Increasing the deep beam capacity by adding a new concrete part on top of it.

[Refaat Faraj]

Dear Colleagues,

I want to increase the deep beam capacity due to the high shear generated by the concentrated beam located at the mid-span of the deep beam (please refer to the attached picture).

It is worth noting that the deep beam dimensions are 4000 mm wide and 2150 mm high (including a 300 mm slab thickness), while the concentrated beam dimensions are 500 mm wide and 1500 mm high (also including a 300 mm slab thickness).

Therefore, I suggested adding a new concrete part above the deep beam and connecting both parts using U-shaped reinforcement, in order to increase the deep beam’s capacity. This is because there is no possibility of doing any work beneath the deep beam due to the presence of a door.

Kindly, I would like to know whether this solution actually increases the deep beam's capacity, or if it simply adds additional self-weight to the beam without providing real benefit?

Thank you in advance for any help.

Refaat

 

[KootK]

Kindly, I would like to know whether this solution actually increases the deep beam's capacity, or if it simply adds additional self-weight to the beam without providing real benefit?

The concept can work but it will be crucially important that you detail your rebar to drag that concentrated load up to the top of the new, deeper beam.

 

[HTURKAK]

It is worth noting that the deep beam dimensions are 4000 mm wide and 2150 mm high (including a 300 mm slab thickness),

- Check your dimensions. 4000 mm wide beam ? ( probably 400 mm)..
- How did you calculate that shear strength of the beam which seems to be not satisfactory ?
- Shear span to depth ratio (4/2.15 ) requires the beam shall be checked using STM method or applicable expressions rather than simple bending equations .

 

[KootK]

crucially important that you detail your rebar to drag that concentrated load up to the top of the new, deeper beam.

This is the story that you want to tell.

 

[Refaat Faraj]

Dear Colleagues,

Thank you so much for your responses.

I have investigated the deep beam capacity using Autodesk Robot Structural Analysis Professional 2025 as follows:

1. I used the Finite Element Method (FEM) with a mesh size of 0.50 m. The element type was set to Tension/Compression only, without considering bending moments (as shown in Picture 01).
2. The deep beam dimensions are 2150 mm in height, 400 mm in width, and 450 mm in thickness.
3. According to Eurocode 2 (Clause 5.3.1), since the span is less than three times the section thickness, I considered it a deep beam.
4. • In the first trial, I investigated the beam capacity based on the existing section(i.e., without the added top part) and provided reinforcement:
     • Vertical reinforcement: Ø16 mm @ 150 mm
     • Horizontal reinforcement: Ø12 mm @ 150 mm
   • After running the analysis, I received a warning message: "Unsatisfactory vertical reinforcement (0.79)" (as shown in Picture 02).
   • Therefore, I decided to increase the shear capacity by adding a new upper concrete part.

1. 5
   • In the second trial, I added a top concrete part with dimensions 1100 mm height × 450 mm thickness.
   • After running the updated model, the shear reinforcement became satisfactory ( 0.94 ) (as seen in Picture 03).
2. 6. Please note that the deep beam has already been constructed, and it is integrated as part of a concrete wall on both sides (as shown in Picture 04).
3. 7. Then may I investigate the new beam as in picture 05

But, I would appreciate your guidance on the reinforcement details needed to properly transfer the concentrated load into the newly added upper concrete part.

Thank you once again for your time and help.

Best regards,
Refaat

 

[bhiggins]

It may be a bit more efficient to bolt a channel to the underside or face of that opening to resist the deep beam tension loads. The bolts would need to be sized for shear flow for composite action.

 

[HTURKAK]

I will suggest you to check the capacity of existing beam with strut and tie method. Probably there are some mistakes for modelling , dimensions and loading.

ACI 318 stipulates that the shear in deep beams should not exceed Phi*10 * SQRT (fck) bwd . If the concrete quality fck =30Mpa , the max shear for the section ( 400mm * 2100mm) should not be more than 2300 kN.

 

[Refaat Faraj]

I will suggest you to check the capacity of existing beam with strut and tie method. Probably there are some mistakes for modelling , dimensions and loading.

ACI 318 stipulates that the shear in deep beams should not exceed Phi*10 * SQRT (fck) bwd . If the concrete quality fck =30Mpa , the max shear for the section ( 400mm * 2100mm) should not be more than 2300 kN.

Regarding the use of Finite Element Method (FEM), Eurocode allows its application for complex cases such as deep beams, as long as the analysis is justified and properly validated.
In terms of shear capacity, according to ACI 318, the maximum shear strength is 2300 kN. Since the maximum applied shear is 1710 kN, this indicates that the section is safe in shear.