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# AS3600 - Column restraints

## AS3600 - Column restraints

(OP)
Hi

It is not clear to me how to brace a slender concrete column or wall in order to reduce its height. Usually with steel column, a restraint is deemed to be satisfactory if the brace capacity is at least 2.5% of the ultimate compression load in the member to be braced. This applies to the flanges of beams, when they require lateral restraint, but can be extended to actual compression members such as columns.

As far as I know in the AS3600 code there's no indication on the design load of restraining members. Many engineers I know extend the 2.5% requirement of AS4100 to RC members too (columns and walls). So for example, if I have a slab connected to a wall on one side only, the design load for the dowels connecting the slab to the wall will be 2.5% of whatever compression load is in the wall. If the compression is say 6000 kN/m in the wall, the dowels will have to be able to transfer at least 3000 x 0.025= 150 kN/m, which may be reasonable.

My issue with this approach is whenever the stiffness of the restraining element is not significantly higher than the restrained member.

Let's say I have a 8 m long concrete column. If in order to halve its height, I connect one continuous beam on two opposite sides of the column at 4 m, the beam will restrain the column very effectively in one direction (parallel to the beam) while in the other direction (perpendicular to the beam) I will have to rely on the out-of-plane beam capacity. I can design the lateral bars of the beam so that the beam is able to take a horizontal point load at mid-span of 2.5% of the column compression. Now, it makes me quite nervous in this case to adopt an AS4100 requirement to concrete elements, because I assume that the AS4100 requirement was formulated with extensive testing so that if a member has enough capacity, its stiffness is deemed to comply for the lateral restrain, however this may not be the case for concrete elements. The only way I can think of designing this is with buckling analysis, but there must be some more easy and practical way to design this.

Does anyone have any clear guidelines on how to restrain compression concrete members, design loads of restraining members, stiffness requirements etc. ?

### RE: AS3600 - Column restraints

The stiffness of the restraint needs to be greater than 2P/L

P = axial load in column
L = distance between restraints

### RE: AS3600 - Column restraints

I would put decent Factor of Safety on bracing elements, as their loss would significantly reduce capacity of the member they brace. AISC provides a more conclusive check on strength AND stiffness for steel members. Also work by Yura and others is readily accessible on the net.
The force will be proportional to the "out-of-plumbness" of column, the stiffness as Retrograde stated. Robustness? what is one of your beams is lost, damaged, removed at a later date as it appears to be doing nothing (I assume no floor on it, hence your problem). Is it not feasible to design column for full height?

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