Preffered Beam Detailing

[SteynvW]

Hi All

We had a discussion in our office and I would like
to know you preffered method.

When to beams connect over a column:

1. Secondary beam top reinforcing is placed lower througout beam
under primary beam top reinforcing and designed with the reduced depth.

2. Secondary beam top reinforcing cranked and placed on
the same level as primary beam top reinforcing and design with reduced depth.

There are good arguments for both, so basically what is the preference.

Kind regards

 

[KootK]

I vote for #1:

1) the cranks will tend to straighten out and spall the concrete over the primary bars. Stirrups in the vacinity of the crank will help with that but I'd rather not rely on that.

2) the cranks add a bit of field complexity.

3) With the cranks, I'd ague that the secondary beam flexural depth is stil the lower d value at the face of the support anyhow.

4) A nice feature of beams is that they are generally not too sensitive to minor changes in flexural depth. If the primary beam has multiple layers of top steel, I might try to plan things out such that the secondary beam steel can run beneath the top layer of primary beam steel.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

Further on the bar fabrication; whenever I put "cranks" or similar in a long bar detail the shop guys always come by my office asking if they really have to do that. The issue they have is it's hard to ensure that a long bar like that is straight after bending it a few times. Often they have to tweak it back and forth a bit until they get it straight and it ends up taking a lot of time if they have to do a lot of them like that.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[rapt]

Definitely no cranks in the top bars near the support, Put in a splice bar that extends to the contra-flexure point to lap with the beam top bars if you want and then the top bars in the remainder of the beam can be deduced in size or number of.

 

[canwesteng]

Number 1 for sure. No one building number 2 is going to be happy.

 

[KootK]

Curious: I don't think that I've ever seen a set of plans where a bar placement order was specified for beams as it generally is for slabs. Nor have I seen one where where rapt's solution was explicitly detailed (I like it, just haven't seen it). Do field crews normally just bend the bars down as required and accept that the stirrups are a bit too tall in spots and the cover greater than specified? There Or are others actually detailing the increased cover for crossing beams explicitly? There could be minor implications for stirrup performance and crack control.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[calvinandhobbes10]

Do field crews normally just bend the bars down as required and accept that the stirrups are a bit too tall in spots and the cover greater than specified?

In my experience yes, they use creative prybar work to marry up the beam cages and are unlikely to revise anything except fish a few more stirrup legs or straight bars into the cage thereafter. I call for alternating top bar layers depth between intersecting beams, although I usually do this with a note in the beam schedule and not a full-on detail.

 

[rapt]

KootK,

We detailed it often in the old days of full detailing. But then we had to think of clashes at high points with PT and sort out these sorts of details and did the same for the reinforcing. And also at end connections with columns etc.

Unfortunately others in the PT industry used to detail tendons in both directions to the same top cover and leave it to be sorted out on site, which resulted in under strength as one direction ended up lower than it was designed. But they did not worry about that as their solution was cheaper, even if it was under designed! And that is one of my biggest gripes with engineers today!