Column Design computes two different kinds of quantity, and each is verified differently. The section capacity is a physical prediction — it states what the section will actually carry. The detailing limits are rules the code lays down. A rule cannot be falsified by a test, because the rule defines what is acceptable. So the first layer is checked against measured columns and the second against the clause itself.
Section capacity
The interaction diagram Column Design uses is the fiber engine of Section Analysis itself. Solving the same column in both macros produces the same mesh fibers, the same bar positions, the same material curves and the same interaction diagram point by point. The comparison was run separately for rectangular and circular sections, and in both the numbers match digit for digit.
In practice this means you can take a column's section into Section Analysis and inspect the same interaction diagram, moment-curvature curve and fiber stress state there. The capacity that enters the strength check in Column Design and the capacity you see there are the same number.
The verification of that capacity against measured columns is on the Section Analysis verification page. Across thirty-nine laboratory specimens, the capacity computed on the design basis never exceeded the moment the specimen carried.
Which diagram the check uses
The strength check uses the code's equivalent rectangular stress block for concrete and the non-hardening elastoplastic diagram for steel. Even when a behaviour curve such as Hognestad or Mander is selected in the request, the strength calculation runs on those two diagrams.
The material factors and are applied to the stress read from the curve after it is built. The modulus of elasticity is not reduced, so the yield strain shifts to .
Behaviour curves exist to produce the expected behaviour of a section and are used in the Section Analysis macro. A code verdict is produced with the code's own diagram.
Detailing limits
Every rule is checked at three points: comfortably inside the limit, exactly at it, and just outside. The value exactly at the limit is the decisive one, because TS 500's limits are inclusive and a rule written with instead of shows up only there. The limit each rule reports is checked as well.
| Clause | Rule | What was checked at the limit |
|---|---|---|
| TS 500 §7.4.1 | Smallest section dimension 250 mm | 250 mm PASSED, 249 mm FAILED |
| TS 500 §7.4.1 | Circular diameter 300 mm | 300 mm PASSED, 299 mm FAILED |
| TS 500 §7.4.1 | Longitudinal bar diameter 14 mm | 14 mm PASSED. 20 mm corners with 12 mm intermediates FAILED, because the smallest diameter governs |
| TS 500 §7.4.1 Eq. 7.8 | Bracketed from both sides | |
| TS 500 §7.4.1 Eq. 7.9 | 0.0103 PASSED, 0.048 FAILED | |
| TS 500 §7.4.1 Eq. 7.7 | Exactly at the limit PASSED, 1 kN above FAILED | |
| TS 500 §7.4.1 | Stirrup diameter 8 mm | 8 mm PASSED, 6 mm FAILED |
| TS 500 §7.4.1 | Mid-zone spacing | Checked at four diameters. At 14 and 16 the bar term governs (168 and 192 mm), at 20 and 32 the 200 mm cap does |
| TBDY 2018 §7.3.4 | Confinement zone spacing | All three terms made to govern, each on a different section |
| TBDY 2018 §7.3.4 | Ductility class | High ductility uses and limited ductility , while the 150 mm cap is unchanged |
| TBDY 2018 §7.3.4 | Confinement zone length | Each of the three terms made to govern on a different section |
A 50 mm floor is applied beneath the ceilings the spacing rules produce. That floor is not the code's rule; it is there to keep the spacing buildable, and it comes into play on small sections.
When a confinement zone length is entered by hand, the formula is bypassed and the entered value applies even if it falls below the 500 mm floor.
Scope of this calculation
What the results above cover, together with the direction each omission leans.
The shear and capacity-shear formulas take both column ends as hinging. The capacity shear is computed as . 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, and the result is on the safe side.
Factored actions, already carrying the overstrength factor, are expected. The combination values entered into Column Design must be the factored ultimate-strength values where they are gravity loads. Likewise, values entered for seismic combinations must already carry the increases the seismic code requires (the equivalent static / modal combination increase, the overstrength factor and the like).
The increased reinforcement ratio limit in a lap splice zone is not applied. TS 500 §7.4.1 raises the ratio limit to 0.06 inside a lap splice zone. The calculation applies in all cases, so a column carrying a ratio between 0.04 and 0.06 in its splice zone is reported as FAILED. The deviation is on the safe side.
Second-order effects are not computed. When slenderness exceeds the TS 500 §7.6 threshold no moment magnification is applied; a warning is issued instead and the relevant check is marked FAILED. Columns that require a second-order analysis need that magnification carried out separately.
Biaxial bending comes from the section engine. The interaction curve at a given bending angle is verified to be the one Section Analysis produces. The resolution of the angle sweep belongs to that engine's own scope.
Bottom cover is a separate input. If top and side cover are entered and bottom cover is left empty, 25 mm is used. The bar layout is then not symmetric about the section centroid. Rebuilding the section in another macro requires entering bottom cover as well to reach the same number.
Whether the code's own rules are adequate is not the subject of this page. What is shown here is that the TS 500 and TBDY 2018 rules are applied as written.