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T-beam over support

T-beam over support

T-beam over support

(OP)
Is there any possibilities that the neutral axis fall inside the flange ? If it’s possible , I think we shall design it as T-beam /flange beam ? This text stated that for T-beam at support, we shall design it as rectangular beam. Can someone explain on this?





RE: T-beam over support

Quote (Is there any possibilities that the neutral axis fall inside the flange ? If it’s possible , I think we shall design it as T-beam /flange beam ?)


Yes possible ...The neutral axis may fall within the flange . In this case the compression zone occupies some part of the fl ange.So, the concrete section at tension side of the neutral axis is assumed to be ineffective, and the beam is treated as a normal rectangular beam.

Quote (This text stated that for T-beam at support, we shall design it as rectangular beam. Can someone explain on this?)


For continuous beam with multi span and beams with rigid connection to columns, negative ( hogging ) moment develops at supports for gravity loading. So, the T beam will be treated rectangular at supports.
You may look to any RC handbook , text book to get the concept.

The following figure is copy and paste from DESIGN OF REINFORCED CONCRETE STRUCTURES ( by. S. NARAYANAN )

RE: T-beam over support

(OP)

Quote (HTURKAK)


How if the neutral axis fall inside the flange for section B-B ( T-beam at support) ? Do we need to consider the compressive part of the small flange part ?

I knew that for section A-A when the neutral axis fall inside the web ( deeper than flange) , we do need to consider the extra web part ...

Do we need to consider the compressive part of the extra small flange part ?

RE: T-beam over support

Quote (dccd)

How if the neutral axis fall inside the flange for section B-B ( T-beam at support) ? Do we need to consider the compressive part of the small flange part ?

If the neutral axis for negative bending falls within the flange (which is unlikely for a typical T beam), theoretically, you would find the centroid of the section by summing the first moments of the areas (A*y) and dividing by the total area.

Proportioning a T beam with a stem/web that small would be very inefficient. The typical solution for this would be to widen the stem to make the section more efficient.

In the real world, the effectiveness of the flange portion in compression would be questionable, as well. If faced with that situation for design, I would ignore the flange and take the compression block as being the rectangular block that's the width of the stem and depth to the neutral axis.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: T-beam over support

(OP)

Quote (BridgeSmith)


So, do you mean for neagtive bending moment case, you will consider whole flange undergo tension despite NA will have chance fall in flange ?

RE: T-beam over support

(OP)

Quote (BridgeSmith)


so, section C-C can be designed as rectangualr beam , while section D-D can be designed as T-beam ?

RE: T-beam over support

You have so much negative moment that your compression block is deeper than your stem depth? That seems like an extremely unlikely scenario.

It would be significantly more cost effective to just widen the stem so that your compression zone is wider allowing a reduction is compression block depth and keep it within the stem.

RE: T-beam over support

Quote (dccd)

So, do you mean for neagtive bending moment case, you will consider whole flange undergo tension despite NA will have chance fall in flange ?

Concrete in tension is typically ignored for design, so the section would consist of the reinforcing in the flange (in tension) and the concrete in the stem (in compression).

Quote (dccd)

so, section C-C can be designed as rectangular beam , while section D-D can be designed as T-beam ?

The question was concerning a T-beam subjected to a negative bending moment (tension in the top), therefore, Section B-B is the applicable diagram. If you had an inverted T-Beam subjected to negative moment, then Section D-D would be applicable.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: T-beam over support

dccd,

The calculation of a T-beam is a straightforward exercise that may be found in any textbook discussing reinforced concrete design. If the neutral axis extends from either the web into the flange (very uncommon), or from the flange into the web (more common), you calculate moment resistance by superposing the resistance of outstand flanges and rebar (lever arm = distance between flange N.A. and rebar) and the resistance of the web and rebar (lever arm = distance from rebar to N.A. of web compressive block).

It would be unwise to design a T-beam of either type "a" or "b" (slab on "top" or "bottom" of the beam) that has neutral axis located very deep in the section (example: type a, section B-B, hogging at supports, with N.A. in the top flange and thus a very large compression block), because it leads to very small steel strain at ULS and subsequently to brittle fracture. This is also explained in university courses on RCS design and in textbooks on RCS design.

I suggest that you brush up on the subject before attempting this sort of design.

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