Help with a confined masonry intervention / rehab

[jcvaldes]

Hello to everyone, this is my first time posting and I come to you with great respect and a request... I have a client that bought an old building (around 40-50 yrs old) of 2 stories and decided to make something new out of it. Now in the ground level it'll become a fast food restaurant and in the upper level he'll make 2 apartments out of it. What is currently the roof will also have a new usage, as it'll have some minor "light" structure, such as a wood pergola and other non-aggressive elements. 

The thing is that he decided not to wait for any advise and already demolished several walls in the ground level, some of which were bearing load walls. So now that we took the project, we need to fix it and ensure that it will not have any problems in the future (especially regarding the seismic effects, which are important in my region). So far the structure seems ok. There's no sign of cracks or important deflections so far. They left the closure ring beam or stem wall (which now is kind of acting like a beam without really being one) but they destroyed the rest of it.

My idea is to reinforce these "false beams" by putting a metal girder directly below them and attaching them to the concrete columns that exist.  

The problem is that I'm not sure if it's a better idea to consider the type of connection as a simple one (pinned), which I think resembles a bit more the original condition or adding a bracket (moment connection) and transferring these new moments to the existing columns by proper joints/anchors. There aren't many walls along one of the directions now, and I'm a bit worried about the seismic behavior, since I'm also changing the rigidity of the system and the center of mass is already displaced. I have no certainty of the structure composition other than the geometrical features. 

I still need to do the calculus to know which IPR section or whatever I'll use, along with the effects of everything that I already wrote. But I wanted to know your opinions and what would you do in my place? (Other than not accepting this kind of clients).

 In the plan, the dotted ones are where there used to be walls...

 

[Ayesha]

On 8/5/2021 at 9:43 PM, jcvaldes said:

My idea is to reinforce these "false beams" by putting a metal girder directly below them and attaching them to the concrete columns that exist.  

Why do you want to reinforce these beams? Also, I am not sure that I understood your post well. You are referring to these beams as false beams. Why do you call them this?

Regarding your idea to reinforce this concrete beam, just placing a steel beam under concrete beam will not transfer any shear from Concrete beam to Steel. The two beams act independently. However, the lateral resistance of frame will increase and that will result in your frame attracting more lateral seismic load than before as stiffness of frame will increase and stiffness attracts load... Doesn't look like a good idea. Do this if you can justify the retrofit and also if you can accurately quantify the effect.

 

[jcvaldes]

I'm calling them "false beams" because they weren't designed for that originally, they were only tie-beams in the upper part for the load-bearing walls of a confined masonry system. 

I need to reinforce them because they destroyed the walls that were carrying the original load, and besides that, they plan to add a terrace and a small room in the 3rd floor (currently the roof).

My original idea was to add metallic frames and get rid of those beams. The thing is that they don't want to lose space because the space for the architectural program is already tight as it is. 

I forgot to tell that the beam would be anchored to the beam, not only put under it. Sorry for that. 

Regarding the stiffness increase, that's what I also thought, which is why I thought maybe it was better to make them as simple connections instead. But I wanted to know if anyone has ever had a similar problem or has another idea of how to attack it. I don't really have too much knowledge of structural rehab / intervention...

Thanks for the reply Ayesha 

 

 

 

 

[Ayesha]

Your reasoning is good. Structural Rehab is nothing different than new design, just a few additional shenanigans to cater for.

I personally like the idea of providing new framing (see below for details) if you can for two reasons.

1) Confined masonry beams would be very weak, and reinforcement in them might not be what you will need to justify a seismic load resisting system. This point is in-terms of ductility requirements that are assigned to different category, like if your frame ends up being an Intermediate Moment Frame (for example), are you confined masonry beams / false beams detailed to give you that ductility. The answer is probably not, so technically speaking, if that is the case you can't rely on them.

2) You have to think of the structural retrofit in-terms of Lateral Force Resisting System (LRFS) and Gravity Force System.

In order to keep the footprint small, it might be better idea to introduce new shear walls that can be connected to floor diaphragms and that can take all the lateral load to foundation, provided, you can provide such shear walls, connected to floor slab and also pour new foundations for them and then your existing system can stay light assuming collector design doesn't poses an issue.

On 8/8/2021 at 8:34 PM, jcvaldes said:

Regarding the stiffness increase, that's what I also thought, which is why I thought maybe it was better to make them as simple connections instead.

If you connect them as simply supported, then they will only help with gravity. One thing as a heads up that, if you are attaching steel beams to concrete columns (post install), the anchors might not be sufficient as I am not aware of the code you are following but say in US, Seismic Codes require beam to column connections in ordinary moment frames to be designed for most probable moments which for post installed steel to concrete connections would be impossible to meet.

Hope this helps.

Edited August 11, 2021 by Ayesha