Column calculations with Frame Analysis

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StruSoft Fredrik L
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Column calculations with Frame Analysis

Postby StruSoft Fredrik L » 29 Jul 2016, 12:40

As you know the EC2 allows three methods to consider flexural buckling for concrete columns and walls. The general method according to 5.8.6, and the simplified methods the stiffness method 5.8.7 and the curvature method 5.8.8. All three methods are allowed according to the Swedish annex.
In all cases minimum reinforcement according to EC2 9.5.2 or 9.6.2 is required.

Frame Analysis can handle all three methods. If only a 1:st order analysis is made the 2:nd order moment in both directions are calculated with the stiffness method or the curvature method depending in the users choice. If a 2:nd order analysis is made the buckling check in the plane of the structure is made with the general method an out of the plane with one of the simplified methods.
The other program in the WIN-Statik package Concrete Column can only handle the simplified methods.

The conditions stated in EC2 5.8.6 for the general method are the following:
A non-linear analysis (meaning considering how cracking effects the stiffness of the members), including geometric non-linearity i.e a second order analysis should be performed.
The effect of creep should be taken in account.
The E-modulus is divided with the safety factor GammacE=1.2.
All these conditions are available in Frame Analysis.

The effect of creep can be taken into account by defining an effective creep value or by connecting the ULS load case to a SLS long time load combination for which a creep has been defined. This is done by defining a dependency for the load combination. This option is appreciated by some clients but most of them does not use this facility and define an effective creep value instead.

Dependency is defined by using the option “Dependent of” and the ID number of the SLS load combination is the displayed for the ULS load combination in question. If a dependency is defined then no effective creep value can be defined for that load combination. If a design is made with this input the utilization differ between the two ULS load combination as it should. As mentioned earlier creep can also be taken account just by defining an effective creep value according to 5.8.6 (4).

Finally about the different buckling situations:

Image

How the member will react is a decision that must be taken by the user and the input is to be made accordingly. If one wants the buckling mode according to the left picture the supports are normally defined as hinged.

If one instead wants the global deformations of a larger structure to have effect on the member then one models it as a frame.
The benefit with Frame Analysis is that one can model a complete frame but of course one can consider only one single member as well.
In e.g. Concrete Column only a single member can be modelled.

If a single member is calculated with e.g. the stiffness method in Concrete Column and in Frame and the max moment is defined in Column the result could differ slightly and be more conservative in Concrete Column as in this program the moment is presumed to act constantly along the column but Frame knows the real moment distribution.

The result for these three methods does differ and for slender compressed walls the general method normally produces more favorable results. The reason could be that in this method the actual stiffness in the structure is used and the program knows when the structure is cracked and not.

In other cases it could be the opposite and the general method is more conservative than the simplified methods.

Finally on our webpage there are instruction videos both for Frame Analysis and Concrete Column which can be useful.
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Fredrik Lagerström
Technical Support and Developement
StruSoft AB

Telephone: +46 (0)40 530 100
Mobile: +46 (0)734 20 20 74
Skype: StruSoft Fredrik Lagerström
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