The STS application performs structural safety analyses of steel column system. Single-piece and double symmetrical cross sections are allowed. The verification is performed on the basis of ideal bifurcation loads for lateral buckling, torsional buckling and lateral torsional buckling.

Column systems
The following structural systems are supported:
- Cantilever column
- Hinged column
- Column pinned on top and restrained on bottom 
- Column restrained on top and on bottom

These supporting conditions always apply to both main axes. Discrete lateral supports and continuous elastic bedding can be defined in addition. For more complex supporting conditions, an interface to the BT II application is available.

Appended hinged column

Cantilever columns are often used as bracing elements in hinged column systems. STS calculates automatically the corresponding equivalent loads and takes the change of the effective length into account.

Basis of calculation
You can perform the design either in accordance with
- DIN 18800
- EN 1993

In combination with EN 1993, the regulations of the corresponding National Appendix are taken into account. 

The following National Appendices are available yet:
- DIN EN 1993
- ÖNORM EN 1993
- BS EN 1993
- NEN EN 1993
- NBN EN 1993
- CSN EN 1993

We currently work on the implementation of additional appendices.

Analyses
The following verifications are performed:
- Elastic or plastic cross sectional resistance. 
- Resistance of the system to lateral buckling, torsional buckling and lateral torsional buckling
- Serviceability

Cross sections
STS allows you to calculate columns with the following types of cross sections:
- Standard I-sections 
- User-defined doublesymmetrical I-sections
- Standard round and square hollow sections 
- User-defined round and square hollow sections

Lateral supports
Lateral supports can be defined to secure the columns against stability failure. You can enter lateral supports in STS either as a continuous support in the form of elastic bedding or as discrete supports in
- the middle of the span
- the third points
- the quarter points or
- at a point x0.

Actions
You can expose the column system to vertical and horizontal loading and define head and base moments. However, you cannot define loading that produces planned torsion. Furthermore, you can define appended hinged columns in the directions of the both main axes and optionally apply the self-weight of the column as a resultant to the column head.

Combinations of actions
STS generates automatically the appropriate load cases and load case combinations, depending on the defined actions, and performs the necessary analyses, whereby the decisive load case combination is determined for each limit state.

Pre-defined loads
In order to simplify the load definition, STS offers a range of pre-defined loads that are sufficient for most systems. 

Optionally available are:
- Vertical loads
- Horizontal loads
- Bending moments 

You can enter
- wind from y/z direction and
- accidental impact loads in y/z direction as span loads applying to the column head.

Calculation
STS calculates the internal forces and the deformations in first-order analyses and determines the ideal bifurcation loads, the slenderness ratios and the reduction factors for the stability verifications in accordance with the equivalent bar method. 
The bifurcation loads are determined numerically with consideration to the torsional rigidities. Therefore, the user can also perform structural safety analyses in accordance with the equivalent bar method for more complex loading situations.

Interfaces to other applications
The characteristic bearing forces can be transferred to the applications FD, ST3 and ST6.
If the real load conditions do not comply with the defined standard or the loading situation leads to planned torsion, you cannot use STS for the calculation. 
If you have a valid licence for the BTII application (2nd Order Buckling Torsion Analyses), you can transfer the structural system to perform second order buckling torsion analyses, if required.

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