Minimum Area of Steel for Beam?

[Redacted]

Hi there,

I am trying to figure out if I have been going about doing beam detailing incorrectly.

For past designs, for simply supported beams, even though top (compression) reinforcement is not required based on my structural calcs, I still normally include the minimum area of steel as the top reinforcement.

I have always assumed that the minimum area of steel is required for top and bot ( I normally design to Eurocode).

Although after looking at my local building code I have noticed that the beams/lintels they spec don’t have top reinforcement until you get to 8’.

I have been including the minimum area of steel as top reinforcement for all beam spans even short spans. For instance for a lightly loaded 5’ beam that is 8” deep, I would have specced 2T12 bot and 2T10 top.

In your opinion is this an issue? In hindsight perhaps I have misunderstood the minimum area of steel term?

Logically, including the top rebar would help from a constructability standpoint e.g. tying stirrups but if the additional minimum area of steel at the top of beams is not beneficial(or even disadvantageous) I would like to know for future detailing.

 

[Lomarandil]

The minimum area of steel provisions (at least in ACI) apply to the total cross section, not to top and bottom sides. Of course, that minimum area should be located on the tension side of the member to be effective and make sure the reinforcement yields.

That said, there are often times when bars are added to the *compression side* of the beam just as you mentioned -- sientirely to be able to anchor the beam stirrups. For lintels, the common solution is to increase the lintel cross section to ensure the shear can be carried by concrete without stirrups, then the top bar is not needed. Your short beams may merit a similar solution -- as long as it won't confuse your contractor too much.

All of this assumes a truly simply supported condition, without incidental restraint at the support (or where some rotation/cracking is allowable).

*(Note that for minimally reinforced beams, steel on the "compression side" is in fact often still deep enough to be stressed in tension, although not always yielded).

----
just call me Lo.

 

[dik]

I usually add 2 of the next lower bar size that's used for bottom reinforcing or 2-15M bars... depending on the mood or application. Adds a little more strength and a little less deflection, both long term and short term as well as something to support the top of the stirrups.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[HTURKAK]

The beam minimum tension and compression reinforcement specified for separately,

EC-2 :
9.2.1 Longitudinal reinforcement 9.2.1.1 Minimum and maximum reinforcement areas (1) The area of longitudinal tension reinforcement should not be taken as less than As,min....

-Tension reinforcement in rectangular beams in bending As min = 0.13% Ac
-Compression reinforcement (if required) in rectangular beams As min = 0.2% Ac

If you are following EC-2 the minimum compression reinf. is applicable if necessary..

 

[KootK]

In North America, the old school approach was to provide minimum top steel somewhere in the neighborhood of 25% to 33% of the bottom steel. That, acknowledging the fact that most CIP beams will have hogging moments at the ends even if the designer chooses to design the beams as though they are simple spanning. Without that top steel at the ends, one risks:

1) Serviceability cracking issues and:

2) Compromising beam shear capacity where moments are reversed relative to the design assumptions.