Large Pad Footing Core, Walls and Columns

[AS3600Eng]

Hi All.

This is my first post on this site so i hope it is in the right section.

I have been given the task to desgin footings for a 20 storey building which sits on rock (Approx 6000kPa bearing).

I have had no trouble with pad footings for the columns however i am now to design the footing for the core and am not quite sure where to start.

This footing has the core, two shear walls either side and two columns that land on the same footing. I have attached a simplistic paint drawing of what it is to look like. (refer to attached picture)

http://files.engineering.com/getfile.aspx?folder=af2e59fb-6c7a-4fe2-82f1-32b2c93a33bd&file=MF.bmp

I have already obtained the moments and axial loads for each element and am now unsure of where to go from here.

My feeling would be to treat this as a one way slab (upside down) and design however being a core and shear walls would there not be uplift etc... concerned with wind and earthquake load cases.

Any help would be greatly appreciated.

Cheers,

 

[hokie66]

Welcome. Yes, the core and shear walls will certainly be subject to overturning as well as gravity loads. As this is a 20 storey building, and you have been assigned this task, I assume you also have a mentor who can assist with your questions.

 

[AS3600Eng]

You are correct i do have two mentors.

However they are out on the other side of the country this week and being a small office i am left to my own for a few days.

I am really just looking for advice on how people would carry out this type of design. Not specific design steps etc... just advice to see if i am working in the right direction and what to be careful of, to ensure i do not miss anything.

Thankyou for your prompt reply.

 

[hokie66]

My approach would be to design the mat foundation for the gravity loads first, then check for the lateral loading conditions. Design the mat for two way action, not one way. It looks like quite a small core for a 20 storey building, so you could have uplift which would require rock anchors. Keying to the rock would probably provide adequate sliding resistance.

 

[asixth]

Two way action. It is a complex interaction between foundation bearing and cap stiffness to determine the distribution of bearing pressures under the cap. Best way is to get stuck into some FE analysis and model the rock-structure interaction.

You can model in an FE program with plate elements using area springs or lots of point springs (my recommendation) or using a frame analysis package and model the raft as a grillage (0.6 stiffness for bending and 0.1 stiffness for torsion). Both plates and frame elements will require a fictionally stiff element to represent the walls because they will help distribute the load out.

The key to these designs is ensuring you have enough anchorage from dead weight and socketing into the rock to prevent overturning. Don't forget to design for shear as well, because it has two-way action, the shear reinforcement requirement is only required when V* is greater than phi*Vuc.

A good design will be one that doesn't overturn whilst not exceeding the geotechnical bearing capacities with a cap that achieves the required stiffness and strength capacities.

6MPa rock probably needs dynamite so be mindful of excavation costs when detailing.

 

[asixth]

Hokie's advice regarding sliding is important not to overlook during concept design. Rafts socketed into 6Mpa rock work good. Piled supported rafts are not so good, particularly those toothpick like driven piles.

A particular tower on the corner of wharf and queen had permanent rock anchors for overturning.

 

[bookowski]

Agree with the others, a one way slab hand calc isn't going to do it.

You should be able to do this in software that you have - possibly what you use for slab design (I'm assuming this is a flat plate building?). If you have SAFE (from CSI) it is very easy to analyze this situation, you can input the soil properties as spring support with compression only properties. Inputting the loads is a bit tricky and I've seen it done different ways. One option is to define all of your walls as rigid or very stiff beam elements and apply your moment and axial to these elements. Depending on the software you have available you may have to come up with something creative to model it. Since it's pretty regular and symmetric you should be have a good sense of whether or not your model is making sense.

 

[msquared48]

Asixth:

Was that for the Tower of London and the overturning from the too frequent use of the rack?

For the record, I do agree with what Hokie says... I was going to suggest further checking the overturning across the diagonals, but not after I saw the sketch.



Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[asixth]

I don't know what you are referring too. I was making reference to a highrise building in our hometown.