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# AS3600 - moment redistribution in columns

## AS3600 - moment redistribution in columns

(OP)
Hi

In checking an existing structure consisting of a concrete floor slab supported on steel beams (UB section secondary beams support slab, WB section primary beams support secondary beams, and concrete columns support the primary beams):

The typical frame for the primary structure = a single span primary beam supported each end on concrete columns:

Am finding that the negative bending moment into the columns by elastic distribution of moments is exceeding the column capacity, but the midspan bending capacity of the beam has loads of excess capacity.

Are there any rules in AS3600 that dictate how much moment redistribution can occur for columns?

ie, so to relax the negative moment at the columns and increase the positive moment at the beam midspan.
The columns are generously sized and not prone to buckling. Longitudinal reinforcement in columns = 1.4%

Thanks
Replies continue below

### RE: AS3600 - moment redistribution in columns

Are you saying that the concrete columns and steel beam form a rigid frame? Are the frames required for lateral loading? How are the beams and columns connected?

### RE: AS3600 - moment redistribution in columns

(OP)
Yes - it is a rigid frame and it is providing lateral stability.

The beam sits on top of the column and has 6 M36 high strength cast-in holding down bolts into the column.
The beam has a 36mm thick baseplate.

Beam = 900WB
Span = 16.0m + 3.0m cantilevers

Column = 700 Dia
Height = 5.5m to underside of beam
Footing = 900 dia pile

### RE: AS3600 - moment redistribution in columns

What I are you using for the column? If you assume the column is cracked, then the moment redistributes. Is the beam sized as a simple span for gravity loading? If so, the column only needs to resist the prescribed nominal eccentricity in the gravity loading condition.

### RE: AS3600 - moment redistribution in columns

Cl 6.2.7 of AS 3600 states requirements for redistribution of moment for strength design. Are these requirements not clear?

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: AS3600 - moment redistribution in columns

I wouldn't be distributing moments out of the column but more so using an appropriate stiffness based on the service moment to cracking moment. By the sounds of it 0.25 to 0.35*Ig for the column would be appropriate.

If the column is exposed (not clad or lined) then you might get some large cracks to the tension face if it isn't reinforced accordingly.

### RE: AS3600 - moment redistribution in columns

#### Quote:

I wouldn't be distributing moments out of the column but more so using an appropriate stiffness based on the service moment to cracking moment. By the sounds of it 0.25 to 0.35*Ig for the column would be appropriate.

The moment distribution provisions are for strength design, i.e. Ultimate Limit State. You still have to check for Serviceability, which in AS 3600 is a stress limit. In a column the cracked stiffness is significantly affected by the axial load, so I'd say you should calculate it (or use a conservative high value), rather than using a typical multiplier.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: AS3600 - moment redistribution in columns

(OP)

The deemed to comply moment redistribution approach in AS3600 is written with beams and slabs in mind. At least, in numerous well known text books, the moment redistribution aspects of the code are discussed in reference to beams and slabs only. Some texts even go so far as to state that redistributing moment out of columns is not the intent. Certainly, redistributing moments out of columns without due consideration for punching shear failure mode (for example) would be poor design. In any case - for many columns - the axial load in the column will increase ku to the point where (under the deemed to comply rules) moment redistribution is precluded, or minimal.

My question is this - has anyone used moment redistribution for concrete columns under AS3600, and if so, please detail how the extent of moment redistribution was justified under AS3600. If clause 6.2.7.1 was used (assuming fracture of reinforcement at the fracture strain), can anyone recommend a reference text that explains this method of modelling, without going into a full blown thesis?

NB:
I am not asking what stiffness factors were used.
I am asking - after an appropriate stiffness factor has been chosen, was there subsequently some moment redistribution (out of columns).

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