Fundamentals of Concrete Beam Analysis Question 1

[mappryan]

Should be pretty simple question. I just never thought about it before and am curious. See attached image. Thanks

 

[JennyNakamura]

When I think of "serviceability" in concrete beam design, I think of deflection. We design our reinforcement based on "strength" design, as the concrete must crack in order for the flexural steel to engage. I don't know the answer to your question, but why would one need the NA of an uncracked beam to check for deflections?

 

[JennyNakamura]

What I meant was, the ACI equations for the effective I requires knowing I_gross and I_cracked when calculating I_effective for the purposes of checking deflections...so why would you need to calculate the NA of the uncracked beam?

 

[IDS]

The strength design NA is based on a rectangular stress block with a known strain and stress.

The serviceability design is based on a triangular stress block with a maximum stress dependant on the applied loads.

Both assume a cracked section with the concrete taking no tension, and ignore the compression steel.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[jhenry]

Ignoring the compression steel for short term deflection is OK, but the compression steel is important for long term deflection control. For serviceabilty design deflection, use the transformed section of the compression (top) steel to determine the total compression area.