Pouring of a Huge beam

[hanihafiz]

We are preparing a method statement for pouring of a huge beam 3.0 (width)x 5.90 (Depth) X 21.0m (Span). The beam shall be poured in stages, each with depth around 1.50m. We are preparing to pour the first bottom 2.0m with the required formwork to support its weight. Then, this beam shall work as a support for the rest of the depth (formwork shall be removed). This is required because this beam is 3 floors higher than foundation, and making formwork to support all this huge mass shall cost a lot in time and money.
I am asking, shall this arrangement affect the final behavior and stress distribution of the complete beam?

 

[TheMoonlitKnight]

I would imagine you designed the stirrups or additional vertical bars to carry most if not all of the horizontal shear at each cold joint? It seems like all should be well in the stress distribution if you detailed that properly.

That is an unbelievably big beam if those cross-section dimensions are in meters!

 

[hanihafiz]

It is a very huge beam (kindly see attachment). Horizontal shear is a good point. I wonder if the bending stresses at the final section shall be affected by such construction sequence. The initial 2.0m depth shall carry the rest of the beam till it gets its strength.

 

[JacksPanic]

This is very very huge. I am interested to know what the application is. Bridge? Is this connected to a column?

 

[Ron]

Agree with MlK....you can also use dowels at the cold joint to make sure you achieve continuity.

I would be concerned about removing the shoring even after the first 2 meters has achieved the desired strength. You have a large dead load consideration that would not be resisted by sufficient compressive concrete. This will "pre-stress" your top rebar in compression when the bottom 2m bends under self load. Check for buckling or displacement of the compression rebar if you remove the shoring.

When the lower 2m has achieved strength, the existing formwork and shoring should support the additional 1m of placement when coupled with the strength of the partially completed beam section.

Pay close attention to the concrete mix design. It is important to make sure you have a proper mix design for strength gain, aggregate size for prevention of voids and for proper encapsulation of the rebar.

 

[TheMoonlitKnight]

Since E is directly proportional to f'c, you (technically) will have a modular ratio throughout the section. If you were to compute the composite section, you would find that the bottom section modulus would be higher than the top...but, it may not be a big deal depending on your design. See attached.

 

[hanihafiz]

JacksPanic, It is a transfer beam supporting multi-story columns over a tunnel. Actually the final load on the beam is much higher than its
Ron, I have added top reinforcement at each cast, working as a compression steel and provide continuity with columns. Actually top bars of the original beam shall be installed after pouring the first and second cast with the upper inverted U-links. I do not understand what is the additional one meter. Formwork is required to be removed to allow finishing work at the lower floors, and to minimize the load supported by it.
MoonlitKnight, I assume that the first 2m supporting all weight above it, and so consider all other as weight only. Is this something wrong with this approach?

 

[BAretired]

You don't need to make the formwork capable of supporting the entire mass, but you could you re-shore the bottom section of beam at third points in order to reduce its span for the temporary load. That would be the usual procedure.

BA

 

[Ron]

Sorry hanihafiz....I misread the depth. At 5.9m, the condition I described is even worse. BAretired explained the correct approach using re-shoring.

How long do you plan to wait between the placements?

 

[dik]

Also consider heat and thermocouples as well as fly-ash or something to minimise heat.

Dik

 

[Ingenuity]

Was a post-tensioned beam solution considered, with staged stressing, given that this transfer girder supports "multi-storey columns"? It may results in several economies of materials, possibly improved construct-ability and member performance.

 

[hanihafiz]

BA and Ron, if we make re-shoring at third point, the reaction on each re-shoring point is around 5000 KN from the weight of the beams and the connected two floors, or 3000 KN from the beam alone. What shoring can support such huge weight? Still not clear to me what is wrong with our approach. The time between the first and the second cast may be two to three weeks, but after that, the period between successive casts may be one week.
dik, We did a mock-up thermal test with the 1.5m depth, and it is OK for the type of concrete used.
Ingenuity, Post-tension may be a good idea. Thanks I shall explore it. Thanks to all.

 

[JacksPanic]

Can you not build temporary concrete columns to act as props?
I would also think a post tensioned beam is a possible better solution...

 

[BAretired]

hanihafiz,

A 2m deep beam spanning 21m with the huge amount of load you are adding just two weeks after it is poured is going to deflect substantially during and after pouring the upper beam section. This will disturb the initial set of the new concrete, resulting in micro cracking.

Third point re-shoring was a suggestion. You could space them closer if you wish. With a 3m wide beam, you would not use a single shore under the center of the beam. You would use two shores about 2m apart.

With shores at 7m centers along the beam, each shore would carry 3000/2 = 1500kN for the weight of the 3.0m x 5.9m concrete beam. A steel HSS, braced at each floor level could easily carry that but I would not quarrel with you if you selected a spacing of 3.5m instead. You would need a foundation under each shore to carry the temporary load, but that is not unreasonable for the project you are undertaking.

BA

 

[hanihafiz]

BA, thank you for sharing your valuable thoughts. The mix is 65 MPa. I have checked the beam for 40 MPa which can be easily reached after two weeks. The resulting deflection (for the cracked condition) equals 35mm, which is not that much for such span. The primary camber shall be around 60mm. If it is not safe, regarding micro-cracking to load at such age, we may leave another one week. This shall be easier than re-shoring which must extend 3 floors down to the raft hindering finishing works for around two months.

 

[steele6707]

Another consideration is reshores for the floor framing on either side of the beam. I may not understand the conditions, but if the floor at the beam soffit is going to carry the upper floor without any reshores you probably want to be very conservative with the live load capacity of the lower floor (significantly higher than the pour DL+LL) and the confirm deflections resulting from the pour loads are within your service requirements.

 

[hanihafiz]

Dear colleagues, I have found in a software capabilities to perform staged construction. I think that what I am looking for. I shall check the results and inform you back.

 

[hokie66]

hanihafiz,

As opposed to some of the advice to reshore, I would approach the construction of this transfer beam in the same manner as you. As Ingenuity suggested, I would use post-tensioning, with at least enough stressing to balance the dead load of the first lift. Staged stressing is typically used in big transfer beams.

 

[BAretired]

I think that Ingenuity and hokie66 have raised a good point. The use of staged pre-stressing may be a better and more economical solution than re-shoring.

BA