Code Based Design Vs Performance Based Design

[Muneeb Badar]

Dear All,

What do you think what are the advantages or disadvantages of code based design

And

What are the benefits/drawbacks of Performance based design which is also Non Linear Design practice.

A short discussion is welcomed.

Thanks

Muneeb

[EngrUzair]

Check out the following link (http://en.wikipedia.org/wiki/Performance-based_building_design). The paragraph titled "Performance approach and prescriptive approach" may give you a brief idea about diffrences between the two approches.

 

Internet search may provide you many more articles and documents dealing with the subject.

 

 

Regards.

 

[Muneeb Badar]

Dear Engr. Uzair,

 

many thanks. But I do not need internet search. I just need to discuss these details on this forum.

 

This post is just for knowledge sharing.

 

Thanks

 

Muneeb

[UmarMakhzumi]

This can only be answered if someone is involved in performance based design, which a lot of people are not. To address your questions, I think both approaches are beneficial where needed. You don't want to design a single story house based on performance based design or a cantilever garage porch. But you may want to consider performance based design for a tall high rise building or structure where you would like to limit the extent of structural damage when subjected to an environmental event because of your Client needs.

 

Also, performance based design is a great topic for undergrad final year students. My friend actually got employed in a firm because he did his undergrad project on performance based design, although he remembered nothing.

 

Thanks.

[WR1]

I dont have any exp in PBD but here are my 2 cents. Please correct me if wrong. 

 

Performance based design is a nonlinear dynamic problem.

 

By this we try to model post-yield ductility and energy dissipation for example under an earthquake.

 

Ductility is very important in seismic design and ductility is actually a ratio of Nonlinear response / linear response 

 

This is handled by R factors in code. Again R factors are for all the modes of vibrations. Which in reality is not true.

 

In linear elastic analysis we design structures for earthquakes for very small forces and actual EQ forces are quite large. But still structures desgined like this have performed well because the structure show a ductile behaviour which is very difficult to model from code based approach. So we assume the remaining portion of forces are handled by ductility. This ductility is handled by proper reinforcement detailing of members and joints.

 

Where as in PBD as the name suggests we design based on performance of a structure. We try to model energy dissipation of structures. For example kinetic energy is applied to structure in an earthquake this is converted to elastic strain energy.

 

Think of this as a car moving (it has kinetic energy) and then it hits a spring fixed on wall...This kinetic energy will be transferred to this spring and the spring will compress and it has now elastic strain energy. Now this SE must be dissipated from the spring otherwise it can react back to car and can throw its passengers outside.

 

This energy dissipation in buildings is handled by damping (really difficult to predict) In buildings damping consists of

 

1. any external equipment like tuned dampers (liquid dampers, mass tuned dampers, braces etc)

2. Material inbuilt properties

3. Joints (e.g. beam column joints)

 

We assume all above 3 to give around 1-5% of Critical damping.

 

Critical damping is the damping where a structure vibrates and never crosses its mean position. Think of it as container filled with liquid and a spoon is immersed in it. Now the spoon will vibrate if external force is applied. Now increase the viscosity of the liquid in container and keep increasing to the point where the spoon vibrates from its mean position to left and comes back to its mean position and stops there (does not go to right).

 

This viscosity of the liquid has critical damping Cc.(that's why we call it viscous damping/dampers). In buildings the damping is 1-5% of this Cc. 

 

In PBD we try to model these dampers.

[Muneeb Badar]

Dear Umar Makhzumi,

 

Thanks for your reply. Well, truly speaking Performance Based Design is not necessarily meant for Tall Buildings. Actually, its about the performance of structure. In some countries we done some projects of 2 or 3 story concrete and masonry buildings. So, we can use these any where. For small buildings it is also of great importance due to the effect of brick masonry walls which can only be predicted by performance based design.

 

So both techniques are not equal. Actually, code based design has too many flaws when considered for the design of a little non rectangular or non square building.

 

PBD is really quite good for undergrads for projects on a small scale.

 

Thanks

 

Muneeb

[Muneeb Badar]

Very Well Explained by Wasim, very nice approach towards PBD