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Re: [SEAOC] Concrete Dilema
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- Subject: Re: [SEAOC] Concrete Dilema
- From: "Hunt, Tom" <tom.hunt(--nospam--at)fluordaniel.com>
- Date: Mon, 15 Apr 1996 08:44:00 -0800
- Priority: Urgent
Eric, As described earlier, the neutral axis in reinforced concrete can move depending on the stress level in your beam. Part of your "dilemma" is to determine what you are going to use the neutral axis calculation for. Typically this is needed for serviceability requirements such as crack control. In this case, you need to use the Unfactored Moment and the actual rebar stress fs and not fy. Using the working stress method here is quite acceptable. However, you can still use the Ultimate Stress Method but it is a little harder and best done with a trial and error table. Assume a rebar stress fs, say 30ksi, then calculate "a" by a=As*fs/.85*fc'*b, then calculate fs=M/As(d-a/2), if ASSUMED fs does not match CALCULATED fs then guess again until they are equal. Now you have the correct "a" and can calculate your neutral actual axis by a=c*B1. You can use a simple table such as: |assumed| |calc'd| |calc'd| | fs | | a | | fs | In an exam, a quick and dirty way to get fs (and hence "a") is to use fs(approx)=M/j*d*As where j is assumed to be 0.9. This is normally pretty close. Good luck on your exam, Thomas Hunt Fluor Daniel TOM.HUNT(--nospam--at)fluordaniel.com ______________________________ Reply Separator _________________________________ Subject: [SEAOC] Concrete Dilema Author: seaoc::(SEAOCAA) at ~FABRIK Date: 4/12/96 10:54 PM From: seaoc(--nospam--at)seaoc.org Date: Fri, Apr 12, 1996 10:54 PM Subject: [SEAOC] Concrete Dilema To: seaoc I am studying to take the PE exam on the 19th, and I have run into a snag on determining where the neutral axis is in a RC beam. The method that I have used in the past is to simply use the expression: a=As*fy/.85*f'c*b and a=B1*c However, in flipping through my concrete book and Lindburg's PE review manual I found another method which uses a factor multiplied with the depth, or: c=k*d where k=sqrt(2*p*n+(p*n)^2)-p*n p= the steel ratio (since rho is not part of my keyboard) n= the modular ratio of E (steel)/ E (conc.) I know that one expression is based on Ultimate Strength and the other is based on Working Stress, but since they both assume a cracked section shouldn't they give similar values? The example that I used was a concrete beam with the following properties: b=18" d=37" f'c=3000 psi fy=60,000 psi As=6.0 in^2 Ec=3,122,000 psi Es=29,000,000 psi n=9.29 k=0.333 The location I get for the neutral axis using the Ultimate Strength method is 9.23". The value I get using the Working Stress Method is 12.34". I have less than a week till the exam and I would really appreciate it if anyone can let me know which method is the correct one. Thanks, ============================== Eric J. Scott escott(--nospam--at)csus.edu ASCE-YMF Membership Chair, Sacramento Cline, Agee, and Swedin Engineers and Architects ============================== ... -------- Received: from lalug.org by portia.fabrik.com with SMTP (AlisaMail V05.1-000a) id SINN.1196023(--nospam--at)portia.fabrik.com ; Fri, 12 Apr 1996 20:10:20 -0800 Received: from arcana.org (arcana.org [198.186.216.151]) by lalug.org (8.6.12/8. 6.9) with ESMTP id SAA08385 for <seaoc-outgoing(--nospam--at)lalug.org>; Fri, 12 Apr 1996 18: 54:23 -0700 Received: (from majordom@localhost) by arcana.org (8.6.12/8.6.12) id RAA13319 fo r seaoc-outgoing; Fri, 12 Apr 1996 17:50:52 -0700 Received: from power.net (touchstone.power.net [198.186.216.131]) by arcana.org (8.6.12/8.6.12) with SMTP id RAA13315 for <seaoc(--nospam--at)arcana.power.net>; Fri, 12 Apr 1996 17:50:51 -0700 Received: from saclink1.csus.edu by power.net with smtp (Smail3.1.29.1 #3) id m0u7uUB-000xDEC; Fri, 12 Apr 96 18:50 PDT Message-Id: <m0u7uUB-000xDEC(--nospam--at)power.net> Received: from u0107-p14.dialin.csus.edu by saclink1.csus.edu with SMTP (1.37.109.16/16.2) id AA009000305; Fri, 12 Apr 1996 18:51:45 -0700 Comments: Authenticated sender is <sac39899(--nospam--at)saclink.csus.edu> From: "Eric J. Scott" <sac39899(--nospam--at)saclink1.csus.edu> Organization: CSU Sacramento To: seaoc(--nospam--at)seaoc.org Date: Fri, 12 Apr 1996 18:50:26 -800 Subject: [SEAOC] Concrete Dilema Priority: normal X-Mailer: Pegasus Mail for Windows (v2.01) Sender: owner-seaoc(--nospam--at)arcana.org Precedence: bulk Reply-To: seaoc(--nospam--at)seaoc.org -------- ...
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