Torsion Releases For Balconies

[Osama Anwar]

Assalaam o Alliakum Everyone!

I was wondering that if we could apply torsional release to the beam between the slab and balcony as shown in the attachment.

Moreover, what about the beams at the side of balconies? What about beam at outer side of balcony?

[Badar (BAZ)]

You can can take away the ability of balcony-beam to take torsion in this case as the beam is not the only load path available. However, you have to design it for cracking torsion at-least as per guidelines of ACI 318.

The beam on sides and the outer end are not needed. Slab is enough to carry forces if it  is normal balcony of less than 6 feet in width.

 

[UmarMakhzumi]

Attached is another solution. You can add new beams. Red and Blue.

The end condition for Red beams can be pinned/ moment released. For Blue, the end towards the red beam can be pinned/ moment released only. The size of blue beam should be same as balcony beam. 

Thanks.

 

[Simple Structures]

Good points made by BAZ & Umar.

Just to add, there is more than one way to skin an animal, as they say – here is another approach to take.

The third option would be not to put in "releases" and to "detail" the beam appropriately for torsion - this means the longitudinal bars in the beam may increase in size, and the links (stirrups) are detailed as torsion links. Transferring “releases” from computer software to the steel fixer on site can be tricky. Concrete behaves as you detail it!

It is the combination of vertical shear and torsion that induce cracking and failure - unless the reinforcement (longitudinal & links/stirrups) are detailed to cater for it for it. In the beam there will be additional longitudinal reinforcement in the top, bottom AND SIDES to cater for any torsion.

The figures below may help you understand the "practical" on site concept of catering for torsion with properly detailed reinforcement.

1.     What is torsion:

 

2.     Below is a torsion "link" or "stirrup" shape – what it looks like:

 

 

Below is a torsional situation: In Pakistan always include a bar in bottom (shown red) of cantilever balcony ALSO, same shape as top bar, but bend up and around towards to top of beam/slab, to allow for load reversal during an earthquake!

 

 

When I built my house in Pakistan, the steel fixer managed to bent one bar thus for the balcony - the workers in Pakistan (where material is expensive and labour is cheap) are very talented, but do not have the technical know-how sadly..... that's the challenge for the likes of you the structural engineer to disseminate down!

 

 

Keep an eye on this forum. There is Pakistan RC Building Reinforcement Detailing Manual in preparation and will include this torsion detail now that it has come up here! This Manual be an advisory document but will assist both "young" structural engineers and reinforcement detailers (draftsmen) to understand reinforcement detailing and fixing!

Passing down knowledge – think of it as “zakat”, then it gets easier then to give and share!

 

 

[Osama Anwar]

On 6/17/2020 at 2:54 AM, Badar (BAZ) said:

You can can take away the ability of balcony-beam to take torsion in this case as the beam is not the only load path available. However, you have to design it for cracking torsion at-least as per guidelines of ACI 318.

The beam on sides and the outer end are not needed. Slab is enough to carry forces if it  is normal balcony of less than 6 feet in width.

 

What if there is a line load along the sides of balcony? I have a 0.65 k/ft line load at sides and 0.3 k/ft at front beam. Btw my balcony is 4' wide.

9 hours ago, UmarMakhzumi said:

Attached is another solution. You can add new beams. Red and Blue.

The end condition for Red beams can be pinned/ moment released. For Blue, the end towards the red beam can be pinned/ moment released only. The size of blue beam should be same as balcony beam. 

Thanks.

 

Should I provide beams if balcony is 4' wide? Rooms are 12.25' x 17.75'. Balcony is 8' long.

I have attached the photo below for visual aid.

I thank you from the bottom of my heart for such deep insights. May Allah grant success to all of you in here and hereafter. Jazak Allah.

[UmarMakhzumi]

1 hour ago, Osama Anwar said:

Should I provide beams if balcony is 4' wide? Rooms are 12.25' x 17.75'. Balcony is 8' long.

You should at-least try the suggested solution. It will fix your model. There are other good points raised by other members that you can consider as well.

If the load on balcony is not much(not sure as I see a masonry facade in the picture) you can also try designing it as a cantilever slab.

Thanks.

[Osama Anwar]

59 minutes ago, UmarMakhzumi said:

If the load on balcony is not much(not sure as I see a masonry facade in the picture) you can also try designing it as a cantilever slab.

Don't Consider the middle part but only the side buildings. You would be able to see masonry facade. 

[Badar (BAZ)]

3 hours ago, Osama Anwar said:

What if there is a line load along the sides of balcony? I have a 0.65 k/ft line load at sides and 0.3 k/ft at front beam. Btw my balcony is 4' wide.

This does not mean you need beam under the line load. The line load at the end will act as point load per unit width of your balcony. The line load at sides can be treated as line for that unit width of balcony. No  beams are needed,which means you will not have forces which need enlarged cross-section. Do your calculations; figure out bending moment and shear at critical section;  you will know.

[Osama Anwar]

3 hours ago, Badar (BAZ) said:

This does not mean you need beam under the line load. The line load at the end will act as point load per unit width of your balcony. The line load at sides can be treated as line for that unit width of balcony. No  beams are needed,which means you will not have forces which need enlarged cross-section. Do your calculations; figure out bending moment and shear at critical section;  you will know.

You mean I convert line load into area load and apply it on balcony slab?

Edited June 18, 2020 by Osama Anwar

[Badar (BAZ)]

You can do that. But I meant something else.

[Osama Anwar]

If there is a masonry wall won't it create serviceability issues? It was the reason a decided to provide beams.

On 6/18/2020 at 6:15 AM, Badar (BAZ) said:

This does not mean you need beam under the line load. The line load at the end will act as point load per unit width of your balcony. The line load at sides can be treated as line for that unit width of balcony. No  beams are needed,which means you will not have forces which need enlarged cross-section. Do your calculations; figure out bending moment and shear at critical section;  you will know.

I'm really sorry can you explain it again. I didn't get it. You mean I design slab as a cantilever beam with a point load at end (equal to UDL at face) + UDL (equal to udl at sides) throughout span of 4'. What about the area loads on slab L.L. L.L etc? 

[Badar (BAZ)]

You can do the same for section B-B

[Osama Anwar]

1 hour ago, Badar (BAZ) said:

You can do the same for section B-B

Thanks.

 

[Osama Anwar]

On 6/18/2020 at 6:15 AM, Badar (BAZ) said:

This does not mean you need beam under the line load. The line load at the end will act as point load per unit width of your balcony. The line load at sides can be treated as line for that unit width of balcony. No  beams are needed,which means you will not have forces which need enlarged cross-section. Do your calculations; figure out bending moment and shear at critical section;  you will know.

 

On 6/19/2020 at 9:02 AM, Badar (BAZ) said:

You can do the same for section B-B

Sorry for the late reply.

I calculated the shear and bending moment at outer face of balcony beam (not at a distance d from balcony beam, just to be more conservative). I solved for section B-B only but also included the Imposed dead load and live load and self weight of balcony. 

Shear came out to be 4656 lbs and Moment came out to be 120384 lb.in. 

Effective depth was 4.72. So 6in slab without beam would be sufficient in my case.

Spacing came out to be 7.06 for #5 bar. I would be going with 6 in c/c.

 

[Osama Anwar]

On 6/18/2020 at 1:24 AM, Simple Structures said:

Good points made by BAZ & Umar.

Just to add, there is more than one way to skin an animal, as they say – here is another approach to take.

The third option would be not to put in "releases" and to "detail" the beam appropriately for torsion - this means the longitudinal bars in the beam may increase in size, and the links (stirrups) are detailed as torsion links. Transferring “releases” from computer software to the steel fixer on site can be tricky. Concrete behaves as you detail it!

It is the combination of vertical shear and torsion that induce cracking and failure - unless the reinforcement (longitudinal & links/stirrups) are detailed to cater for it for it. In the beam there will be additional longitudinal reinforcement in the top, bottom AND SIDES to cater for any torsion.

The figures below may help you understand the "practical" on site concept of catering for torsion with properly detailed reinforcement.

1.     What is torsion:

 

2.     Below is a torsion "link" or "stirrup" shape – what it looks like:

 

 

Below is a torsional situation: In Pakistan always include a bar in bottom (shown red) of cantilever balcony ALSO, same shape as top bar, but bend up and around towards to top of beam/slab, to allow for load reversal during an earthquake!

 

 

When I built my house in Pakistan, the steel fixer managed to bent one bar thus for the balcony - the workers in Pakistan (where material is expensive and labour is cheap) are very talented, but do not have the technical know-how sadly..... that's the challenge for the likes of you the structural engineer to disseminate down!

 

 

Keep an eye on this forum. There is Pakistan RC Building Reinforcement Detailing Manual in preparation and will include this torsion detail now that it has come up here! This Manual be an advisory document but will assist both "young" structural engineers and reinforcement detailers (draftsmen) to understand reinforcement detailing and fixing!

Passing down knowledge – think of it as “zakat”, then it gets easier then to give and share!

 

 

I have not changed torsional coefficient to 0.01 for my balcony beam so I guess I could go with your configuration. What would be the development lengths in this case?

Someone said to me that another alternative could be to just extend the bars into the slab by same length (in my case it would be 4'). What is your comment on that?

 

[Simple Structures]

Designing buildings via such forums can lead to engineering failures - its a dangerous thing.

This forum is for general advise, and then you as the engineer need to make the necessary engineering judgement based on your knowledge and experience.

However, as rules of thumb:

1. Which code are you designing the concrete to? American or European?  Follow the rules given in the code!!

2. Here is general advice, based on European Codes.

For say 30N/mm2 concrete strength, and straight ribbed bars:

ANCHORAGE:

Good concrete/rebar bond conditions, good quality control: 40 x bar diameter;

Poor concrete/rebar bond conditions, poor quality control: 60 x bar dai

 

LAPS: 30 N/mm2 conc

Bar laps, good bond, good conc quality control on site: 60 x bar dia

Bar laps, poor bond, poor quality control on site: 90 x bar dia

 

Below are defined "good" and "poor bond" conditions. If your slab is less than 250mm thick, then you have good bond condition, provided there is good concrete quality control regime on site. Quality is not something Pakistan construction sites are renowned for from my limited experience.

 

 

 

 

 

[Osama Anwar]

7 hours ago, Simple Structures said:

This forum is for general advise, and then you as the engineer need to make the necessary engineering judgement based on your knowledge and experience.

1. Which code are you designing the concrete to? American or European?  Follow the rules given in the code!!

 

The project I am working on is kind of an assessment. It is my first time so I want to explore different options and alternatives to expand my engineering knowledge. I am very thankful to all of you for your guidance and deep insights. I am very happy to find you guys who can guide me regarding the problems faced by structural engineers in Pakistan. I hope to benefit from you guys in the future.

I am using ACI. Slab thickness is 6" and fc' is 3000psi.

Edited June 25, 2020 by Osama Anwar

[Kamranullah]

On 6/25/2020 at 7:52 AM, Osama Anwar said:

The project I am working on is kind of an assessment. It is my first time so I want to explore different options and alternatives to expand my engineering knowledge. I am very thankful to all of you for your guidance and deep insights. I am very happy to find you guys who can guide me regarding the problems faced by structural engineers in Pakistan. I hope to benefit from you guys in the future.

I am using ACI. Slab thickness is 6" and fc' is 3000psi.

These Type of problems and Deep technical Ambiguites should/must be shared  to get the Experience Str Engineers opinions . I appreciate you on highlighting this issue and Thanks to all the Engineers who responded to your post