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.

 

[JedClampett]

Depends on what is causing the moment and what direction it's in. If it's dead load and live load perpendicular to the edge, you've got a problem. If it's seismic and wind parallel to the edge, you can add bracing or a shear wall.

 

[ishvaaag]

At these sizes you should be able even to produce very specific details to deal with the problem, this is the high range of spans within buildings.

This meaning that you should be able to modify the general scheme (path of forces, lateral forces resisting system) to allow to deal effectively with your problem with some alternatives, the simple of which to place some hinge there, and if unwanted, then have bigger columns, something even bigger from where to hang or from some nucleus with some cables, very variegated ideas might appear.

I have already said of one case where I dismissed a idea for precast members L T etc that required 60 MPa and awesome sections, well, 10 years later someone has seen opportunity in it and someone has put money enough to keep functioning a glistening precast plant just on these principles. Most of us that do not deal with monster structures are not enough acquainted with the monstrosities of piercing the bottom of the seas with giant pipes, or make shear walls of steel over a foot thick. Some get opportunity of such possibilities and resolve such problems maybe in their case not otherwise more easily resolved.

 

[hokie66]

I assume your beam is 500 wide x 1200 deep, and the column is 500 wide x 800 deep. Correct? What limits the width of your beam and column? What is the spacing of the columns in the other direction and how thick is the slab between the beams? With better understanding of the whole structure, we can possibly give some ideas to help.

 

[BAretired]

One idea is to post tension the beam to balance some or all of the dead load.

A second is to create a hinge at the top of each edge column and design the beam as a simple span.

A third is to reinforce the beam as a restrained but not fully continuous span and use a reduced amount of reinforcement in the column, ensuring that the sum of the positive and negative moments in the beam satisfy statics. Some cracking may be expected in the column.

BA

 

[slickdeals]

I agree with BA. I have done this in the past. The bigger you make your column the more moment it will attract. Hence, you are better off using the third option as suggested by BA. Make sure you provide closely spaced shear reinforcing at the top of the column and provide additional bottom reinforcing in the beam.

In your structural model, you can model this by reducing the moment of inertia of the column by using crack factors.

 

[hokie66]

Good points, BA and slickdeals. But we still need more information from the OP to give good advice. He needs to answer Jed's query and mine. The other thing which should be considered is to add an additional column. 20 metres is a long span, and 10 metre spacings can be accommodated in most buildings.

 

[BAretired]

Good point, hokie. If a column at midspan can be accommodated, we don't have a problem.

BA

 

[RLC32681]

Thank you BA and slickdeals, however i consider already that option and requires me a big rebar diameter in order to meet the moment requirement.

BA,

Can you explain how to introduce a hinge at the top of the column. How should it be detail? Sorry for the ignorance.

Jed,

the load that governs is the combination of dead load and live load. it is parallel with the span of the 20m beam.

Hokie,

Intermediate column is not possible is not advisable because of its occupancy. for the beam, 500mm is the width and 1200mm is the depth. you are also correct with the column, 500mm wide and 800mm depth. The distance between columns in the other direction is 6.2m and the slab thickness is 150mm.

we can adjust the width of the column and beam but i think it would increase the dead load and one thing more the moment is acting perpendicular to the width of the column. Please correct me if i'm wrong.

 

[BAretired]

RLC32681,

I don't believe a hinge at each column is your best option because the beam is too shallow for a simple span of 20m unless it is prestressed.

You said

however i consider already that option and requires me a big rebar diameter in order to meet the moment requirement.

You do not have a specific moment requirement to meet. Ignore the elastic analysis and consider:

Simple span moment = wL²/8.

M_neg = wL²/24 (minimum recommended).

M_pos = wL²/12.

I would be inclined to boot that up to M_pos = wL²/10 for good measure.

In the circumstances, that is a better solution than a hinge at each column, but you must expect some cracking in the column.





BA

 

[slickdeals]

You can always add compression steel in your beam which will also help in deflection control.

 

[hokie66]

Try turning the column 90 degrees so the 500 dimension is parallel to the span. As a general rule for constructability, it is not good practice to use the same width beam as column, as the reinforcement clashes. And I agree with BA, the beam should be post-tensioned. Balance all the dead load with the prestress, and use bars for the live load.

If you have other means of coping with the horizontal forces, you could design the beam to column connection with a bearing, similar to bridge construction. This would eliminate bending from the connection.

 

[rapt]

hokie66,

You cannot ignore all of the dead load in that case. Factors still need to be applied so

1.2 * SW + 1.2 * SDL + 1.5 * LL + 1.0 * PT (factors vary depending on code)

So, if you balance SW, there is still
.2 * SW + 1.2 * SDL + 1.5 * LL

if you balance all DL, there is still
.2 * SW + 2 * SDL + 1.5 * LL

remaining.

Also, I would question a slab depth of 150mm spanning 6.2m. You might be able to make it work with PT, definitely no as RC.

 

[BAretired]

I agree with rapt. The slab depth is not adequate for the span. It seems to me that the OP is trying to stretch permissible spans far beyond where they ought to be. It would be prudent to take a look at the building code in your area.

BA

 

[hokie66]

rapt,

Yes, I agree. My suggestion was too simplified. And I also agree that the slab is too thin.

 

[francischacko]

Is it possible to space the columns on the other direction to 3.1metre instead of 6.2 metres.More over 150 mm thick slab for 6.2 metres span will not be sufficient and by reducing the spacing there will be much reduction in loads/moments

francis

 

[RLC32681]

Post tension is a very good solution for this project; however, the contractor which is our client demands to use reinforced concrete. I have made a further analysis and incorporated your opinion guys, I came up with an idea of extending the rebars coming from the beam to the column to satisfy the required moment. FYI, huge moment in the column occurs only at a distance 2m below the beam. May I know your comments guys with my solution. I would appreciate if you can add some recommendations.

BA, Francis, Hookie

Thanks for reminding me about the slab thickness, I will check the deflection if it is okay otherwise increase it to 180mm.

 

[hokie66]

RLC32681,

Is there a reason you cannot turn the column 90 degrees as I suggested? That would reduce the column moment considerably, at the same time increasing the beam moment.

It will increase the beam deflection also, and 20/1.2=16.7 is already pushing it for a simple span reinforced floor beam. Some compression reinforcement will help, but the beam really should be tensioned.

 

[rapt]

I think you need to discuss these things with your supervising engineer.

Extending the beam bars is possible but the details depend on the code you are using. And the bars will have to develop past the point where they are needed using full tension development. And the builder will hate you.

RTe the concrete thicknesses, we are not suggesting the slab thickness might not work, we are saying it WILL NOT work. If you design shows it will, change design tools!

I hope you client does not mind deflections!

 

[phuduhudu]

I can assure you, you will find it very difficult to detail the beam column connection. Your max moment is on the outside top edge of the column/top end of the beam so you will end up having to take your column bars round the corner into the beam. The internal radii of these bends need checking for concrete bearing strength. If you have more than 1 layer of reinf then you have additional problems. And if you have a perpendicular beam coming in as well then you have another problem. For this type of detail it is the detailing that can determine the member sizes rather than plain member section size calcs. The beam span just does not seem practical.