# ASCE 7 Section 15.4.4



## McEngr (Aug 23, 2011)

Section 15.4.4 – why does it state that Section 12.8.2 can be used and that the equations of 12.8 cannot be used. It seems to be stating opposite things. Can someone please explain? Thanks!

Also, example 50 uses a 386 kip/in/sec2 and I think this must be a shortcut based upon someone's in-house experience or a Roark's Formula or something. Could someone help?

Thanks!


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## ipswitch (Aug 23, 2011)

McEngr said:


> Section 15.4.4 – why does it state that Section 12.8.2 can be used and that the equations of 12.8 cannot be used. It seems to be stating opposite things. Can someone please explain? Thanks!
> Also, example 50 uses a 386 kip/in/sec2 and I think this must be a shortcut based upon someone's in-house experience or a Roark's Formula or something. Could someone help?
> 
> Thanks!


Seems like 12.8 equations are for buildings while 15.4.4 is for nonbuilding structures. I.e., 12.8-9 is for masonry or concrete shear wall structures.


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## Ble_PE (Aug 24, 2011)

I think what it is stating is that the fundamental period shall be determined using a properly substantiated analysis, i.e. using a structural analysis program like RISA. The equations from section 12.8 are approximate equations that are only valid for building structures, so these cannot be used. You can use Eqn 15.4-6 to calculate the period in lieu of a structural analysis.


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## McEngr (Aug 25, 2011)

Ble_PE said:


> I think what it is stating is that the fundamental period shall be determined using a properly substantiated analysis, i.e. using a structural analysis program like RISA. The equations from section 12.8 are approximate equations that are only valid for building structures, so these cannot be used. You can use Eqn 15.4-6 to calculate the period in lieu of a structural analysis.


Sorry if I'm making this more difficult than it has to be: it appears that equation 15.4-6 is the period equation for the first mode of vibration. Is this correct? In the Seismic Design Manual Vol 1, they use a number within the equation that simplifies this calculation. I figured that for a water tower, it would be very similar to a pendulum type of relationship whereby all the mass is mostly at the top. Is this a correct understanding?


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## Ble_PE (Aug 25, 2011)

I assume you're talking about Example 50 in the SDM Vol 1. They did simplify equation 15.4-6 by using m/k under the square root. They did this since you were given the stiffness of the tower in the problem statement. At work we have a earthquake guideline that explains eqn 15.4-6 is simply modelling the mass and stiffness of the structure, hence the use of m/k. I find the code explanation of the equation and the variables used to be very poor and there is no help in the commentary either.

Does this help any? :dunno:


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## McEngr (Aug 25, 2011)

Ble_PE said:


> I assume you're talking about Example 50 in the SDM Vol 1. They did simplify equation 15.4-6 by using m/k under the square root. They did this since you were given the stiffness of the tower in the problem statement. At work we have a earthquake guideline that explains eqn 15.4-6 is simply modelling the mass and stiffness of the structure, hence the use of m/k. I find the code explanation of the equation and the variables used to be very poor and there is no help in the commentary either.
> Does this help any? :dunno:


Ble,

It does help. Thanks for the background. Hopefully, I don't need to derive a new equation like you have in the office eq guideline.

Much appreciated. Study on!


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## ipswitch (Aug 25, 2011)

McEngr said:


> Ble_PE said:
> 
> 
> > I think what it is stating is that the fundamental period shall be determined using a properly substantiated analysis, i.e. using a structural analysis program like RISA. The equations from section 12.8 are approximate equations that are only valid for building structures, so these cannot be used. You can use Eqn 15.4-6 to calculate the period in lieu of a structural analysis.
> ...


Seems like the free vibration equation but it's set up so your sigma f*delta doesn't have to be a fraction as in i.e., k= kips/in. I imagine the other section 12 equations have dampening built in???


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