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RE: Flexural reinforcement "Ad Libitum"

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Jesus,

There are essentially two limits for minimum reinforcing that you must
satisfy.  The first is shrinkage and temperature, rho = 0.0018 on the
gross section.  The second is for flexural purposes to avoid having a
section so lightly reinforced that it actually behaves as an
unreinforced member (ie the reinforcing never gets activated and the
member fails in a brittle fashion).  The minimum for this condition can
be the smaller of:

1. Increasing the calculated required flexural reinforcing by 1/3.
2. rho minimum of 3 sqrt(f'c)/fy (=0.0033 for f'c = 4,000 psi and less) 

Taking your case, you previously satisfied the second case when the beam
was shallower.  With the increased depth, the calculated required
reinforcing for flexure should decrease.  If this new As multiplied by
4/3 is less than what you have already provided, then you have satisfied
the code.

Hope that is clear.  Let me know if not.

Eric

Eric R. Ober, PE, SE
Associate
Holbert Apple Associates
3423 Olney-Laytonsville Rd., Ste. 6
Olney, Maryland  20832
(301)570-1460 (Phone)
(301)570-1462 (Fax)
 
 

 

-----Original Message-----
From: Jesus Gomez [mailto:jgomez(--nospam--at)schnabel-eng.com] 
Sent: Thursday, May 18, 2006 4:36 PM
To: seaint(--nospam--at)seaint.org
Subject: Flexural reinforcement "Ad Libitum"

I have a "philosophical" question of important implication in a current
project.

I will not use real quantities. Also, I will not describe the problem
entirely and focus on a particular aspect of it. Say a beam of given
dimensions requires minimum reinforcement to handle a given bending
moment. Say that the beam is 100 in (d) by 10 in (b) and that the
minimum reinforcement is thus 3.33 in2. 

For constructability, the contractor would need (not prefer) to make the
beam with d=150 in instead of 100 in

Do I need to provide now 4.95 in2 instead of 3.33 in2 ?

The typical answer is "yes". However, how can the cracks become larger
for the same moment, same steel, and a larger section? Also, if I keep
increasing the size of the section, I would need unrealistically large
amounts of reinforcement.

Just for information, I am retrofitting an existing pile cap with
additional piles (micropiles). I am providing a new "pile cap" on each
side of the existing pile cap connected by dowels, which would work as
flexural and shear reinforcement at the interface. The dimensions of the
added pile cap are pretty much defined by external factors and I only
have a limited space available for dowels.

Thank you 




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