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kHURRAM ALI

What Modifier Should We Take For Servicibility Check (Deflection And Drift)

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aslamwalekum , when we design reinforced concrete elements , for beam we take 0.35 , for column and wall we take 0.7 and for slabs we take 0.25 as modifiers , but i read in aci that for servicibilty analysis these modifiers has to increase by 43% which means for beam it become 0.5 , for column and shear walls it become 1.0 and for slab it become 0.35 , by doing this model drift and deflection both reduce to almost half of its original value.

 

one thing more when we do manual check we dont take crack sections , for e.g for simply supported beam the deflection is 5\384 WL^4\EI , here the I (inertia ) is not the for the crack section ,its for the uncracked section , so what modifiers should be use 0.35 or 0.5 or 1.0

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but i read in aci that for servicibilty analysis these modifiers has to increase by 43% which means for beam it become 0.5 , for column and shear walls it become 1.0 and for slab it become 0.35 , by doing this model drift and deflection both reduce to almost half of its original value.

 

Please post the reference from where you have read about increasing serviceability factors for drift checking?

 

 

 

one thing more when we do manual check we dont take crack sections , for e.g for simply supported beam the deflection is 5\384 WL^4\EI , here the I (inertia ) is not the for the crack section ,its for the uncracked section , so what modifiers should be use 0.35 or 0.5 or 1.0

 

You should consider cracked section while doing manual checks too. Just reduce your beam/ slab inertia by factors as per ACI Code. 

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i have posted a few lines mention in the ACI 318 code refrence R10.10.4.1 below ,please review and confirm for 43% increase in service modifires , i belive drift is also a kind of deflection and deflection always calculate at service level , thats why i think for drift modifiers will not be same as they use for ultimate design

 

the code lines are

 

Section 10.10 provides requirements for strength and
assumes frame analyses will be carried out using factored
loads. Analyses of deflections, vibrations, and building
periods are needed at various service (unfactored) load
levels10.37,10.38 to determine the serviceability of the structure
and to estimate the wind forces in wind tunnel laboratories.
The moments of inertia of the structural members in the
service load analyses should be representative of the degree
of cracking at the various service load levels investigated.
Unless a more accurate estimate of the degree of cracking at
service load level is available, it is satisfactory to use
1.0/0.70 = 1.43 times the moments of inertia given here for
service load analyses.

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You should upload a snapshot and Highlight it. What it looks to me is that you are taking things out of context. The clause appears to be applicable to where wind tunnel studies have been done. I doubt if that is the case for you.

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Many engineers are confused with stiffness modifiers stuff in ETABS including me. Let me add my cent here.

 

1. Any reasonable set of stiffness can be used as explained in ACI 318.

 

2. You have to determine what type of frame you are analyzing; braced or un-braced

 

3. Whats the analysis type? Strength, service?

 

4. Code allows use of 0.35,0.7 factors on inertia in chapter 10 of ACI 318 for slenderness effects.

 

5. Also according to code, you can use the above same model in step 4 for lateral deflections.

 

6. Now for lateral deflections, if the lateral load is service (like Wind load of ASCE 7-05 and previous) multiply above factors by  1.43 or 1.40 (see code). For strength lateral load like Earthquake, no need to multiply by this factor.

 

7. Some times you really dont need to apply modifiers at all. For example for strength design.

 

8. For a two-way frame with membrane slabs at top, you just need to apply 0.5 factor to beam elements. Code allows this which says for strength design you can either

 a. use the same modifiers as used for slenderness

 b. use 0.5 for beam stems only

 

 

In simple words, modifiers are factors to reduce inertia for cracked sections. But if you are doing strength design, why you need the cracked inertia.

 

And as far as serviceability is concerned, authors like Nilson even argue that a factor of 0.5 as we applied in step 8 can also be skipped because of the following reasons.

 

 a. For positive moment, beam is designed as T section but in ETABS we use rectangular section, Stiffness of T = approx 2 x stiffness of rect. So no need to apply 0.5 inertia factor to beams because T compensates for that. Just use the rectangular section in ETABS.

 

b. For negative moments, beam is designed as rectangular, and we also model the rectangular section in ETABS. Now the cracking in this section is offset by the continuation of bottom bars into support which have stiffening effect.

 

I hope that helps. Let me put it as a summary here;

 

 

1. Strength & service design with membranes (slab on rigid beams as compared to slabs).

 

Beams = 0.5

 

or beams = 0.35 & columns = 0.7 (or whatever ratios just keep it constant).

 

or no modifiers at all

 

walls = 0.7 for un-cracked

            0.35 for cracked

 

2. Strength & service design with shells (flat plates etc).

 

beams and walls same as above

slabs  = 0.25 for out-of-plane m factors.

 

just multiply above factors by 1.43 for service lateral deflections.

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On 9/15/2015 at 11:49 AM, Rana said:

Some times you really dont need to apply modifiers at all. For example for strength design.

I think for single column or single beam, This statement seems true but for a frame in ETABS, relative difference between stiffness of column and beam must be provided so in any case 50% difference between stiffness of both beam and column must be provided. Am I right?

On 9/15/2015 at 11:49 AM, Rana said:

And as far as serviceability is concerned, authors like Nilson even argue that a factor of 0.5 as we applied in step 8 can also be skipped because of the following reasons.

 

 a. For positive moment, beam is designed as T section but in ETABS we use rectangular section, Stiffness of T = approx 2 x stiffness of rect. So no need to apply 0.5 inertia factor to beams because T compensates for that. Just use the rectangular section in ETABS.

 

b. For negative moments, beam is designed as rectangular, and we also model the rectangular section in ETABS. Now the cracking in this section is offset by the continuation of bottom bars into support which have stiffening effect.

I have few questions here.

1) Does ETABS design beam for positive region as T-beam or Rectangular beam? Mean does it takes into account monolithic behavior of beam with slab and make part of slab acting as flanges or not?

2) If it only design as rectangular beam, why it has algorithm of T-beam design in its manual?

3) If it design as T-beam also depending on neutral axis depth, we will have to provide 0.35 modifier for beams because here T beam is being taken as T beam in positive region so its stiffness must be les than 50% than column so we will have to use 0.35I for beams.

Edited by Engr Waqas

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On 8/22/2016 at 11:28 AM, Engr.Suliman said:

Capture2.PNG

The statement of the quoted document (who is the author btw?); "Therefore, finally all columns and rectangular beam section's stiffnesses shall be reduced by a factor of 0.7..." is erroneous. It should have been as explained previous posts;

On 9/15/2015 at 10:49 AM, Rana said:

a. For positive moment, beam is designed as T section but in ETABS we use rectangular section, Stiffness of T = approx 2 x stiffness of rect. So no need to apply 0.5 inertia factor to beams because T compensates for that. Just use the rectangular section in ETABS.

 

b. For negative moments, beam is designed as rectangular, and we also model the rectangular section in ETABS. Now the cracking in this section is offset by the continuation of bottom bars into support which have stiffening effect.

 

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On 1/24/2017 at 0:04 PM, Rana said:

I am sorry, my statement is wrong. You are right in previous post. Ig of rectangular beam in FEA should have 0.70 factor.

I assume this is just because FEA software takes beams as rectangular section instead of original T section so we are omitting to maintain 50% difference between beam and column and considering same modifier for both. But what if in manual calculations we are using same T section instead of taking it as rectangular section? I assume then we should use 0.35 for beam and 0.7 for columns. Moreover i also doubt if FEA softwares consider rectangular beam purely rectangular. After all deflection of beam is reduced due to participation of slab stiffness with beam. So i think ultimately we should consider 0.35 for beam and 0.7 for columns.

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