Property Modfiers_Etabs!!!

[Waqar Saleem]

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??

[WR1]

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.

[UmarMakhzumi]

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

[WR1]

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.

[UmarMakhzumi]

please explain more as i here im a little confused about your statement.

Deflections are affected, not the reinforcement. you can check that manually.

[WR1]

Deflections are affected, not the reinforcement. you can check that manually.

WITH MODIFIERS (MORE INERTIA) MORE STIFFNESS SO MORE MOMENT

[WR1]

WITHOUT MODIFIERS LESS MOMENT LESS STIFFNESS

[WR1]

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.

[WR1]

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)

[Badar (BAZ)]

Umar is right. Use section modifiers for serviceability checks.

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.

[WR1]

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).

[Badar (BAZ)]

What you do is according to code. There is nothing wrong about it.

[UmarMakhzumi]

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.

[Waqar Saleem]

@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?

[Waqar Saleem]

please differentiate shell and membrane ?how do we say that one slab is membrane and other is shell or plate ?

[UmarMakhzumi]

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.

[Badar (BAZ)]

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.

[Badar (BAZ)]

@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.

[UmarMakhzumi]

@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.

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.

[UmarMakhzumi]

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.

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.

[Badar (BAZ)]

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.

[UmarMakhzumi]

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.