Buckling Restrained Braces
 

Alright, so whats the big deal about buckling restrained braces? I understand that by eliminating euler buckling as a failure mode the brace can be made to exhibit similar behavior under tension and compression loads, and the size of the axial force bearing member can be reduced greatly. However, isn't the material saved by downsizing the axial member more than spent on the sleeve that prevents the buckling?

http://hamidsoltani.com/seismic/brbf.png
Hamid Soltani

In the schematic shown (with the outer steel casing) Euler buckling is not eliminated but simply improved (by increasing the I-value). The only real way to eliminate buckling is to reduce the effective length (kL) to the point of being zero or sufficiently small.

The Euler buckling criteria:

Pcr = pi^2 * E * I / (kL)^2

It is my understanding that the aim of of BRBs is to make the effective length zero by surrounding the load carrying core in a much stiffer sheath where the sheath does not carry any axial load and only carries the P-delta moment.

That may be the intention, but the sheath itself remains unbraced and now falls prey to Euler's buckling criterion.

The only way to improve Pcr (Critical or Euler buckling load) is to:

- increase moment of inertia (I) as this arrangement does, or

- reduce effective length (kL).

Assuming of course E is constant. The ways to reduce kL are:

- shorten the distance between the ends,

- add one or more mid-length supports (add say another brace to the brace), or

- increase the degree of fixity at the end points.

The first two options work to reduce L while the last works to reduce k.

BRBs aren't used to minimize material usage, rather they are engineered for good cyclic seismic performance. They really are designed not to buckle and the specs take into account the important characteristic by making the engineer check that they don't buckle even after a story drift 2 times the design story drift. For the design loads they deal with they the steel core always yields in tension or compression before any buckling occurs. Because BRBs yield in both tension and compression they can develop great ductility and energy dissipation in cyclic loadings. They try to combine the stiffness of a normal braced framed with the ductility of a moment frame.

Thanks, your answer has helped me understand better why BRBs are preferred.