Purpose of top bars for haning ties (links)

[bratty]

Hi

Is a thought provoking question on hanger bars. If there is a bottom load acting on a RC beam, hanging ties must be provided and tie to the top bar to support the load. Please see sketch in Case 1.

In case 2, what happen if there is a step and causes discontinuity in the top bar?

Must the top bar behave like beam supported on both ends?


In case 3, instead of hanging links, some recommends using V bars and they are supported by the top comressive force

 

[r13]

For both beam configurations, you have a constant shear, that equal to P/2, on the spans left and right to the load. If P/2 is greater then the allowable shear, then you need to provide stirrups for the entire beam, not locally as shown, otherwise, stirrup is theoretically not required. For both case, depends on how the load is applied, you might need to check local pull out strength, and provide additional reinforcement as required.

 

[Agent666]

In case 2, how were you detailing/developing the top reinforcement between the two depths. As you just show it stopping.

Irrespective of this I think provided you can drag the load to the top you've achieved the intent required for hanger reinforcement requirements. However I guess strut and tie can be used here to demonstrate a load path for the hanger force and the transition of forces between the two depths occurring simultaneously.

Never seen diagonal hangers before, but I guess you can check them based on your codes requirements for inclined stirrups because that's all it really is. Alternatively strut and tie can no doubt substantiate their effectiveness.

 

[KootK]

In case 2, what happen if there is a step and causes discontinuity in the top bar?

While the step obviously need to be accounted for in the design and detailing of the beam, I agree with Agent666 that you've satisfied the intent of the hanger bars in running them vertically to the top of the beam as you have. See some ideas below with regard to appropriate detailing and strut and tie modeling.

Must the top bar behave like beam supported on both ends?

No, the rebar does not need to behave like a beam. And that's good given that the rebar would make for a pretty terrible transversely loaded member. Often, prudent detailing practices give the impression that we're hanging load from transversely loaded reinforcing but that is almost never the case in reality.

Once you've dragged the load to the top of the beam you can expect it to make its way into the body of the beam via compression struts emanating down from the load and into your stirrup reinforcing as described by retired13. Again, see the strut an tie model proposal shown below.

 

[r13]

Is this a notched (in the bottom) beam?

 

[KootK]

I've been interpreting it as an upstand girder supporting an upstand beam. It's an uncommon condition to be sure.

 

[KootK]

In case 3, instead of hanging links, some recommends using V bars and they are supported by the top comressive force

1) I've seen similar detailing in older drawings and, in the world of precast, we use similar strategies where loads are light and the width of the incoming member is narrow.

2) While the mustache/hairpin bars may be able to be proven adequate, my recommendation would be to stick with the concentrated vertical stirrups. I believe that alternative preferable for a few reasons:

a) I feel that it's easier to build with less congestion issues.

b) I feel that it's a more direct, and more tractable, load path.

c) I feel that you'll struggle to get the capacity you need with the mustache bars because it forces all of your incoming beam load to travel through only two points of support as shown below.

 

[bratty]

Thanks for the contribution.

I am more focus on the hanging ties and the top rebar rather than strut and tie throughout the beam. In most conventional secondary beam is sitting on the girder acting on the top and bottom rebar together, hanging ties are not required. But as in this case, since the heavy loaded secondary beam just sitting on the bottom bar of the girder, the concrete in the girder above the secondary beam will be in tension. My Senior told me while the hanging links are required but the top hanger bars need not to be treated like a beam. It can be discontinued where the 450mm top notch occurs. the reason given is top of beam is under compression.


As for Kootk's load path of mustache bar, where does the development length begins in order to determine if the anchorage is adequate?