edge column design 3

[RLC32681]

Hi guys..

I am presently designing a two storey with basement sub-station made of reinforced concrete frames. i have a problem in the edge columns in the upper floor because columns are only provided in the perimeter and support a 20m beam. the beam size is limited to 500x1200mm. Based on my analysis, the moment in the beam support that transfer to the column is very large, thus giving a steel requirement of 50-25MM dia. for the column. the column size is limited to 500x800mm.

Your opinion on how to reduce the column moment is highly appreciated. I would appreciate also if you can share any alternative to design this frame.

 

[RLC32681]

hokie,

I try rotating the column by 90 degrees but im sorry to tell you that column moments doesn't decrease, rather it increases.

Rapt,

Im using ACI code, it seems that extending the bars is more economical option since huge moment is only near the roof beam. i try to check the deflection but it does exceed the req't of the code. Regarding slab thickness, can you please discuss further why it will not work. Sorry if i did not get your point.

phuduhudu,

You are correct that detailing is the problem but i'm planning to increase the size of the beam and column at the point where the huge moment occurs by tapering it.

Thanks guys for your opinions, additional comments and recommendations would still help. I appreciate if you can share engineering solution for this project.

 

[BAretired]

The column stiffness of a 500 x 800 column is least when the 500 dimension is parallel to the beam. You should find that this results in the smallest moment. If not, you are doing something wrong.

BA

 

[apsix]

Perhaps the column was originally orientated to be 500 deep and rotating it 90deg increased its stiffness, hence the moment increased.

Regarding the slab thickness; a 150 thick slab spanning 6.2 m will have L/d of about 50; deflection will be excessive.

 

[rapt]

RLC32681,

If you do not have the experience to know, as apsix has pointed out, that an L/d of 50 for an RC slab will not work, then you should not be sizing this structure without doing the calculations to justify it. One way RC slabs definitely will not work at this L/d. An experienced design engineer would know this. An inexperienced design engineer should be talking to his supervising engineer about it, not us!

In fact, PT slabs will not work either. They will fail in vibrations, probably ductility and be very expensive to make work in deflections. For an RC slab, you have no chance in deflections either!

If you have changed your column to 500 in the direction of the beam span and 800 wide, the moments must reduce in the end column and increase in the beam!

 

[rowingengineer]

The ACI has minimum requirements for thickness of members for a slab take a look at TABLE 9.5(a)

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[rapt]

rowingengineer,

I think you should point this out to RLC32681, not me. But I think these numbers back up our arguement with minimal calculation required. For a 6200 end span, 6200 / 24 = 260mm thick slab!

Now the beam is in trouble due to extra load, and the columnn moments just got a lot bigger!

I do not base design on L/D ratios, I believe in calculating estimates of real deflection (not kcs fudges either).

 

[rowingengineer]

Rapt, my comment was aimed at RLC32681, as he had stated earlier that he was and is using ACI code.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[RLC32681]

I think ACI equations 9-12 or 9-13 must be use in the computation of minimum slab thickness since it is two way slab.

 

[seny]

9-12 or 9-13 for minimum thickness - yes, but it's not final required thickness check for particular case. after you need to do strength & deflection calculation to prove it (or to go w/ thicker)

 

[BAretired]

RLC32681,

You said:

I think ACI equations 9-12 or 9-13 must be use in the computation of minimum slab thickness since it is two way slab.

I don't have the ACI code to check the equations you cited but I understood this to be a one way slab.

BA

 

[rowingengineer]

I was of the opinion this is a one way slab as well.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[apsix]

We were told of beams spanning 20m at 6.2m centres, with a 150mm slab.

We should have insisted on a sketch at the onset.

 

[rapt]

apsix,

A case of garbage in, garbage out!

But then there must be secondary beams! If not, it is still a 1way slab.

Even so, 150 thick will not span 6200mm as a 2way slab!

And I would like to see the calculated deflections for 1200 deep 500 wide beams spanning 20m as an end span!

And interestingly, hokie66, when I run it with the columns about their weak axis, the column moments drop a lot! Who would have thougt that!

 

[80smetalfm]

RLC32681,

I strongly feel the 20m span beam would have a maximum positive moment & somwehat reduced moment at the column location as this is an end span & a very long end span!

FYI I have used 150 thick slab for 7m x6.5m two way slab using AS3600 with compression reo for an office block which is still rock solid! If your slab is oneway, i would be thinking around the 180-210 thk mark using 40mPa rc concrete.

Would like to see a plan sketch.

 

[rowingengineer]

what moments did you ahve to get compression reo in a 150mm slab? What % of the compression reo did you manage to engage? I have never heard of compression reo in a 150mm slab before.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[BAretired]

I agree that a sketch is in order. The OP has not stated that this is an end span. I thought it was a single span with an edge column each end.

BA

 

[RLC32681]

sorry if I did not mention erlier that I have an intermediate beams. As requested I attached here the framing plan for further reference.

seny,

As stated in the ACI code, the minimum thickness of two way slab is computed using either equations 9-12 or 9-13 depending upon the value of alpha. Slab thickness less than the minimum required by 9.5.3.1, 9.5.3.2, and 9.5.3.3 shall be permitted where computed deflections do not exceed the
limits of Table 9.5(b).

 

[80smetalfm]

Aha!
nice sketch, the 150thk slab sounds right to me based on that sketch. i also note that its a roof slab. Would there be a BBQ plate up there with some VB beer? I would be calling & rechecking for a 4kPa live load if it is. Just a thought.


________________________________
Use RAPT for Slender RC Columns

 

[rowingengineer]

80smetalfm,
I would like to here how you got compression reo to work in a 150mm slab when you have the time.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[rapt]

80smetalfm,

Actually, I would like to know how he got it into compression at all.

A lot of designers assume that reinforcement at the top at mid span is "compression reo". Unfortunately it is "compression face" reinforcement only. It is not in compression and cannot be used to reduce kcs in the deflection calculations in AS3600. You have to PROVE that the reinforicement is significantly in compression in the first place, and you would not be able to do that in this case.

Would be interesting to see the deflections, and the sketch!

In my opinion, another case or garbage i, garbage out, this time into code formulae.

80smetalfm
I would not be boasting about this to anyone. I cannot believe that the deflections are acceptable.

RLC32681
Now that the slab spans are 2way and about 4m, yes they should work at 150, depending on loading. But I would still be worried about the beams. I noticed on your plan they are now 1450 deep, instead of 1200!