, where it is desirable to prevent nonlinear action in the structure, effective stiffnesses representative of pre-yield behavior may be appropriate. For earthquake-induced loading, the level of nonlinear deformation depends on the intended structural performance and earthquake recurrence interval.

As UBC 97, or any other building code, wants us to compare design level drift to maximum inelastic drift, one should use stiffness modifiers that reflect the yielding of the member. If one is using performance based approach, these modifiers can change. (I am talking about seismic forces)

[Ayesha]

I think your question has been answered above. I will only add that even for performance based design, the stiffness modifiers shall be different for each performance level. I think that is implied by the commentary.

You an also have a look at the following thread to see how drift control and stability requirements are enforced by code in general design practise. It is not 100% related to your question but it gives you a nice overview of how certain elements that we talk about are incorporated into the design with us taking the fore granted. You might find it usefel.

 

[rummaan17]

Thanks Baz and ayesha for sharing your valuable opinions.

Also, I found answer to my question as well also in ACI 318-14 as the least value of stiffness modifier that can be used to account for second order effect/Cracked analysis.