Concrete structure- Shell Utilization

Replies to This Discussion

Permalink Reply by Karamba3D on January 23, 2017 at 1:54pm

Dear Aryan,

the acceptable range of the utilization computed for reinforced concrete (RC) shells depends on whether these are susceptible to buckling or not. RC shells can be quite sensitive to buckling: cracking decreases the stiffness of the cross sections (by a factor of roughly two), creep increases displacements over time.

Since calculations in Karamba 1.2.2 are based on linear elastic material behavior, cracking and creep are not included. Thus I suggest as a rough approximation a maximum utilization of 0.1 in case you use characteristic loads and characteristic material properties.

Best,

Clemens

Permalink Reply by Aryan Shahabian on January 24, 2017 at 3:10am

Dear Clemens,

Many thanks for your reply.

Actually in my case, the utilization output shows large numbers in comparison with 0.1 (average about 0.2, some nodes more than 0.5 and in rare cases even more..)

How important is it to check utilization in early stages of design?

As my main question:

What are the very main items to be checked in Karamba, to see if a concrete structure is acceptable, in early stages of design? 

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More info in my case:

Actually as part of my academic research, there are 3 alternatives of a tall structure which are going to be compared with one another in terms of their mass. All of them are similar 40 stories diagrids with central core and radial beams under slabs, joining the core with the outer diagrid. The main difference between the 3 setups is their materials (C40, C55 and C70) 

The models could be simplified as much as possible, since in the end, only the total mass of each alternative would matter for the comparison. So I won't go into details and the models aren't required to be very accurate but at the same time they shouldn't be very far from reality either. 

What I've done till now is:

- All the elements, defined as concrete shell.

- All the joints, defined as rigid (just for simplification; because in reality, hinges - connecting the radial beams to the core and the diagrids - could function better).

- Approximate dead, live, seismic, and wind loads are defined.

- Using galapagos and a python script, the height and/or thickness of the elements are going to be optimized in a way that in all three alternatives, the maximum displacements would be the same, while minimizing the total mass.

So what I've checked till now, is the displacements to see if they are in standard ranges.

What else is essential to be checked in very early stages of design to get sure that the models are not far from reality and they are acceptable?

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Thank you for your time.

Best,

A

Permalink Reply by Karamba3D on January 26, 2017 at 12:43pm

Dear Aryan,

I think in the early stage of design of a concrete structure you should not rely too much on the local utilization of some elements since you can control it by placing reinforcement there.The approximate amount of reinforcement would be more useful. Since this is not included in Karamba yet (in version 1.2.2.) using the maximum displacement is a good idea. Another performance indicator for comparing different design would be the first eigenfrequency (the higher, the better).

In highrises the floor slabs usually account for a large percentage of the total mass. Thus one is trying to make them as lightweight as possible. With higher grade concrete it is possible to make the floor slabs thinner and thus lighter. However the slenderness of slabs can also be increased by e.g. applying post tensioning or hollow floor decks.

In Karamba the displayed utilization is the ratio of actual stress and the strength of the material and builds on the assumption of linear elastic material behavior (see manual). However concrete is a highly non-linear material - it cracks under tension, and has a non-linear load-displacement curve under compression. Therefore the utilisation values shown in Karamba need to be interpreted with care. Use e.g. Response2000 to calibrate your model.

Best,

Clemens