Congested Reinforcing in Hub of Cantilevered Beams 1

[waytsh]

I am struggling to find a good way to place the reinforcement in a pedestal with cantilevered beams protruding in all directions. I am running into a problem when I try to bring the top steel in the beam to the far side of the column to get proper development. All of the steel is intersecting at the center of the column making it impossible to place properly. I can stop the steel short and hook it down before they interfere but I am not comfortable with this even though they would hook down inside of the column reinforcement.

Is there a recommended detail for conditions such as this? Is it acceptable to bring the steel down on the front side of the column as long as it is encased by the column steel? I would greatly appreciate any thoughts you all may have.

 

[dik]

Can you weld a 'head' on the end of the reinforcing to act as a headed stud? and just extend the reinforcing vertically into the mess of reinforcing? If you don't have the clearance for that, then you likely have to increase the size of the members to obtain clearance.

Dik

 

[waytsh]

Dik,

Thnks for the input. My isssue is more with the convergence of all the horizontal bars more than it is with the verticals projecting out of the pedestal. I am thinking about sweeping adjacent horizontal bars together so that I do not have to create as many layers. For instance I would let one of the beams reinforcing run straight through to the opposite side of the column. Then the two adjacent beam reinforcing would sweep into line with this steel as they approached each other. Does this sound reasonable? Then instead of having 6 layers stacked on each other I would only have two.

~waytsh

 

[dik]

not sure... you might want to draw up a cad detail for placing to confirm that the bars fit.

Dik

 

[jike]

I am not sure that I understand. Why can't the top steel run thru to the opposite beam in both directions? Wouldn't this be only two layers of horizontal bars (one in each direction)?

Be careful about turning the bars 90 degrees to connect with the other beam steel. The steel on the inside face could break out the corner of the pedestal. Outside face bars are not usually a problem.

 

[waytsh]

It is a little hard to explain but there are actually 7 spokes on this "pinwheel" that encompass 180 degrees of the wheel. The other side is against another structure. So there are really only two beams that are opposite each other and will allow the steel to run straight through. The other 5 spokes will all come in at a different angle each one 30 degrees away from the last. So if I would run the steel straight through it would mean 6 layers of steel at the center of the column.

I was hoping that I could create two groups and that a 30 degree bend for the two adjacent beams would not be too hard of an angle once it enters the column.

I laid the entire thing out on AutoCAD and it looks like it will work as long as there is not some reason this should not be done.

Is it possible to post drawings somehow to show you exactly what I mean?

Thanks,

waytsh

 

[jike]

Could you pair up 6 beams and therefore have only 3 top layers of bars? Then you would need to terminate only one set of bars with 90 hooks in the far face of the pedestal.

 

[Tomfh]

What are the dimensions? Is there a slab?

 

[waytsh]

jike,

Yes I suppose I could go to three layers if 30 degrees is too much of a bend to make. Thanks for the suggestion.

Tomfh,

The beams are 17' cantilevers and the slab is 8" thick except for the 11' diameter central hub which is as deep as the beams (36"). The center column is 5' in diameter and the contactor has requested that I use #10 bar for the beam reinforcing since he has an overstock of that size. It actually ends up being a good thing because then I only have 4 bars per beam coming into the center. Although they are going to be much harder for him to bend.

waytsh

 

[jike]

I think that there is an ACI provision to distribute negative top beam steel in the adjacent slab, which you may also want to take advantage of.

 

[JAE]

waytsh - if I understand you correctly, you have a series of beams cantilevering out of a center axle such that there are few, (only two you say) conditions where there are beams directly opposite any one cantilever -

and this creates a condition where each beams top steel must extend into the center pedestal (or axle of the wheel) and be developed into it so as to provide the necessary top negative moment steel...is this correct?

And you are supposing that you could take two beams which are "sort of" opposite one another and bend the top steel so there would be a sort of continuity...right?

I would be hesitant to do this - for the reason that any kind of bent rebar placed in tension will try to straighten out - thus, popping off any concrete cover and possibly failing altogether.

One idea would be to create a steel plate hub where all the bars would interconnect via welds, or threaded couplers welded to the center hub. Lenton has some tension couplers that would work here as well as other companies. This would minimize congestion within the axle pedestal and also negate the difficulty in getting development via hooks into the pedestal.

Also - your suggestion to run the top bar into the pedestal and hook it down 90 degrees before it crosses any other top bars from the other cantilevers doesn't work - hooks have specific development lengths too and I doubt you have a large enough pedestal to allow the hooks to all develop without any bars crossing....but you never stated the size of the pedestal...

 

[miecz]

Even if you get the reinforcing to fit, I have a hard time seeing how concrete will get into the little cavities. If you use a hub, you should think about perforating it for the same reason.

 

[JLNJ]

Sounds like a job for structural steel!