Modifying frame stiffness. Is that necessary? 1

[dollarbulldog]

Dear Guys

When modeling the building frame structures for concrete design using ACI318-99, shall the frame stiffness be modified according to ACI 10.11?

Thanks

 

[slickdeals]

For strength design, yes.

 

[StructGen]

This is a genral misconception, if you look at ACI 10-11 title "Maginified moments". This Article presents an alternate method of P-Delta analysis, stiffness of members are reduced to obtain higher deflection to get the affect of second order moments.
Yes you should reduce stiffness as per 10-11 if you want to do P-Delta analysis by this method otherwise Etabs have other options, mass based and load based. Mass based is more stable and quick.

 

[WillisV]

StructGen - I agree that 10.11 is for an alternate P-delta method (more likely to be used for hand calcs etc); however, using the stiffness modifiers presented in ACI 10.11 even in an automated P-delta analysis is a generally accepted way of meeting the conditions of 10.10.1 which requires a second-order analysis that considers cracking among other things as there is little other guidance.

 

[structuresguy]

I agree with WillisV. we always use the stiffness modifiers in Etabs, and other software applications.

 

[bazflexure]

I have seen some models in which the designer has used 0.01 as trosional constant in etabs.. why this value has been used.. any reference ..etc??

 

[StructGen]

WillisV, ACI 10.11 factors if used for flexural cracking will be excessive and kind of an over kill. There are other approches to accomadate flexural cracking in concrete members to estimate the loss of stiffness. We have developed some our own, based on available research papers.

 

[rowingengineer]

bazflexure read what asixth has to say:

thread507-255117

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[slickdeals]

@StructGEN:
Mind providing more information? The research is lacking in this regard. I think I read somewhere that based on experimental data for shear walls, you rarely will need to reduce more than 0.6 Ig.

 

[seny]

despite, big number of things to be taken into consideration,
Structural Members / Service / Ultimate
Beams / 0.5EcIg / 0.35EcIg
Columns / 1.0EcIg / 0.70EcIg
Walls /
Uncracked 1.0EcIg / 0.70EcIg
Cracked 0.5EcIg / 0.35EcIg
Flat plates / 0.35EcIg/ 0.25EcIg.
Flat plates (equivalent slab-beams consideration)/1.0EcIe / 0.70EcIe
This is basically according 10.11.1 and R10.11.1 x1.43 for service as per ACI 318.

 

[bazflexure]

so we should always consider the stiffness modifiers for service and for seismic we should consider factors listed under the ultimate ..
rite??? i guess so