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Concrete Strength with stiffness modifiers

 
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saahil
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PostPosted: Sun Aug 18, 2019 6:16 am    Post subject: Concrete Strength with stiffness modifiers Reply with quote

What will be the effect of stiffness modifiers (used in earthquake analysis) on compressive Strength of concrete?
Will the reduced stiffness have any impact on design section?
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Dr. N. Subramanian
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PostPosted: Mon Aug 19, 2019 1:49 pm    Post subject: Re: Concrete Strength with stiffness modifiers Reply with quote

saahil wrote:
What will be the effect of stiffness modifiers (used in earthquake analysis) on compressive Strength of concrete?
Will the reduced stiffness have any impact on design section?



Dear Er Saahil

Please note that strength and stiffness are two different items. Of course the reduced stiffness will have an effect on the designed section but will not affect the strength of concrete.

Stiffness [N/m] is an indicator of the tendency for an element to return to its original form after being subjected to a force.
Strength [N/m2] is the maximum stress that the material can resist before deformation or fracture.

It can be especially easy to confuse the difference between strength and stiffness. If a material is stiff, then it must be strong enough to support a large load, right? Therefore, aren’t strength and stiffness the same? The answer is a resounding no! To understand this concept, let us see the stress-strain diagram.

The ratio of stress to strain is the elastic modulus—stiffness, but the stress, and only the stress, defines the strength of the material. Remember, strength measures how much stress the material can handle before permanent deformation or fracture occurs, whereas the stiffness measures the resistance to elastic deformation.

See also the two figures taken from Cornell University website, which compare the stiffness and strength of different materials:





Best wishes
NS
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Dr. N. Subramanian
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PostPosted: Mon Aug 19, 2019 4:19 pm    Post subject: Re: Concrete Strength with stiffness modifiers Reply with quote

A comparison of stiffness modifiers in different codes is enclosed

This is from a paper from the STRUCTURE Magazine of ASCE. It is also enclosed for those who find it difficult to read the table I posted earlier!



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Comparison of Stiffness Modifiers in Different codes.PNG



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Wong-Jan17-Effective Stifness for Modeling RC Structures.pdf
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Last edited by Dr. N. Subramanian on Fri Aug 23, 2019 6:50 pm; edited 1 time in total
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saahil
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PostPosted: Thu Aug 22, 2019 3:08 am    Post subject: Concrete Strength with stiffness modifiers Reply with quote

Thank you sir for your comprehensive reply.
Since stiffness reduction is due to cracks development is there a possibility of Strength being effected by cracks if these penetrate the compression zone
especially in case of beams and slabs
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Dr. N. Subramanian
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PostPosted: Fri Aug 23, 2019 6:54 pm    Post subject: Re: Concrete Strength with stiffness modifiers Reply with quote

Dear Er Saahil,

You may note that the Limit States Design is based on the capacity of the section and not on working loads. Hence, the NA depth as given in the code is based on the final cracked section. Hence do not worry about it.

Best wishes
NS
saahil wrote:
Thank you sir for your comprehensive reply.
Since stiffness reduction is due to cracks development is there a possibility of Strength being effected by cracks if these penetrate the compression zone
especially in case of beams and slabs
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View user's profile Send private message
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