What are stiffness modifiers in Etabs? What are stiffness modifiers in Etabs? 112
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Are they used to account for reduced sectional properties due to cracking? Is there any mention of these in IS 456? Will I be overestimating strength if I do not apply stiffness modifiers ? What values are typically used for concrete members (Slabs, Beams, Columns) ?
Karanpal Singh, Structural Engineer Written Mar 20, 2016
Yes, you are correct. The stiffness modifiers are used to take into account the cracking of RCC sections. IS 456:2000 does talk about stiffness of members in Clause 22.3. However, it does not provide the modifier values to be used. You can refer Code Section 10.11 of ACI 318 for stiffness modifier values suggest by ACI. Stiffness modifiers can have significant effect on the behavior of the structure. In absence of stiffness modifiers the structure would be stiffer and thus attract higher lateral forces due to earthquake. So you might end up with heavily reinforced shear walls , moment frames etc. At the same time you might also underestimate the drift. Stiffness modifiers will also affect force distribution among different members of the structure.
Nishant Rathi, Structural Engineer Written Dec 8
The concept of stiffness modifier is not related to Etabs. Stiffness modifiers are used to simulate the behaviour of structure in cracked stage. while there is nothing mentioned about stiffness modifiers in Indian standard, values mentioned in ACI 318 can be used.
Georgios Bekas, 8 years of experience. Licensed Civil Engineer. Updated Oct 3
In seismic retrofitting the stiffness of structural elements according to various codes is reduced to reflect a reduction in the strength of the material, as a result of aging and environmental conditions
Sujaykumar R SanglikarPhysically Human,Mentally Zombie...! Works at Smartminds Engineering Pvt.Ltd.Bangalore. Studied at Sri Dharmasthala Manjunatheshwara College of Engineering and Technology Lives in Bengaluru, Karnataka, India 134.5k answer views4.5k this month Follow227 Turn On Notifications Sujaykumar R Sanglikar, worked at Structural Engineering Written Mar 15, 2016
If the structure is designed using LIMIT STATE METHOD (LRFD/LSM/LS) the section is designed as cracked section. So the section after crack shall have less stiffness compared to the gross section stiffness (Also the stress carrying capacity), the same is assigned for in the model. However, there is no clear indication of this in our IS codes,may be the FOS for dead load is considered as 1.5 against 1.2 of the ACI. I believe that for linear first order or flexure analysis of non-slender structures, the stiffness reduction need not be used. Any consistent assumption of equivalent stiffness should suffice according to 8.7 ACI-318-2011.
Stiffness modifier for beam, column, wall, slab Stiffness modifier for beam, column, wall, slab structech08 (Structural) (OP) 7 Dec 10 21:26 Hello, What is the importance of stiffeness modifier for the beam, column, slab , wall etc to ETABS analysis and section design? What is the purpose of using the stiffness modifier?
Why is it that I get more reinforcement when no stiffness modification is applied to the columns, beams, wall and slab? When I put the stiffeness modifier as per ACI 318 requires, I got lesser area of steel reinforcement for the beams and columns. Thank you for your response. Structech08
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RE: Stiffness modifier for beam, column, wall, slab slickdeals (Structural) 8 Dec 10 07:22 Stiffness modifiers are used to account for cracking in concrete. It is the nature of concrete to crack and hence, it is important to modify its stiffness to accurately get deflections and P-Delta moments. In a building with shear walls and beams/columns (which I presume is your case), cracking the columns/beams will reduce their stiffness and more moments/shears are transferred to the shear walls based on relative stiffness. Stiffness attracts force. As a result, if your column/beam is relatively less stiff than the shear wall, it will have less moments and shears. This will result in lesser reinforcing. Hope that makes sense. Post if you have other questions.
RE: Stiffness modifier for beam, column, wall, slab structech08 (Structural) (OP) 10 Dec 10 06:54 Hi slickdeals, I really appreciate your response, your explanation is technically clear. Thank you so much. structech08
RE: Stiffness modifier for beam, column, wall, slab zstone (Structural) 21 Dec 10 11:38 Dear friends Regarding to fema 356 It is required to reduce also stifness properties of walls For flexure 0.5 to 0.8 EI, and for shear 0.4EA, My question is how to put this effect into walls by stifness modifiers, Which parameters i need to reduce, Because if i reduce f11, f22, or f12 i affect also axial rigidity ( which i dont like to affect axial rigidity because by fema 356 it will bee 1.0EA) If i reduce m11, m22, m12 i affect out of plane bending stifness of plate, which also is not in my. Kind Regards
RE: Stiffness modifier for beam, column, wall, slab zstone (Structural) 21 Dec 10 11:39 Or i maybe need to reduce stifness only at beams and columns. Many Thanks
RE: Stiffness modifier for beam, column, wall, slab
jenofstructures (Civil/Environmental) 22 Dec 10 02:20 Is it based from the stiffness values stated on ACI?
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RE: Stiffness modifier for beam, column, wall, slab zstone (Structural) 22 Dec 10 04:51 Right Both FEMA and ACI propose values for this purpose, But how to include this values into walls, For example FEMA recomend to reduce Flexural rigidity of walls by 0.8EcIg My question is: how can include this effect into ETABS through stifness modifiers,or any other way maybe this values are available if you model walls as column elements ( onedimensional elements) Regards
RE: Stiffness modifier for beam, column, wall, slab slickdeals (Structural) 22 Dec 10 13:52 1. You can change Ig (if required) using the shell stiffness modifiers. 2. You can change Ec using material properties (specify 0.8E as your concrete Ec) That way what you end up with is 0.8 Ec Ig
RE: Stiffness modifier for beam, column, wall, slab zstone (Structural) 23 Dec 10 05:14 Dear friends, thank you for your response It is a little bit a problem. If i reduce modul of elasticity i affect all rigidities Flexural, shear and axial Exactly im trying to do what fema 356 - chapter 6 - concrete - Table 6-5 - Effective Stiffness values recomend: Walls uncracked ( on inspection) Flexural Rigidity 0.8 EcIg, Shear Rigidity 0.4EcAw, Axial Rigidity EcAg Walls - cracked Flexural Rigidity 0.5 EcIg, Shear Rigidity 0.4EcAw, Axial Rigidity EcAg Now if i reduce modul of elasticity , i affect also axial rigidity and reducing values for flexure and shear are not same , which value to reduce E, with 0.8 or 0.4. Regards ZG
Property Modfiers_Etabs!!! Started by waqar saleem, October 3, 2012
Property modifiers etabs
22 posts in this topic Top of Form waqar saleem
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250 468 posts LocationRawalpindi University:SCET,wah cant(affliated with UET taxila),NUST Employer:URBAN PLANNING AND DESIGN (Pvt.) Ltd, Islamabad Posted October 3, 2012 Respected Members, what is the main purpose of property modifiers in defining farme sections in etabs and how we can relate them to theoretical knowledge?? 0
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Rana
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795 747 posts LocationDubai University:NUST Posted October 4, 2012 Property modifiers in etabs are used to model cracked behaviour of concrete sections. They are only applied to concrete members because of cracking.
Gross moment of inertia is bd^3/12 for a rectangular section, but when you make this member of concrete, it will experience cracking when loaded after some time. This cracking will happen when concrete reaches its tensile capacity which is about 7-10% of its compressive strength. Formula to calculate cracking moments are given in ACI. For example 3000psi will have only 300psi of tensile strength. Actually the reinforcement starts its work when concrete cracks because of tension. After cracking concrete is no longer able to carry tension so steel starts taking the tension. So now if concrete cracks after 300psi the moment of inertia will be reduced because of cracking. If moment of inertia is reduced, its stiffness is reduced, taking less moment, and its deflection increases because of less stiffness. This moment which the cracked beam is not taking anymore will be re distributed to other structural members based on their stiffness. If you read ACI chapter 10, there are many sets of modifiers used for different types of analysis. ok im leaving from office, if you have more doubts i will write in later. 4
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UmarMakhzumi
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835 926 posts LocationCanada University:NUST, U of A Employer:SEFP Posted October 4, 2012 (edited) The use of these property modifiers coefficients should be for serviceability. Update: Members should be checked for strength checks also using cracked section properties. Edited November 12, 2014 by Umar Makhzumi Updated Answer 3
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Rana
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795 747 posts LocationDubai University:NUST Posted October 5, 2012 On 04-10-2012 at 8:34 PM, Umar Makhzumi said: The use of these property modifiers coefficients is for serviceability checks only, not for strength checks. please explain more as i here im a little confused about your statement. 0
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926 posts LocationCanada University:NUST, U of A Employer:SEFP Posted October 5, 2012 On 05-10-2012 at 8:20 PM, Rana Waseem said: please explain more as i here im a little confused about your statement. Deflections are affected, not the reinforcement. you can check that manually. 0
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795 747 posts LocationDubai University:NUST Posted October 5, 2012 On 05-10-2012 at 10:42 PM, Umar Makhzumi said: Deflections are affected, not the reinforcement. you can check that manually.
WITH MODIFIERS (MORE INERTIA) MORE STIFFNESS SO MORE MOMENT
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795 747 posts LocationDubai University:NUST Posted October 5, 2012 WITHOUT MODIFIERS LESS MOMENT LESS STIFFNESS
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795 747 posts LocationDubai University:NUST Posted October 5, 2012 So just compare the two images, model having modifiers have less stiffness, so take less moment, that moment is distributed to other stiff elements. so the point is if you reduce the stiffness as in case of cracking, it will affect deflection as well as moments. so the reinforcement values depend upon the moment. I agree with you that in beam design formula, it depends upon b and d. Now this b and d are not with modifiers rather full values.
But the moment which has to be used in this formual is less because of less stiffness due to reduction in modifiers. 0
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LocationDubai University:NUST Posted October 5, 2012 another thing. if you put the same modifiers for every thing like walls, columns, beams, floor etc..so the reduction will be uniform it will not affect the results. but if you make something stiffer and another less stiffer so you basically are creating differences in stiffness so is the difference in the moment that will goto these elements. like in this example i have applied beams = 0.35,0.35,0.35 (j,m22,m33) cols = 0.7,0.7 (m11,m22) slab = 0.25,0.25,0.25 (m11,m12,m22) walls = 0.7,0.7(m11,m22) 0
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BAZ Major
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387 249 posts LocationRawalpindi University:NUST, Politecnico Di torino Employer:CIIT Posted October 6, 2012 Umar is right. Use section modifiers for serviceability checks. Quote beams = 0.35,0.35,0.35 (j,m22,m33) cols = 0.7,0.7 (m11,m22) slab = 0.25,0.25,0.25 (m11,m12,m22) walls = 0.7,0.7(m11,m22) This set of modifiers are mentioned in section 10.11.1 of 05 addition which is dedicated to computation of lateral deflections of frame. Remember that code specifications are based on worst case scenario, and these values are worst case scenario for lateral deflections of frame. For elastic analysis of frame it is OK to use gross properties based on rectangular section as it is done in ETABS . We provide rectangular beam section properties in ETABS, but cast in place beam has T section in positive region while rectangular section in negative region, so using rectangular section along entire length compensates for that. Moreover if bottom reinforcement of beam is developed in column, as it is normally done, it increases stiffness of beam in negative region. Amount of reinforcement provided in section also plays its role and we dont know how much reinforcement will be required before starting analysis.
Bottom line: it is complex topic and one have to use assumptions. Even if one is using 0.35 and 0.7 factors to size the xsection of member, structure should still stand provided assumptions are uniform through out the analysis, as concrete has this ability to distribute moments according to provided reinforcement. 2
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747 posts LocationDubai University:NUST Posted October 6, 2012 I know in the code these modifiers are for lateral deflection. but then how would you justify the reduced moment of inertia after cracking? Are you talking about membrane slab which has no out of plane stiffness? Yeah in membrane the modfiers will not affect but in shell if you dont apply the modifers, the slab will carry all the moment. I just want to be more clear about the concept, because we apply modfiers in every model. and according to ACI we can use the same modifiers set for lateral and for strength design. (for wind serviceability the modifiers are multiplied by 1.4 in a separate model). 0
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BAZ Major
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387 249 posts LocationRawalpindi University:NUST, Politecnico Di torino Employer:CIIT Posted October 6, 2012 What you do is according to code. There is nothing wrong about it. 1
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UmarMakhzumi
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835 926 posts LocationCanada University:NUST, U of A Employer:SEFP Posted October 6, 2012 On 06-10-2012 at 6:14 PM, Rana Waseem said: I know in the code these modifiers are for lateral deflection. but then how would you justify the reduced moment of inertia after cracking? Are you talking about membrane slab which has no out of plane stiffness? Yeah in membrane the modfiers will not affect but in shell if you dont apply the modifers, the slab will carry all the moment. I just want to be more clear about the concept, because we apply modfiers in every model. and according to ACI we can use the same modifiers set for lateral and for strength design. (for wind serviceability the modifiers are multiplied by 1.4 in a separate model). I think, considering LRFD load factors provides enough factor of safety. Specifically you can never "justify reduced moment of inertia after cracking" and no one knows
how real the exact cracking would be, what would be its extent and how different the load distribution would be after that. 2
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waqar saleem
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LocationRawalpindi University:SCET,wah cant(affliated with UET taxila),NUST Employer:URBAN PLANNING AND DESIGN (Pvt.) Ltd, Islamabad Posted October 6, 2012 @RANA when we say that uncracked section has more inertia and more stiffness and more moment taking but cracked section has less inertia and less stiffness and less moment taking,inertia is the geometric property moment taking is related to strength, uncracked concrete section has less strength than cracked section so sir Rana what do you say about that?also in case of concrete when section is cracked it means it has started taking loads otherwise its strength is not fully generated so what about the stiffness of cracked and uncracked section? 0
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waqar saleem
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250 468 posts LocationRawalpindi University:SCET,wah cant(affliated with UET taxila),NUST Employer:URBAN PLANNING AND DESIGN (Pvt.) Ltd, Islamabad Posted October 6, 2012 please differentiate shell and membrane ?how do we say that one slab is membrane and other is shell or plate ? 0
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UmarMakhzumi
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835 926 posts LocationCanada University:NUST, U of A Employer:SEFP Posted October 6, 2012 On 06-10-2012 at 11:11 PM, waqar saleem said: please differentiate shell and membrane ?how do we say that one slab is membrane and other is shell or plate ? for simplicity, remember this if it has out of plane stiffness its shell, if not its membrane. 1
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387 249 posts LocationRawalpindi University:NUST, Politecnico Di torino Employer:CIIT Posted October 6, 2012 On 06-10-2012 at 11:11 PM, waqar saleem said: please differentiate shell and membrane ?how do we say that one slab is membrane and other is shell or plate ?
membrane is capable of taking tensile stresses only like our skin or tarpal (urdu word). Word slab used in structural engineering cant be membrane. shall is capable of taking shear and moment. Any thing that can take shear or moment can also take direct tension or compression. plate is shell/slab loaded in its plane like shear wall. 3
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387 249 posts LocationRawalpindi University:NUST, Politecnico Di torino Employer:CIIT Posted October 6, 2012 On 06-10-2012 at 11:09 PM, waqar saleem said: @RANA when we say that uncracked section has more inertia and more stiffness and more moment taking but cracked section has less inertia and less stiffness and less moment taking,inertia is the geometric property moment taking is related to strength, uncracked concrete section has less strength than cracked section so sir Rana what do you say about that?also in case of concrete when section is cracked it means it has started taking loads otherwise its strength is not fully generated so what about the stiffness of cracked and uncracked section? flexural stiffness of section is ability to attract moments. so cracked section will attract lesser moment as only part of section is available to resist rotation. Do not confuse moment attracting ability( stiffness) of section with its strength. Section with lesser stiffness can have greater strength as it depends upon material strengths, reinforcement and size and shape crossection. 3
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UmarMakhzumi
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835 926 posts LocationCanada University:NUST, U of A Employer:SEFP Posted October 6, 2012 Quote @RANA when we say that uncracked section has more inertia and more stiffness and more moment taking but cracked section has less inertia and less stiffness and less moment taking,inertia is the geometric property moment taking is related to strength, uncracked concrete section has less strength than cracked section so sir Rana what do you say about that?also in case of concrete when section is cracked it means it has started taking loads otherwise its strength is not fully generated so what about the stiffness of cracked and uncracked section? Quote
flexural stiffness of section is ability to attract moments. so cracked section will attract lesser moment as only part of section is available to resist rotation. Do not confuse moment attracting ability( stiffness) of section with its strength. Section with lesser stiffness can have greater strength as it depends upon material strengths, reinforcement and size and shape crossection. To sum it here, design your members for augmented loads(with load factors) and check them for serviceability with cracked section.. its a conservative way, super conservative to get good design. All members are good, even if they crack and meet serviceability requirements (which may be different upon their intended use). Update: Members should also be checked for strength using cracked sections. Its more safe to apply to both serviceability and strength conditions. 2
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UmarMakhzumi Colonel
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835 926 posts LocationCanada University:NUST, U of A Employer:SEFP Posted October 6, 2012 On 06-10-2012 at 11:25 PM, baz said: membrane is capable of taking tensile stresses only like our skin or tarpal (urdu word). Word slab used in structural engineering cant be membrane. shall is capable of taking shear and moment. Any thing that can take shear or moment can also take direct tension or compression. plate is shell/slab loaded in its plane like shear wall. here is an excerpt from on of my fav books on structural design, I read it some 3 years back, and it always reminds me how we structural engineers can exploit material properties to achieve robust designs. Quote Because concrete is easily formed and shaped, its compression strength makes it the ideal material for shells of any nature. The work of Felix Candela in developing plates and shells wherein he spans 100 m with a 5 cm concrete shell truly opens the possibility for the free form Catia-driven shapes of Gehry Buildings in concrete.
Concrete is perfect for membrane stresses. The potential was realized in my design of the 120 ft diameter roof of the Lebanon Senior High School. This roof was flat and was spanned using a 20 in. thick hollow slab. This span and thickness of concrete were made possible by the fact that as a circular shell deflects, it creates compression in membrane stresses, thus minimizing the deflection. Dr. Timoshenko, the father of concrete plates and shells, shows that the span of a circular slab is represented by the radius of the circle rather than its diameter. 5
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BAZ Major
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387 249 posts LocationRawalpindi University:NUST, Politecnico Di torino Employer:CIIT Posted October 6, 2012 clearly he defined membrane member which can also take compression. What is terminology used for members that can only take tension (i am not talking about cables) like tarpal. This span and thickness of concrete were made possible by the fact that as a circular shell deflects, it creates compression in membrane stresses, thus minimizing the deflection. Did you get the picture of section he might have used for roof slab? I mean there has to something between compression and tension parts of slab. 0
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UmarMakhzumi
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835 926 posts LocationCanada University:NUST, U of A Employer:SEFP Posted October 6, 2012 On 07-10-2012 at 0:08 AM, baz said: clearly he defined membrane member which can also take compression. What is terminology used for members that can only take tension (i am not talking about cables) like tarpal. This span and thickness of concrete were made possible by the fact that as a circular shell deflects, it creates compression in membrane stresses, thus minimizing the deflection.
Did you get the picture of section he might have used for roof slab? I mean there has to something between compression and tension parts of slab. It doesn't specifies, it just says hollow slab 20 " thick. There might be. Bottom of Form
