# RE: ACI318-99 load combination question

• To: seaint(--nospam--at)seaint.org
• Subject: RE: ACI318-99 load combination question
• From: "Lutz,James" <JLUTZ(--nospam--at)earthtech.com>
• Date: Thu, 24 Aug 2000 10:15:22 -0700
```For seismic effects associated with groundwater, I agree your geotechnical
engineer should be including this in his dynamic soil pressure
recommendations. I was thinking more of the situation where you have large
tanks integrated in the building structure.

I do a lot of water storage and wastewater treatment structures in seismic
zones, and you have to consider the seismic loads created by the mass of the
water. For simple circular and rectangular tanks on grade, there is some
"simplified" methodology for estimating base shear and pressure profiles
that was developed by George Housner of Cal Tech in the 50's and lives on in
American Waterworks Association Standards.

Housner's methods were published in some monographs and later in TID-7024, a
publication related to estimating loads for nuclear structures. You may also
find them referenced in books on structural dynamics, which sometimes
include a little discussion of hydrodynamic loads for dams and tanks. There
are lots of good papers on the subject, but I've yet to run across a
textbook devoted to the topic which really lays it all out for you. If you
have access to a decent university engineering library and want to look up
experts who have published on this subject, some recurring authors are
Housner, Haroun, Veletsos, Wozniak, Clough.

The mathematics involved in direct solutions are excruciating, and require a
very sophisticated level of math expertise (way, way over my head.) "Exact"
solutions involve partial differential equations and take the form of long
formulas full of Bessel functions and mind-numbing infinite series. Only a
couple of reservoir shapes have published solutions. Even Housner's methods
are based on approximate methods. For odd shaped tanks and reservoirs,
software is now available to solve for the wall pressures using
computational fluid dynamics, but the software is expensive, and most jobs
wouldn't justify the cost involved.

In addition to wall pressure and floor pressures created by horizontal
movement (and don't forget those sloshing waves), vertical accelerations
effectively increase the specific gravity of the fluid, creating more wall
pressure. Building codes avoid prescriptive methodology for determining
these kinds of loads. You are basically on your own, and have to research
the subject to come up with defensible numbers.

-----Original Message-----
From: Michael Bryson [mailto:bryson(--nospam--at)civil.ubc.ca]
Sent: Thursday, August 24, 2000 8:32 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: ACI318-99 load combination question

What exactly would the seismic effects be on the fluid? There is the
static effects on basement walls and hydrostatic uplift. But I think the
geotechnical engineer would give you the seismic effects indirectly with
some coefficient on the static effects?

On Wed, 23 Aug 2000, Lutz,James wrote:

forget
> seismic effects on the fluid, which are part of the E loads.
>
> -----Original Message-----
> From: Zachary Goswick [mailto:ZachG(--nospam--at)angusyoung.com]
> Sent: Wednesday, August 23, 2000 2:57 PM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: ACI318-99 load combination question
>
>
>
> Does anybody know whether equation 9-6 "U=1.4(D+T)" or the commentary on
the
> same section saying equation 9-6 is "U=1.05(D+T)" is correct?  It appears
to
> me that they are saying two different things, or am I not seeing this
> correctly?
>
> Also, I am looking at section 9.2.5 and reading that 1.4F should be added
to
> ALL combinations that have live load in them.  The commentary says use
just
> two equations "U = 1.4D + 1.7L + 1.4F" and "U = 0.9D + 1.4F".  This does
not
> seem to me to be ALL of the combinations with live load in them.  Wouldn't
> this be all of the combinations given in paragraphs 9.2.1 through 9.2.7.
as
> follows?
>
> U = 1.4D + 1.7L + 1.4F                          - from equation 9-1
> U = 0.75(1.4D + 1.7L + 1.7W) + 1.4F             - from equation 9-2
> U = 0.75(1.4D + 1.7L + 1.7(1.1E)) + 1.4F        - from paragraph 9.2.3
> U = 1.4D + 1.7L + 1.7H + 1.4F                   - from equation 9-4
> U = 0.75(1.4D + 1.7T + 1.7L) + 1.4F             - from equation 9-5
>
> It appears as if the second equation given by the commentary would not
even
> be included in what the actual code says.  Am I reading this incorrectly?
> Can anybody shed some light on these issues?
>
> I can figure out what would be the worst cases, but I am just trying to
>
>
> Zachary Goswick
> zachg(--nospam--at)angusyoung.com
>
>
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