Seismic performance of infill frames 3

[Blackstar123]

This is a question that always pop into my mind when I see a 10+ story RCC building, with all stories filled with masonry infill walls except those floors which are reserved for parking.
How do they take into account the effect of masonry infill in the seismic analysis of the frames?

My major design experience is in cement plants which mostly don't have frames with infill walls. So if someone have designed tall buildings, i would really appreciate if they will share their design practices.

Euphoria is when you learn something new.

 

[Blackstar123]

Akee, thank you for sharing the research paper link. I’ve always found it easier to understand the result of comparative research studies as the one you shared. I just wished they had also studied the seismic performance of a 3rd specimen that is without any infill at all. It would have clarified if the behavior of isolated infilled frame and a bare frame is really the same.

I think it's more probable that a masonry partition wall will fall (and have a great risk of killing people) vs a new designed RC structure to collaps.

And after reading the following paper, I seem to have lost faith in the design adequacy of Moment Frames for seismic loads or as you say “Newly designed RC Strucure”. This author has proposed a solution for the infilled walls that seems like a viable solution even for simple partition walls, but unfortunately, there is no experimental data to back it. I must confess, in all this time I’ve been searching about infilled frame, I never thought about the problems that could arise from the instability of a simple partition wall.

https://www.conservationtech.com/RL...9+ICEC'19/Langenbach_ SACEE+ICEC_Keynotes.pdf

In all the searching I’ve been doing on this topic, I’ve not yet come across anyone who has specifically designed the structure by modeling the infill struts. All the works I’ve seen so far are either the research studies of behavior of infill frames or non-linear analysis and assessment of existing buildings with infilled frames. After reading some very good research works I’ve come to the conclusion that If you are in designing face, it is definitely better to isolate the infill wall from the frames.
But if someone is interested in modelling of masonry struts, I find this paper has some good references on proposed models to capture the behavior of infill walls.

https://www.researchgate.net/public...-ductile_structure_with_nonlinear_shear_model

Euphoria is when you learn something new.

 

[Akeee]

I am living (and also designing) in a high seismic activity area and in all of our residential buildings these types of partition are used. Worst part of my job... I do hate them a lot but what can I do.. To get things even harder our loved architects now to make some super space economy's for the clients they use very thin and slender walls - 15cm thickness (even 11cm), 2.5-3.0m height and 4-6.0 long. SO you can imagine what happens on 10th story inside the apartments where this "membrane" walls are one a big earthquake shake...
Blackstar123 - I understand you perfectly - I searched on the internet and in books for 2-3 years to find something about this problem - with unfilled partition masonry walls - I didn't find anything good.. So this days I am thinking to do a different approach : search for big earthquakes and then search for damage reports and then see if I can draw some conclusions about the walls that felt/dmg .. geometry, position , why etc.. But this is only to get a big idea not to find a way to calculate them..

After reading some very good research works I’ve come to the conclusion that If you are in designing face, it is definitely better to isolate the infill wall from the frames.

Now the problem with the partition isolation from structure : at first it looks promising to split them and have a clean structure that behaves like you designed .. BUT (I know because I done it in design and in execution) there are some hidden problems : 1. stability (it's more unstable a wall that haves free ends then a one that doesnt - so if you are not careful you may end up with making the situation a lot worst..You would say that the reinforcement will be put on every row and connects the masonry to the RC column -... yes indeed but the masonry walls are built after the RC structure so to do that you need to put chemical anchors in the RC columns at every rows of the masonry walls .. on one hand its expensive and on the other it's a high probability that in the execution phase they will not do it - will just put the rebar without the chemical part. in my building code is mandatory to put this rebars but is one thing when the wall is in contact with the RC structure and this reinforcement act with dowel effect and is other thing when you have this rebar + 2 or 3cm gap.. no more dowel effect if you dont have that chemical anchor (the rebar will just bend).... so you may end up with a very dangerous walls ) 2. cracking - at low intensity earthquakes all of your soft joints will crack like hell - so every apartment will have a repairing job to do and you might be waking up with a lawsuit 3.soundproof.. But lets just ignore point 2.and 3. and lets say that you isolate ALL of the masonry from the RC structure and its done properly. That means that on every intersection with the RC structure will be a vertical gap and all of the walls are not connected on top with the slab. Now you have to take every wall and calculate it for a seismic perpendicular force (from its mass) and you will have the top edge free. From my calculations the seismic force increases with the height so on 3..4th story your walls will not hold for bending and you have to reinforce them with mesh's.. all of then.. again you end up with a strange situation, nobody does it... for a 10 story building from 4th story to start reinforce with mesh all of the partition walls will be something ...
Because I encounter this problem many many times I draw some conclusions :
- masonry partitions are not made for this (for >4th story buildings in a high seismic zone)
- you can reduce the risk a lot by not making RC frames but rather using RC walls - the rc walls are very stiff... I limit the SLS drift to 0.5% and for SLU max. of 1.5% then I reduce a lot the risk of masonry walls falling
- I dont let any partition walls that are longer then 3m to have a free edge - there I put a RC small column between the slabs
- I am very careful with the facade walls where the windows are - my seismic code mandates that if a hole if bigger then 1.5 m2 to put rc columns left and right
- I wrote on plans a minimum class of mortar and a good type of bricks
- Pray for the best... what can I do if nobody knows how to do it properly, and I asked a round a lot...

The list was for wall that are not infilled in RC frames.. For the ones that they are :
- I am looking not to have soft story's - very dangerous
- I am modeling with diagonal struts to check if they gave my global torsion - important for the R factor
- I check the compression strut (not all the time)
- If there are problems I am thinking of isolation solution

 

[bones206]

I recently designed a RC frame building with participating reinforced 12” CMU infill in a non-seismic area.

Correction: I recently designed a RC frame building with participating reinforced 12” CMU infill in a non-seismic seismic area.

Miami Buildings Shake After Powerful Earthquake Between Cuba and Jamaica

 

[Blackstar123]

Akee,
I have no experience in designing of masonry infill so I apologies in advance if I seem terribly ignorant.

To get things even harder our loved architects now to make some super space economy's for the clients they use very thin and slender walls

If seen in a global sense, isn’t this a good thing? The lesser the seismic weight of the building, the lesser will be the base shear and inevitably the less seismic force on components.

That means that on every intersection with the RC structure will be a vertical gap and all of the walls are not connected on top with the slab. Now you have to take every wall and calculate it for a seismic perpendicular force (from its mass) and you will have the top edge free.

I don’t understand why walls are not connected at top with slab? I thought isolation was being achieved from column ends only and any lateral pressure on wall will be resisted because of friction force at the wall ends. This way it could span in one-way direction between top & bottom beam/slab. Also, the vertical load from beam/slab will help reduce the tensile stress produced due to lateral loads and if the walls were designed to be adequate in shear & flexure, wouldn’t stability issue get resolved automatically? Can you give me some numbers about the expected resistance of an unreinforced or reinforced masonry wall for a typical story height and magnitude of pressure that is produced due to any earthquake load?
About the vertical gap, I was also wondering how long this gap should be and what is this compressible material they filled it with. Does the end result successfully withstand the environmental conditions like rains.

cracking - at low intensity earthquakes all of your soft joints will crack like hell - so every apartment will have a repairing job to do and you might be waking up with a lawsuit

By soft joints do you mean contraction joints? I didn’t know these kind of joints are also provided in masonry walls.

Because I encounter this problem many many times I draw some conclusions :

Your deduction about the whole topic is impressive and all your conclusions seems like a good design practice.

I am modeling with diagonal struts to check if they gave my global torsion - important for the R factor

I’m curious how you consider the effect of torsion on the R factor. We usually increase the eccentricity by a certain factor to increase the torsion forces.

I check the compression strut (not all the time)

Do you define a separate material model for these struts? if yes? then can you share it. And if axial force exceeds the compressive strength of strut then what do you do? Because in reality the bracing force will be released when the axial force exceeds the strength.

Euphoria is when you learn something new.

 

[Blackstar123]

bones206,
But you later said that you designed without considering infills and one of the reason to do this was because it was a non-seismic area?

Euphoria is when you learn something new.

 

[bones206]

I thought it was a non-seismic area until a couple days ago when they had a seismic event. But either way, I put a lot of consideration into the infill. I pulled my hair out over it and researched the hell out of it. Spoke to industry "experts", including the guy responsible for the new infill provisions of the TMS 402 code. In the end, I used engineering judgement to design the infill. I designed it tight to the bounding frame and pretty heavily reinforced. It will be fine for minor seismic events, behaving more like a shear wall building.

 

[Akeee]

Blackstar123,

If seen in a global sense, isn’t this a good thing? The lesser the seismic weight of the building, the lesser will be the base shear and inevitably the less seismic force on components.

No, because the capacity is dropping way faster then the reduction in mass (force). And also the thin bricks have a nut and feder connection that is worse then normal one.

I don’t understand why walls are not connected at top with slab? I thought isolation was being achieved from column ends only and any lateral pressure on wall will be resisted because of friction force at the wall ends. This way it could span in one-way direction between top & bottom beam/slab.

This only work with wind - you have the top and bottom end fixed and the left and right free and so its a one way slab and the wall will bend and take the force. But with earthquake it's other thing - you have what is called story drifts that means the top slab will displace 2-4cm horizontally (not vertically) away from the bottom slab. So if the wall is fixed top and down a displacement like that will break it.

Can you give me some numbers about the expected resistance of an unreinforced or reinforced masonry wall for a typical story height and magnitude of pressure that is produced due to any earthquake load?

Gperete is the weight of the wall in kN and Fcns is lateral force. So for example a 15.6kN (1.56 tones) masonry wall 2.5m height, 3.7m long and 11cm thickness will have 11.6kN (1.16tones) demand. This is the 4th story and with every story the forces are increased by about 20%.

About the vertical gap, I was also wondering how long this gap should be and what is this compressible material they filled it with. Does the end result successfully withstand the environmental conditions like rains.

Something like rubber. There are some special membranes. From the top to the bottom.

By soft joints do you mean contraction joints? I didn’t know these kind of joints are also provided in masonry walls.

See below top view section :

In the left is the RC element and on the right is the masonry wall. Between them is a gap of 3cm filled with soft material (like rubber) so that the 2 parts are not structural tight.

I’m curious how you consider the effect of torsion on the R factor. We usually increase the eccentricity by a certain factor to increase the torsion forces.

In my designing code there is a limit of torsion the building can have (maximum of 25% displacement on the edges vs the middle - its a rotation check). If the limit is exceeded then the R factor is 3. So I model with diagonal struts the infilled masonry and then recheck the torsion. If I need to reduce the R factor from 5 to 3 then I am thinking about the separation option.

Do you define a separate material model for these struts? if yes? then can you share it. And if axial force exceeds the compressive strength of strut then what do you do? Because in reality the bracing force will be released when the axial force exceeds the strength.

I have clear indications in my seismic code about the modeling of these diagonals.
You can read here (and btw this is a good study too about infills effect):

https://www.dropbox.com/s/2wok7s4s2wn50ta/v7n3a06.pdf?dl=0

If the diagonal doesnt hold you either increase the quality of the masonry (bricks and mortar) or its thickness or think about detaching from the structure... Or make the structure more stiff (use RC walls) and then you decrease the force in masonry walls.

Hope that helps you.

 

[Akeee]

Watch the bare wall on the right between 1:00 and 1:04 in this video. It kind of illustrates what I mean:

https://youtu.be/Wd4rQ48w-ug

I know this video, thanks for sharing. I think the masonry here is build in a nonseismic way from the start. Look here :

There is no way and arch will form with a 2cm middle gap. Its crazy.. and the PGA is 1.0g - extreme.

 

[Akeee]

Btw I think that this software is nice and would help us but I dont know if it could be implemented for our buildings and for current design:

https://www.youtube.com/watch?v=TduSTyD3r7g

 

[bones206]

That simulation is amazing but you're right, it doesn't help much for designers. It just confirms how poorly the masonry performs in an earthquake. Hopefully the ongoing research in this area will lead to some life-saving (and economical) design innovations in the future.

 

[Blackstar123]

In truth before this post, I never thought about the out of plane stability of infill. When it was first mentioned in one of the shared link, my first thought was "Why do we need to check this? This is not going to affect my structure.". But this whole discussion have made me realized that to make sure that infill does not fall down in out-of plane bending is as important as its in-plane effect on the adequacy of the structure.

I did beef up the columns and beams for the amplified shear from the struts.

There are so many uncertain factors in the performance of infills that I agree, as far as the design of structure is concerned, this is the best we can do to make sure the frame does not collapse.

I've learnt a lot since I started this post. I want to thank you all, especially Akee and bones206, for trying to help me understand such a complex topic.




Euphoria is when you learn something new.

 

[Akeee]

Bones206 - can you share some pictures with how you reinforced the wall? I am curios. You are lucky that you have one wall. Usually in my buildings are about 30-50 walls/story so 200+per building. I would love to have the time to deal with them all.
Blackstar123 - In very rare cases (compared to the risk of falling) the masonry can really affect the structure. And as I said before I think it's more probable that a masonry partition wall will fall - out of plane - (and have a great risk of killing people) vs a new designed RC structure to collaps.
The only good thing about masonry is that increases the rigidity of the structure. Now this good thing has an exception - when you are on the other side of the response spectrum and increasing the rigidity will increase the earthquake - response spectrum analysis.
And yes Blackstar123 this is a very complex topic and I dont think anyone truly can calculate these partition walls correctly. My only hope is that from past experience they behave good - they did all fall.. So there is hope, but we will see with the next seismic event.