Differential settlement of soil in two connected buildings of different mass

[kellez]

Hi,

I have two buildings of different mass that are connected together by foundation beams (at foundation level) and by beams (at storey level)

Its a reinforced concrete frame of only one storey. I know these are not big structures but do you think there would be any issue with differential settlement due to the difference in mass between the two buildings? Do you think its ok to connect them together as shown in the pictures?

 

[kellez]

I just had one thought regarding the differential settlement, I think i might have this completely wrong in my head,
I should be thinking in terms of kN/m2 and not the mass of the building as a whole right?
Therefore since both buildings have only one storey their load in kN/m2 is about the same therefore there shouldnt be a problem for differential settlement,

However if one of the buildings had 2 storeys then in that case there could be a problem with differential settlement.

 

[EireChch]

Hi Kellez,

Yes you are correct. It doesnt matter about mass. Only matters about what the increase in contact pressure that the soil "feels". The difference between one storey and two storey residential construction isnt that much and i woudlnt be worried about differential settlement between the two portions.

I would be more concerned with the ground conditions and if they are consistent across the site.

Have you any borehole info?

PS-it looks a very modern house design, are all the wall openings going to be glass?

 

[kellez]

Hello there and thanks a lot for the reply, actually this is just the structural model of the building showing only the RC frame, please look at the pictures of the architectural model below. Its a small house with a private chapel.
I am a civil engineer and this is actually my first project, i have done the architectural design using a BIM software and now i am doing the structural design in order to produce construction drawings and reinforcement detailing.

Actually no i do not have any borehole info, thats another concern i have, here at my country they do not usually carry out site investigations through bore hole data when the project is only residential of 2 storeys. i dont really know why they do not carry out site investigations, the only thing they might do is to use a digger to dig a maximum of 2 meter hole to check the soil. But even this rarely happens, I think it could be the fact that the geology in my country is pretty well known.

However they do build a good base from good grade gravel of about 35cm thick for the foundation to sit on. They also build very strong foundations on all residential projects, They use 45-50cm thick raft foundation with foundations beams embedded within the raft foundation connecting all columns together....You cant possible get a stronger foundation design than this other than also adding piles underneath the foundation slab.



However I very well know that site investigation is the most important part of a project and could save a lot of money if done properly and will allow for issue to be discovered early in the design phase.

 

[EireChch]

Ha of course it is, my mistake. I was thinking it was some Grand Designs (UK home building tv show) type house. One thin is for sure, your BIM modelling looks very good!! Although it seems overkill for a residential in my opinion, the money would be better spend on SI.

Do you know anything of what soils you can expect? Clays sands etc, groundwater in the area. Is it a flat site?

I should clarify, I did not mean a machine borehole, I meant a small MM hand auger borehole. However this may not be common practice for your area. As you have said, some money spent on geotech could save a lot of head aches down the line. What if you build over a peat layer or a layer of man made fill? The house will start settling pretty fast and the house owner will come looking for someones head.

If you have an experienced engineer that knows the geology and has experience in local soils then you may get away will him/her supervising the excavation of footings during construction. I would even recommend doing a trial pit off as there is still the unknown. Side note - remember to backfill the trail pit properly.

i would try gather as much information as you can, geology maps, do a walk over of the site/area. Have a look for any construction sites close to your site. You might be able see an excavation etc.

At the end of the day, i would be asking for two trial pits to 3m depth with some pocker penetrometers on the side walls. If the house owner says no then at least you have it as back up down the line (if you get it in email form).




Good luck.

 

[fattdad]

there's nothing to worry about if the ground is good.

If the ground is bad, well, then there's something to worry about! Nothing that can't be addressed in design; however.

f-d

ípapß gordo ainÆt no madre flaca!

 

[kellez]

I will have to agree on the overkill comment regarding the BIM modelling but only on the modelling of the furnitures and fixtures and fittings.
On the contrary, BIM modelling saved me a lot of time when we had to change something on the design, basically once you have a 3D model of the
building and once you setup your drawing preferences all drawings (sections, plans, elevations, isometric) are automatically created, and the
best thing is that when you change something on the 3d model it's automatically reflected on all drawings. That is what i like most with BIM
software. However this is mostly for Architects

Also it helps me a lot with visualisation and understanding of the building, it also helps a lot with communicating ideas, but yes this is just a
residential project however once you learn how to use the software you become really fast pretty quickly. And of course I can also use the architectural
model to model the frame of the building to be used for structural analysis.

Regarding the site investigation, I will certainly ask for a trial pit of 3m deep, and i will certainly gather all the information i can,
I still have a lot of studying to do on the site investigation, and i will certainly come back with more questions, i just hope you are here to help.

Can you suggest a good accurate pocket penetrometer? I found this one from a simple google search but seems very cheap, what do you think?

https://www.globalgilson.com/pocket-penetrometer

 

[cvg]

you will need to do one of the following:

a) assume everything settles uniformly and ignore any moment or shear at the beam connections
b) assume it does not settle uniformly and design so that moment and shear at the connections is handled by the beams
c) don't connect the two buildings and allow them to settle independantly

method c is probably the more common approach. the grade beams and story beams going across the ends of the patio / breezeway would typically be designed as a fence and not connected to either structure

 

[oldestguy]

Funny no one mentioned temperature effects. With the many different units, each one may have to be somewhat separated by joints to allow for temperature expansion and shrinkage effects. That alone may be sufficient treatment for any settlement differences.