Whether a column is adequate is not settled by a single check. The strength of the section under axial load and bending is one question, its shear and torsion strength is a second, and whether the reinforcement satisfies the code's detailing rules is a third — often the governing one. Once the seismic code applies, confinement zones form at the ends of the column, and inside them the stirrup spacing and confinement reinforcement follow different rules.
Column Design in StructuralMind answers those three questions separately and reports every rule on its own line. The reinforcement can be selected automatically from the candidate diameters you supply, or your own reinforcement can be verified. When a load case does not pass, the rule that failed is shown with its value and its limit.
Two working modes
| Mode | What it does |
|---|---|
| Design automatically | Searches the longitudinal and stirrup candidate lists you supply for the smallest reinforcement that satisfies every load case |
| Check reinforcement | Runs every check on the longitudinal reinforcement and stirrup layout you entered |
When no diameter in the candidate list meets the demand, automatic design reports the result as infeasible and states whether the infeasibility comes from axial load, shear or torsion.
What is checked
| Check | Against what |
|---|---|
| Axial load and bending | The capacity read from the interaction diagram. Under biaxial bending the curve at the demand angle is used |
| Shear | In both directions separately, with a separate stirrup layout for the mid and end zones |
| Torsion | When it stays above the code's threshold for neglecting it, and it is then combined with shear |
| Detailing | Minimum section dimension, bar diameter, reinforcement ratio limits, axial load limit, stirrup diameter and spacing |
| Confinement zone | When the seismic code is selected: zone length, spacing limit and confinement reinforcement |
| Slenderness | Screened from the effective length factor and the clear column height |
Every check reports its own demand/capacity ratio. When a ratio exceeds 1, that check is reported as FAILED.
Which diagram the section strength uses
The code's equivalent rectangular stress block is used for concrete and the non-hardening elastoplastic diagram for steel. The material factors are applied as and .
The section solution is the fiber engine of the Section Analysis macro. The same column's section can be taken into that macro to inspect the same interaction diagram, its moment-curvature curve and its fiber stress distribution. There, behaviour curves such as Hognestad, Mander and trilinear can be selected and the expected behaviour of the section is shown. The strength check itself runs on the code's own diagram, because TS 500 defines section strength with that diagram.
Section, reinforcement and stirrups
Rectangular, circular and hollow circular sections are solved. The reinforcement layout is symmetric: corner bars and the intermediate bars in each direction take their own diameter and count. Beams with different top and bottom reinforcement are outside this macro, because the seismic code's detailing rules are written for a symmetric arrangement.
Stirrups are defined separately in two zones.
| Zone | What it governs |
|---|---|
| End zone | The confinement zone at the column ends, where the spacing limit and the confinement reinforcement are tighter |
| Mid zone | The region between the two confinement zones |
A rectangular stirrup takes its leg count separately in each direction and can be single, double, triple, diamond or cross-tie. On a circular section a circular hoop or a spiral is used.
The confinement zone length is the largest of three values: 1.5 times the larger section dimension, one sixth of the clear height, and 500 mm. On a 400×500 mm column with a 3 m clear height that length comes out at 750 mm. When a value is entered by hand, the calculation uses the entered value.
Seismic code and ductility class
With no seismic code selected, only the TS 500 rules apply. When TBDY 2018 is selected, a ductility class is stated as well and the confinement zone rules come into effect.
The ductility class sets the bar-diameter multiplier in the stirrup spacing limit: six times the smallest longitudinal bar diameter at high ductility, eight times at limited ductility. The other two limits, on the section dimension and at 150 mm, are independent of the ductility class.
Out of scope for this calculation
| Topic | Status |
|---|---|
| Load factors and overstrength | The calculation applies no factor of its own. The combination values entered must be the factored ultimate-strength values where they are gravity loads. For seismic combinations the increases the code requires (the equivalent static / modal combination increase, the overstrength factor and the like) must already be applied before entry |
| Beam hinging in capacity shear | The capacity shear is computed as , taking both column ends as hinging. Where the beams framing into the column are weaker and hinge first, the beams' plastic end actions are not distributed to the column ends in proportion to the members' stiffnesses. The deviation is on the safe side |
| Second-order effects | No moment magnification is applied. When slenderness exceeds the TS 500 §7.6 threshold a warning is issued and the relevant check is marked FAILED; for those columns the magnification has to be carried out separately |
| Increased reinforcement ratio in a lap splice zone | The ratio limit of 0.04 is applied in all cases. A column carrying a ratio between 0.04 and 0.06 in its splice zone is reported as FAILED |
| Asymmetric reinforcement layout | Sections with different top and bottom reinforcement are solved in the Section Analysis macro |
| ACI 318 and Eurocode 2 | Only TS 500 and TBDY 2018 are applied at present |
| Beam-column joints | The column section is solved; the joint region is not |
| Foundation and anchorage | The anchorage length at the column base and the foundation connection are out of scope |
| Fatigue, creep and shrinkage | Time-dependent effects and cyclic fatigue do not enter the solution |
Endpoints
| Endpoint | What it returns |
|---|---|
POST /api/v1/rc-column/design | Automatic design: the longitudinal reinforcement layout, the stirrup design and every check result |
POST /api/v1/rc-column/check | Verification of the reinforcement you entered: demand/capacity ratios per load case and rule-by-rule results |
POST /api/v1/rc-column/rebar-positions | Bar coordinates computed from the geometry and the reinforcement layout |
POST /api/v1/rc-column/stirrup-layout | Stirrup layout coordinates |
GET /api/v1/rc-column/norm-bundles | The supported design codes and the seismic codes that can be selected with each |
Adding audit to the request returns a step-by-step breakdown of the calculation. Each step carries
its own clause reference, its inputs, its limit value and its PASSED/FAILED result. The language
field sets the language of that generated text and does not affect the numeric results.
Which rule is checked at which limit, and the verification of the section capacity against measured columns, are on the Column Design verification page.