Stiffener Modifiers for Shear wall in STAAD Pro

[Ahmed Waqar]

Respected All,

Am doing shear wall analysis and design. Property reduction factors are easily incorporated in Frame elements. but for shear walls property modifiers i didnt find any. Please guide me that how can i change the property reduction factors of shear walls in STAAD Pro 

[Ayesha]

Did you figure out a way to resolve the issue yet @Ahmed Waqar?

[Ahmed Waqar]

YUP. Its an indirect way. As stiffness is the function of EI/L, Instead of changing I, just change E to 0.7 results will be same. 

[EngrUzair]

Dear Ahmed Waqar,

Thanks for sharing.

IMHO, whenever someone asks a question on the Forum, but later on reaches some good answer by himself or herself, it is always a good idea to share the answer at the Forum as well, for the benefit of other colleagues & users.

Regards.

Edited April 18, 2016 by engruzair

[Ahmed Waqar]

Agreed, @engruzair

[UmarMakhzumi]

9 hours ago, Ahmed Waqar said:

YUP. Its an indirect way. As stiffness is the function of EI/L, Instead of changing I, just change E to 0.7 results will be same. 

Make sure you also change your shear modulus (when you change your E) to have consistency.

Thanks.

[WR1]

I think in most of the programs G is automatically calculated based on E and v but double check. And it is not always good idea to change E instead of I for stiffness reduction. Because E can change EA, EI and GA as well.

[Ahmed Waqar]

Thanks for the above feedback. I dont know of other softwares but in STAAD Pro, You have to give all the properties of the material if there is any change. Now, E of shear wall is the change material. And we know the pupose of shear wall is to reduce lateral displacements and to take lateral load as per his stiffness and that will be the fuction of EI. So only E will be change and all other parameters will be on E original characteristics. Correct me if I am wrong.

[UmarMakhzumi]

Ahmed,

10 hours ago, Ahmed Waqar said:

I dont know of other softwares but in STAAD Pro, You have to give all the properties of the material if there is any change.

So you will have to enter modifier shear modulus manually.

For the above mode, you need to define at least 3 materiel properties. One for Beams (0.5*E), one for slabs(0.25*E) and one for columns and shear walls (0.7*E). I totally agree with Rana that tweaking modulus of elasticity is not a wise thing to do, but you can at least give it a shot and do a quick comparison with ETABS to see how off your results are.

Quote

Now, E of shear wall is the change material. And we know the pupose of shear wall is to reduce lateral displacements and to take lateral load as per his stiffness and that will be the fuction of EI. So only E will be change and all other parameters will be on E original characteristics. Correct me if I am wrong.

Your understanding is not correct. When you change E, everything for that material would be based on modified E, not original E.

Thanks.

 

[Ahmed Waqar]

14 hours ago, UmarMakhzumi said:

Ahmed,

So you will have to enter modifier shear modulus manually.

For the above mode, you need to define at least 3 materiel properties. One for Beams (0.5*E), one for slabs(0.25*E) and one for columns and shear walls (0.7*E). I totally agree with Rana that tweaking modulus of elasticity is not a wise thing to do, but you can at least give it a shot and do a quick comparison with ETABS to see how off your results are.

Your understanding is not correct. When you change E, everything for that material would be based on modified E, not original E.

Thanks.

 

Thanks for the feedback. 

Frame elements like line elements and plate elements stiffness modifier will easily work. Now we left with shear walls. What i am saying is create new material profile for shear wall concrete. its stiffness for flexural desformations are 3EI/L (cube) and varies to 12EI/L(cube). Greater the shear walls greater is the trend towards cantilever nature of wall.Now we remain ourselves is elastic analysis of first and second order. As far as material is elastic its properties are well defined as per established principles of mechanics. Now we just change I to 0.7 of I which is stiffness modifier. In staad we cannot achieve this with ease like other elements (as per my knowledge of staad) So my proposal is just to change E to 0.7 of E instead of I. You will get the same results. Its simple mathematics. But this E is of shear wall only and precisely in flexural deformations only. Which is primarily over concern,

[UmarMakhzumi]

What you have stated is provisionally correct.

All, I am asking you is to compare your Staad Pro results with a ETABS results. The reason being that E is used in a lot of places, and we might not be fully aware of the implications of this method. Once you have done the comparison (let it be a very high level), you have done due diligence on your end. You will at-least have a basis to compare your results too.

Thanks.

 

 

[WR1]

Yes you are correct as Umar said but you results might not match with ETABS as you would be reducing everything by E (A for shear and axial, I for bending). Generally reduction in GA is not done. But it all depends on what type of stiffness is dominant. Or these squat walls where shear stiffness governs or tall walls where flexural deformation govern?

 

If flexural deformation, you could change E and the effect from shear would be minor. But remember that axial and bending stiffness in a wall are inter-dependent (regardless of the software). Changing one affects another.

 

There might be many possibilities.

 

1. Change E (affects EI, EA and GA as G depend on E unless you put different or original G).

2. Change thickness (affects EI, EA and GA). This is the case of ETABS modifier. m modifiers internally affects t³ and f modifiers internally affects t. But if you change t outside in material properties like in STAAD, you would affect all the m and f modifiers.

So from the above discussion, I would go for option 1. Change the E (only for walls), but dont calculate G based on new value of E. It depends if you want to reduce shear stiffness too or not. Another point now is that what value you would choose for E? Is E=0.7Eold...

The easy answer is use your judgement (:p).

Another way is to compare results with ETABS, but would you make another model in ETABS? Because comparing with simple example might not be efficient in this case. At the end you need relative stiffness of other members too.

 

To go a little deeper;

 

A) Axial stiffness of wall = in-plane bending stiffness of wall = Eh/(1-v²)

f modifiers in ETABS affect this h. (f11,f22,f12).

You can change E here as well (but make sure to check the value of G as it is affected by E in some programs like ETABS). But in ETABS there are 3 modifiers, in STAAD if you change E or h, it would change all f11, f22, f12.

So you have to make sure is it flexural dominant wall (f22)? a spandrel (f11?) or shear dominant wall (f12?). Other wise h or E in staad will affect all f.

 

B ) Out of plane stiffness of wall = Eh³ / 12 (1-v²)

So in ETABS m modifiers affect this h³. Meaning 0.7m is equal to 0.7 (h³).

Again here, reducing E or h directly reduces everything (m11, m22, m12). Its like you just using a new all with reduced h and all properties are based on this new h.

 

At the end, see what case you have?

#1 Are you designing the wall as in-plane (shear wall) or out-of-plane (basement wall)?

#2 Which stiffness is controlling?

x. Shear

y. flexure

z. shear+flexure?

 

Lets assume its an in-plane shear wall.

For x:

Change G only

For y:

Change E only

For z:

Change G and E both

 

This is purely my own understanding. Others are welcome to suggest.