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# Drift Criteria / Cracking - Wind Loads

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• To: seaint(--nospam--at)seaint.org
• Subject: Drift Criteria / Cracking - Wind Loads
• From: "Anantha Narayan C.K." <anant_27(--nospam--at)yahoo.com>
• Date: Mon, 29 May 2006 15:08:05 -0700 (PDT)

Greetings,

I am looking for answers from experienced engineers for a few questions and this listserv has been more than helpful in many previous instances.

The first question is aimed toward drift criteria for wind loading in buildings. I work in Miami and Miami-Dade county requires all buildings to be designed for wind speeds of 156 mph (3 second gust). There is no prescribed load combination to check for drift under service loads in ASCE 7-02 nor FBC 2004 except a load combo in the commentary of ASCE 7-02 which requires drift to be checked for D + 0.5 L  + 0.7 W. Typically we have been checking drift under service wind and D + L + W. Are there any other specified means of checking for drifts at service loads?

The second question is aimed towards cracking in concrete at service loads. Chapter 10 in ACI gives stiffness modification factors for columns, beams and walls. However, I am going around in circles trying to figure out the accurate means of establishing cracking in concrete and its associated reduction in stiffness (increased drift). Typical buildings that we work with might not have appreciable change in stiffness due to cracking, but I do wish to understand the rationale in the code.

1. For shear walls, ACI requires you to use a stiffness modification factor of 0.7 (even prior to cracking?), run your analysis under service loads, check for cracking under service loads and then use 0.35 in case there is cracking and re-run. Also the code suggests using 1.0/0.7 = 1.43 for service load analysis. Does this mean you are increasing your stiffness to a value greater than Ig? What is the suggested means to model a realistic behavior? Why is the stiffness reduced by 20%-30% for axial members and by around 50% in flexural members? Are there any experimental values that these are based on? List of references/sources will be very helfpul in understanding the same.

2. Moment-curvature diagrams can be created for columns, beams etc. Mcr can be calculated based on Ig (not including the increase in Ig due to reinforcement) per Mcr = fr*Ig/y. Based on this and the service load moments, degree of cracking can be computed. This can then be used as a stiffness modification factor. Is this approach of any value in trying to compute accurate cracking and reduction in stiffness.

I would appreciate your responses in this regard.

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ANANTHA NARAYAN, E.I.
Structural Engineer
Bliss and Nyitray Inc.
Miami, FL - 33134

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ANANTHA NARAYAN, E.I.
Structural Engineer
Bliss and Nyitray Inc.
Miami, FL - 33134