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RE: Concrete Tank Design Help

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Mark:

At interior wall conditions, I generally use standard hooks on the vertical
wall dowels and continuous steel in the base slab. Check CRSI to make sure
that the vertical dowels have enough embedment to be fully developed with a
standard hook. If not, use smaller bars at closer spacing or thicken the
base slab to provide the required embedment.

At exterior walls, I typically use a full lap splice both in the vertical
wall and the base slab.  I require the same dowels on both wall faces since
tanks are generally hydrostatically tested before backfilling, so large
forces can occur in each direction. I lap both dowels with the bottom layer
of steel in the base slab.

You do not want to lap an inside bar in the wall with a top bar in the base
slab. This is an "inside to inside" connection that if stressed will pop out
the corner. Extend those bars to the outside faces and hook.

How much fixity to use at the base is a matter of judgement. In reality, no
connection or its resiting elements can provide perfectly pinned or fixed
conditions.

You could do a sophisticated analysis by a finite element model in 3D with
compression springs to model the soil. This may be easy or difficult
depending on what software package is used.

A simpler anaylsis would be to design the wall for the maximum steel given
by each analysis (pinned or fixed) whichever is greater. Then provide dowels
to fix the base. Design the base to resist this overturning as if it is the
base of a retaining wall. Another analysis that you could use is a beam on
elastic foundation (Enercalc)with an end moment and vertical load. You may
have to thicken the base out a distance away from the wall to resist these
forces. The outward forces (caused by hydrostatic tesing) are harder to
resist by the base so I would probably consider this condition pinned. In
other words, conservatively bound the problem and design for the maximum
forces produced by either (pinned or fixed) analysis, whichever is greater.

I hope this helps!

Jim K.

-----Original Message-----
From: Mark Seifried [mailto:mseifried(--nospam--at)ggjengineers.com]
Sent: Friday, September 27, 2002 7:11 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Concrete Tank Design Help


My question was a general one for all tank walls: interior, exterior, and
different loadings. Further questions:

*What about preventing the transfer of slab moment into the walls due to the
fixed condition between the wall and slab?
*Partial fixity cracks with 90d hooks; at what plane in the joint would
adequate tension development end? It would seem that this point would not
occur until the bend in the hook, wouldn?t this moment dissipate as it has
traveled through the slab?
*I have run analysis of structural models with many different degrees of
fixity at the wall base, what percentage moment transfer could a 90d hook
pinned joint provide along the base of the wall? Zero?

Thank you for the response. I was beginning to think my question was too
easy, but I have looked through many ACI & PCA design guides that have not
touched on this subject. This list server has given provided tremendous
knowledge within many areas over the last year, especially since I?ve only
been out of school a couple of years.
Mark Seifried, EIT

From: Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com]
Sent: Thursday, September 26, 2002 10:59 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Concrete Tank Design Help


Is the wall to slab connection at an interior wall or exterior wall? If at
an exterior wall, is there exterior soil load which will put the exterior
face in tension at the base of wall?

At interior walls, the wall dowels may use standard hooks into the base
slab, assuming continuous reinforcement in the base slab.

At an exterior corner between a wall and base slab, and assuming moment
transfer with tension at both faces, the outside vertical wall reinforcement
should be lap spliced to the bottom slab reinforcement and the inside
vertical wall reinforcement may be doweled with a standard hook. A standard
hook at the outside of the corner does not provide adequate continuity if in
tension. (At any plane cut thru the joint, you should have continuity of the
outside tension reinforcement. If you just provide a standard hook on such a
bar, you may get cracking in the joint due to partial restraint without
adequate tension development.)

Also, the slab reinforcement must be able to resist the moment transferred
from the wall.

William C. Sherman, PE
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com

-----Original Message-----
From: Mark Seifried [mailto:mseifried(--nospam--at)ggjengineers.com]
Sent: Wednesday, September 25, 2002 8:29 AM
To: seaint(--nospam--at)seaint.org
Subject: Concrete Tank Design Help


My question deals with the transfer of moment from the base of a tank wall
into the tank slab. I know that this connection can be assumed closer to
pinned of fixed depending on the reinforcement detailing.

But, if this connection is assumed close to fixed, what kind of
reinforcement is necessary to complete this transfer of moment into the
slab? I typically see 90d standard hooks used as dowels at the same spacing
as the vertical wall rebar... Is this tension development length provided by
the hook adequate for transfer? Or does the standard hook EF result in more
of a pinned connection since the length of rebar parallel with the bottom
slab rebar is very small?

Thank you in advanced for your help or any direction you can provide?   Mark
Seifried, EIT

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