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[shirishmehta3]

Dear Sefians

I am interested to know how shear design of an inverted beam differs from that of a drop beam. As I see; while casting an inverted beam there may exist a cold joint in the slab which is supported by the beam at the beam - slab interface . Will this affect the concrete shear capacity?? Is it ideal to ignore shear strength of concrete and assume that the entire shear will be taken by the beam stirrups only??


Regards
Shirish.

[thirumalaichettiar]

Er.Shirish,

Normally it is advisable to cast the upper portion of the beam on the same day with less time interval.

It is usual that the shear due to construction problem is not taken into account while designing.

If the design is done for EQ or by not taking into the concrete shear Vc then it is the stirrups which will take the entire shear and no problem.

It is only a cold joint formed  that is a concrete problem and in most cases it is better to avoid .


T.Rangarajan.

[sspawar]

Dear Sir T. Rangarajan and all,

In my first field Job - yr1982-83, I was working as a Client Engineer Trainee with RANBAXY, at that time consultant firm was M/S PHIROZ KHUDUIANWALA for a PHARMA Project spread in 120 Acre land.
There was two long blocks called AMPICILLIN and NON AMPICILLIN of sizes 120m by 60m. These two blocks were completely constructed with Inverted beams and slabs with number of columns and spans.
The work was carried out straight forward in two tiers.
First is Slab only.
Then is remained Beam web. Time difference of casting was one day.
That time Consultant Mr. HEGDE from Mumbai was around 45yrs old, explained us that shear force takes place vertically or diagonally.
Here critical issue is separation of Horizontal layers, which is unavoidable and we have considered this difficulty of construction in design itself by providing extra stirrups
Horizontal construction joint /cold joint near neutral axis are not a matter of worry while extra stirrups and crank bars are provided as per designed aspects considering such incidents. So that total depth of inverted T beam will remain theoretically monolithic.
I was little confused on this issue. But second time I never come across of such incident.
Regards

[drnsmani]

Hi All,

If the T- or L-beams are provided with upstanding beams, the flanges will not contribute in resisting the positive BM at the mid-span, as they will be in the tension zone; hence you have to consider only the extended rectangular section in the design (as is done in the support points of down standing beams). But for shear design, as Er Rangarajan has pointed out, there is no difference in design.

I do not think there will be any problem in concreting such an upstanding beam- it should be concreted along with the slab. Such upstanding beams are quite common in raft slabs!

ACI 318-08 states the following: Transverse reinforcement over the lengths identified in 21.5.3.1 shall be proportioned to resist shear
assuming Vc = 0 when both (a) and (b) occur:

(a) The earthquake-induced shear force calculated in accordance with 21.5.4.1 represents one-half or more of the maximum required shear strength within those lengths;
(b) The factored axial compressive force, Pu, including earthquake effects is less than Agfc′ /20.

21.5.3.1 — Hoops shall be provided in the following regions of frame members:

(a) Over a length equal to twice the member depth measured from the face of the supporting member toward midspan, at both ends of the flexural member;
(b) Over lengths equal to twice the member depth on both sides of a section where flexural yielding is likely to occur in connection with inelastic lateral displacements of the frame.

Best wishes
NS

[sspawar]

Dear Sir,

First and formost I agree with you  that concrete should be in one pour.
But It is an ideal situation.
In practical it never be like so. ( As I experienced and worked with various design and concrete cunsultants including Late A P REMEDIOS - thrice in different projects)

Still my concern deviate towards statement of Mr. SHIRISH : Is it ideal to ignore shear strength of concrete and assume that the entire shear will be taken by the beam stirrups only??


I just go practical and put my words here:

If Inverted beam with slab is designed so that shear stress is found less than the concrete shear strength and as per codes provision nominal shear steel is provided.

More over in any design method shear is no concern in middle half of span.

Now if casting is done in 2 layers - slab and web.

Will it be relible structure?


I also agree that Raft Foundations are also an example of such structure in case considerable over burdened earth is there.
Generally open basements or shallow foundations are made with  raft footings, and in case to recieve the upword pressure beam may be designed as inverted tee beams.

Please do correct me if I am wrong.

Warm regards

[vikram.jeet]

Shear design:

Beam under bending is acted upon by shears in :
Vertical direction
as well as  
Horizontal direction

The resultant of these two forces give rise to Diagonal tension
for which shear reinforcement is provided in form of Stirrups
or Bent up diagonal bars

In case of layer(Horizontal ) joint due to time - lag in concreting:
(i.e. joint in the cross-section of beam)
Here the Horizontal shear needs to be tackled

The practice of developing a construction joint as per codal provisions
may result in aggregate locking and horizontal shear resistance is not
jeopardized

In case of proper construction joint possibility not certain, it is better to consider
additional vertical stirrups reinforcement for the horizontal shear at the surface of joint.
Horizontal Shear at joint H = F Ay' / I Kg/cm
Additional Ast for stirrups =h *100/t cm2 per meter

H - Horz shear at joint in Kg/cm
F - Vertical shear force in Kg
A - - Area of concrete section above joint
y' - - Cg Of above area from NA
I - MOI of section
t - permissible tensile stress for shear reinf.

In case of vertical joint due to time - - lag in concreting
(i.e. joint in the span of beam):
Here the vertical shear needs to be tackled

The practice of developing a construction joint as per codal provisions
may result in aggregate locking and vertical shear resistance is not
jeopardized
In case of proper construction joint possibility not certain, it is better to consider
additional horizontal reinforcement for the vertical shear at the surface of joint.

Horizontal skin bars at joint embeded (2* Ld) on either side of joint ,may help,

Area of the top & bottom reinf( provided for flexure) could be enhanced to take
take care of additional vertical shear at joint

Additional horizontal Ast = F / t ( to be provided half at top and half at bottom)

OR to be provided as Skin bars .

- - - - - - - - - - Subject to corrections from sefi members

regards

vikramjeet



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[drnsmani]

Dear Er Pawar,

In your posting you have written like this:
Quote<<If Inverted beam with slab is designed so that shear stress is found less than the concrete shear strength and as per codes provision nominal shear steel is provided.

More over in any design method shear is no concern in middle half of span.

Now if casting is done in 2 layers - slab and web.

Will it be relible structure?>> Unquote


I agree with you that at mid span shear is not a problem, unless a shear force is acting near midspan.

If proper care is taken to avoid cold joint or proper preparation is done there should not be any problem of the two layers. moreover the shear will act perpendicular to that joint.

Regards
Subramanian