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How to M odel Cr acked Behavi our of Shear W al l s i n ETABS
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How to Model Model Cracked Behaviour of Shear Walls in ETABS
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How to Model Cracked Behaviour of Shear Walls in ETABS
#1
03-06-2015, 08:37 PM
Hussein.Rida Principal Moderator
How to Model Cracked Behaviour of Shear Walls in ETABS Flexural and and axial behaviors for shell wall elements can be modified in ETABS by using either f11 or f22 property modifiers, depending on the orientation modifiers, orientation of your local local axes. The shear behavior is controlled by f12 property modifier. The terms f11 or f22 would correspond to to modificati modifications ons of EI or EA and f12 would correspond to modifications to GA shear. The code recommendations in ACI3 18-05, Section 10.11 are related to slenderness effects where flexural deformations govern so the code recomm ends modifying EI (corresponding to f11 or f22 for shear walls). Furthermore, ACI318-08 Section 8.8 includes inclu des recommendations for member’s properties modification factors to be used for lateral loads analysis. There is no recommendation for reducing the GA shear. Modifiers for f12 can be used where deterioration of shear stiffness is expected. The above discussion applies when the local axes 1 and 2 of the shear wall area object are either vertical or horizontal. This is under user control. When drawing walls in ETABS, the default is to have the 1 axis horizontal and the 2 axis vertical. This means that the flexural modifier for EI should be applied to f22 for wall piers and to f11 for spandrels.
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The designer should keep in mind the following: 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 (fr=0.62*sqrt(f'c)), the analysis should be repeated with I =0.35Ig in those stories where cracking is predicted using factored loads.
We can summarize the followings: 1. For shell elements pier-shear walls with default orientation of local axes, the main modifier affects directly on flexural stiffness "EI" is "f22". 2. For shell elements spandrel- beam with default orientation of local axes, the main modifier affects directly on flexural stiffness "EI" is "f11". ACI318-08 code declared in its commentary “R.8.8.2“ that the modulus of shear modulus may be taken as 0.4Ec, so the shear stiffness modifiers "f12" could be reduced as well.
In general, we can use the following stiffness modifiers for pier-shear walls: f11=1, and f22=f12=m11=m22=m12=0.7 for un-cracked walls. f11=1, and f22=f12=m11=m22=m12=0.35 for cracked walls. For spandrel shell-modeled beams: f22=1, and f11=f12=m11=m22=m12=0.35 For shell-modeled deep wall spandrel-outriggers under high level of horizontal and vertical stresses: f11=f22=f12=m11=m22=m12=0.35 Sometimes, the designer may go lower than those values of stiffness modifiers mentioned in code. This decision depends on designer's judgement on the degree of cracking and the expected degradation in element's stiffness under the cyclic loading and level of developed stresses.
http://struc ructuralral-e expe xperts. rts.c com/s m/showth wthrea read.php?65-Ho -How-t w-to-Mo -Model-Cr l-Cra acke cked-Be -Behavio viour-o r-of-Sh -Shear-Wa r-Wallslls-in in-E -ETA TAB BS
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How to Model Cracked Behaviour of Shear Walls in ETABS It is good to highlight the followings: 1. Against the expected, ACI318-08 code doesn't discuss the issue of reducing the flexural stiffness modifier under chapter "21" adopted for Earthquake Resistance Structures, even though this issue is quite related to the ductility and design of structures under the attack of earthquake waves. However ACI code discuss this issue under the clause of slenderness effect in compression members, and to be more specific, when it talks about the design of long/slender columns which are extremely affected by the second order displacement/moment result from lateral load such wind & earthquake load. In this regard: it is so clear that reducing the flexural stiffness will lead to increase the lateral displacement caused by lateral load and then increasing the second order moment effect "P-Delta" called-phenomena. 2. Reducing the flexural stiffness affects directly on structure stability index (equation 10-10 in ACI318-08). 3. Ductility of structure may measure by the degree of flexural cracking takes place under the reversal/cyclic seismic load. These cracks grow up from cycle to the other result in degradation in element’s stiffness. And for high-ductile special structures the degree of degradation quite differs from this observed for low-ductile structures. However ACI code releases up to 2005 edition have no such distinction in the value of stiffness modifiers between special, intermediate and ordinary structures, whereas the latest edition ACI318-08 start show such difference as shown on equations “10-8” & “10-9”. Hussein Rida
Last edited by Hussein.Rida; 03-06-2015 at 08:48 PM.
Thanks given by: mhardman87 (01-06-2016),mhsala7 (05-05-2015),usama_usama2003 (03-07-2015)
#2 03-06-2015, 09:23 PM
Hussein.Rida Principal Moderator
To determine whether the Walls are cracked or not, we need to do the followings:
1. In the first analysis step, assume that walls have no flexural cracks under combined effect of gravity and lateral loads, so enter 0.7*Ig as a value of flexural stiffness modifiers. 2. From the outputs of first analysis step, extract the wall-shell elements tension stresses (S11, S22, S12 | principal stresses Smax) from load combinations that have both gravity and lateral loads. 3. Compare wall's tension stresses values with concrete modulus of rupture [fr=0.62 * sqrt(f'c) MPa] which denotes the value of tension stresses in concrete where the cracks start to be developed after reaching it. 4. If the extracted wall's stresses are greater than concrete modulus of rupture, this means that wall section will have cracks, so we need to repeat the analysis again with value of flexural stiffness modifiers equal to 0.35 * Ig.
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Originally Posted by ayelamayem77
1-are the stresses (s11 , s22 , s12 , s max ) which are obtained from Sap2000 or Etabs are based on ultimate load combinations To determine whether the Walls are cracked or not, or they are obtained from service load combinations??
Since the seismic load per the latest codes UBC97, IBC2009, or ASCE7-10 is already given at ultimate design stage, therefore you have to use the ultimate load combinations include both gravity and seismic load with their scale factors as described in relevant codes. Originally Posted by ayelamayem77
2-is this procedure in workable in case of modeling ground tanks?? or we run the model at the first time without any reductions of the stiffness modifiers and get the tension stresses from service loads combinations and compare with the modulus of rupture to ensure that the section is not cracked?
I understand –correct me if I am wrong- from ground tank that you mean underground water tank. According to the hazardous of liquid material filled inside tank, so you need to determine whether you need to analysis and design the tank under elastic or inelastic seismic load as required by code. The proper case is to design the tank under elastic limit (i.e.: R=1) so no reduction in stiffness shall be made since the cracks should not be occurred even under seismic load. In this case, service load combination that include gravity load with full magnitude of seismic load shall be used to extract the tensile stresses, and then to com are with concrete modulus of ru ture and should not exceed it.
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How to Model Cracked Behaviour of Shear Walls in ETABS Again, my speech here is for very hazardous liquid containers. Originally Posted by ayelamayem77
3-what is the actual value to ensure that the section in UN-cracked for ground water tanks ? (modulus of rupture or concrete flexural tensile stresses as per (1816.4.8 Cracked section considerations in ubc -97)
The flexural tensile stresses given in UBC97 under clause 1816.4.8 is for two way pre-stressed slab only and not for underground water tank unless the walls of tank having pre-stressing action in both ways. The more modern approach per ACI standard -refer to ACI318-11 clause 18.3.3- requires that all two way prestressed slabs shall be designed as class “U” that have no cracks with modulus of rupture not exceeding 0.5*sqrt(f’c). Originally Posted by ayelamayem77' pid='1078' dateline='1340869 050
4-in case of solving a precast concrete wall by sap2000 or etabs[/b][/color] the actual value to avoid cracks is to divide the modulus of rupture by 1.5 as per pci-6 eq(4.2.2.2) so are the steps mentioned by engineer Hussein reda have to be compared with this new value (modulus of rupture /1.5) ? to check the cracking conditions.
I understand here that you are talking about pre-stressed precast concrete wall. Once the concrete section is subjected to pre-compression caused by stands force so this will try to reduce/eliminate the tension cracks under seismic load. Here there are many schools to estimate the reduced stiffness modifiers. The most popular ones is to consider stiffness modifier equal to (1) for un-cracked wall, and (0.5) to cracked wall. Above values are equal to those mentioned in ACI code for walls after multiplying by 1.43. And this will give almost same approach as you mentioned above to divide the modulus of rupture by 1.5. To conclude: use (1) as stiffness modifier for pre-stressed wall. Do the analysis and compare the tensile stresses with concrete modulus of rupture [0.62*sqrt(f’c)]. If it exceeds, then repeat the analysis using (0.5) as stiffness modifier. Hussein Rida
Last edited by Hussein.Rida; 03-06-2015 at 09:26 PM.
Thanks given by: jclcereno87 (09-08-2015),usama_usama2003 (03-07-2015)
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