Here are all the resources from the hands-on workshop. The full session is available to watch again, along with the presentation slides (PDF) and the Q&A report.
Moment Curvature Response - Theory and practice with web based tools, for columns and beams
Moment Curvature using Column Designer & GEAR - Comprehensive tools for RC Column and Beam
Questions & Answers
Generation of the Moment-Curvature (M-φ) Curve using Column Designer and GEAR | Q&A | Hands-on Workshop, 06 August 2026
QNo 1
Can Column Designer generate moment-curvature and model the irregular-shape sections of bridge piers, including the rebars? Also, the parametric shape pier- can we do that in Column Designer?
Ans
Yes. Column Designer can generate moment-curvature for essentially any pier configuration. It includes a library of parametric bridge-pier sections that can be used directly, and for irregular or non-standard shapes the section can be built by merging or drawing custom geometry. Rebars can be placed either parametrically (pattern-based) or manually at any location, and confinement zones can be defined independently per region (for example, two circular confined zones plus one rectangular confined zone within a single elliptical pier section). Once the section, materials, and confinement are defined, moment-curvature is generated using the same engine used for standard building columns.
QNo 2
Is this moment curvature directly related to the back bone curve of a non linear material to be used in the non-linear pushover analysis? If yes, how can we relate specially in bridge piers?
Ans
The moment-curvature curve from Column Designer is monotonic (first-cycle response). It is not itself the cyclic backbone used in nonlinear/pushover models. To obtain the analysis backbone, the monotonic M-φ curve must be combined with a degradation/hysteresis model, which typically reduces capacity at large curvature under repeated cycles. This modified backbone is then converted to a moment-rotation relationship (using an assumed or calculated plastic hinge length) and assigned as the hinge property in ETABS, SAP2000, or CSiBridge.
For piers specifically, since M-φ is both axial-load and direction dependent, more rigorous analyses use fibre sections that regenerate the M-φ backbone in real time for the current axial load and bending direction during the analysis, rather than relying on one pre-computed curve.
QNo 3
Do you anticipate that it will remain open source in the future or are there plans to transition to a commercial product?
Ans
To clarify: Column Designer is not an open-source project. It is a web-based product that follows a freemium commercial model, so there is no open-source version to transition away from.
How the model works:
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Free tier: every app can be used without registration to try the full workflow, and creating a free account allows work to be saved between sessions. This tier remains available for evaluation and learning.
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Paid tiers (Standard and Advanced): unlock advanced capabilities such as unlimited projects, ETABS/SAP2000 import, CDB legacy import, composite columns, confinement zones with the Mander model, Word/Excel export, and priority support.
Regarding the future: because product plans and pricing can evolve, only the current offering is described here, without commitments about future source-code availability, pricing, or the free tier.
QNo 4
(a) How do we select and assign the material properties for concrete confined and unconfined to the section?
(b) How to define confinement?
Ans
Materials / stress-strain: navigate to the material definitions (Define, or Project Explorer > three-dot menu) and open the Stress-Strain tab. Concrete options include Unconfined, Mander Confined (Rectangular or Circular), or a General user-defined curve; steel has its own equivalent options.
Confinement zones: under Design, choose Auto Confinement for an automatic zone, or manually add a Rectangular/Circular Confinement Zone matching the shape, specifying tie size, spacing, and leg arrangement (or spiral, for circular sections).
Toggle for M-φ analysis: ensure the shape’s assigned stress-strain curve is toggled to Confined so the engine uses the Mander parameters for the moment-curvature and ductility evaluation.
Note: code-based P-M capacity interaction surfaces remain based on code stress blocks, per design code requirements, regardless of the confinement model assigned.
QNo 5
Can this Column Designer be used directly to model and design a prestressed concrete girder, or is it intended only for defining the cross-sectional geometry and conventional reinforcement?
Ans
Not as a dedicated prestressed-girder design tool, but the essential ultimate-strength effect of prestressing can be approximated: convert the post-tensioning force into an equivalent axial load applied to the section, and model the PT tendons as high-strength ‘rebar’ using the prestressing steel’s stress-strain properties. The applied axial load represents the prestress force, since the initial tendon prestrain is not tracked in the section analysis.
This gives reasonably good results for ultimate capacity and moment-curvature. However, time-dependent effects such as creep, shrinkage, and relaxation are not captured by this approach and require separate long-term analysis models.
Column Designer is intended for cross-section geometry, conventional reinforcement, and this approximate PT representation. It is not intended for full prestressed-girder design including long-term losses.
QNo 6
Can different material properties be applied to individual rebars to simulate corrosion?
Ans
In principle, yes. The formulation supports assigning different concrete models to different zones of a section, and the same applies to individual rebars, so a corroded bar could in theory be represented with reduced area and/or reduced strength while adjacent bars keep their original properties.
However, this per-bar material assignment is currently not exposed in the Column Designer interface and will be added in next update.
QNo 7
Could you please clarify why when axial load changes, the Moment-Curvature also changes?
Ans
Because the section must satisfy force equilibrium at every point on the curve. With zero axial load (pure bending), internal compression must equal internal tension. Once an axial compressive load is applied, equilibrium requires internal compression to equal tension plus the applied axial force, so the neutral axis depth and the entire strain state of the section change.
The effect is not one-directional. Below the balance point, moderate axial compression actually increases the moment capacity, because it delays cracking and engages more of the concrete in compression; what it consistently reduces is the curvature ductility, since the extreme concrete fibre reaches its crushing strain at a smaller curvature. At high axial loads, both moment capacity and ductility reduce, and the failure mode shifts towards compression control. This compression-controlled regime is exactly where confinement is most effective; at low axial load, failure is typically tension-controlled and confinement of the compression zone contributes less.
This is also why seismic codes commonly cap the axial load ratio on columns (typically in the range of 0.3 to 0.6 of the axial capacity, depending on the code and ductility class) to preserve enough ductility and prevent brittle compressive crushing.
QNo 8
Moment-curvature analysis is a sectional analysis. How do you justify using it alone for performance-based assessment when actual column failure is governed by member-level instability, P-Δ effects, and frame interaction?
Ans
Moment-curvature is deliberately a section-level tool, not a complete performance assessment on its own. It is the first link in the chain: material → section (M-φ) → member (moment-rotation, via plastic hinge length) → structure (frame with P-Δ and interaction).
Member-level instability (second-order P-δ effects along the member length), storey-level P-Δ, frame load redistribution, and biaxial interaction that changes with drift direction all require this section behaviour to be carried into a member/frame-level nonlinear model, e.g. fibre-hinge or lumped-plasticity elements in ETABS/SAP2000/CSiBridge that explicitly include P-M-M interaction and geometric nonlinearity.
So moment-curvature is not used alone for performance-based assessment; it is the required input that is then propagated through a P-Δ-capable frame analysis. Used in isolation, without that follow-through, it would indeed be insufficient to capture member instability or frame interaction. This is exactly why the workflow explicitly includes the moment-rotation and structural-analysis steps beyond the section curve.
QNo 9
Is axial load assumed constant throughout the analysis? During earthquakes axial force fluctuates significantly. How is this incorporated?
Ans
A standalone moment-curvature curve is generated for one specified axial load and one load combination at a time; it is a section-level, not time-varying, calculation.
During an earthquake, however, column axial force can change substantially due to frame action and overturning, particularly in exterior and corner columns, even when the vertical component of ground motion is not included. A simple uniaxial moment hinge calibrated at a single axial force does not capture this P-M interaction.
How it is incorporated: for analyses in which axial-force variation matters, a fibre-section/fibre-hinge or an interacting P-M-M formulation is used. These regenerate the moment-curvature relationship in real time for the actual axial load and bending direction at each analysis step, rather than relying on a single pre-computed curve. A separately verified axial-force-dependent hinge model can also be used.
If a fixed moment-curvature / moment-rotation curve is instead assigned as a hinge property, it applies to one axial load only; capturing several axial-load levels then requires generating multiple curves (some programs, such as SAP2000, accept many curves and interpolate between them).
QNo 10
Is the arching effect between column longitudinal bars considered in the analysis and moment curvature plot?
Ans
Yes, provided the Mander confined-concrete model is used with the actual transverse-reinforcement and longitudinal-bar geometry. Arching action is then accounted for automatically.
The Mander formulation uses a confinement-effectiveness concept to represent the non-uniform confinement and the arching that develops between supported longitudinal bars and transverse reinforcement. The effectiveness coefficient (kₑ) captures two components:
- Vertical arching: the clear spacing (s′) between transverse tie/hoop layers.
- Horizontal arching: the clear lateral spacing (wᵢ) between longitudinal bars restrained by tie corners or cross-ties.
Concrete in the parabolic arching zones between bars and ties is treated as ineffectively confined, so the gross core area (Acc) is reduced to an effectively confined core area (Aₑ = kₑ · Acc). This lowers the effective confining pressure and therefore the confined concrete stress-strain response (peak stress f′cc and ultimate strain εcu) used in the moment-curvature analysis.
Importantly, the confined response depends on the full confinement model and reinforcement parameters (bar layout, tie size, spacing, and arrangement), not on the effectiveness coefficient alone. In this way the actual bar layout and tie distribution realistically influence the computed section ductility.
QNo 11
Can you add additional rebar layers (e.g., 2 or 3 layers) in negative reinforcement zones?
Ans
Yes, multiple layers of reinforcement can be modelled.
While the program does not have a dedicated ‘layer’ object or an automated multi-layer pattern generator, multiple layers can be defined manually:
- Manual / line rebar tools: use the Point Rebar tool (for individual bars at specific coordinates) or the Line Rebar tool (for rows of bars) to place bars at different depths from the concrete face, creating 2, 3, or more layers in tension or compression.
Note: regardless of how many layers are placed, the moment-curvature (M-φ) and P-M interaction engines fully account for each individual bar’s depth (dᵢ) and material properties during analysis (full strain-profile support).
QNo 12
Can we get the moment curvature curve for the entire building in 2D or 3D for different loading and combinations?
Ans
No. Moment-curvature is a section property, not a building property. It is specific to one axial load and one bending direction, and it changes with both.
There is no single M-φ curve that represents an entire building. For building-level performance, the equivalent measure is the global load-deformation response (base shear vs. roof displacement, or drift ratio), which is assembled by carrying the section behaviour (moment-curvature) up through member behaviour (moment-rotation) and into a frame-level nonlinear analysis such as pushover or time-history.
QNo 13
(a) How can we define the strains like DU, DX, DR, and DL for concrete and rebar materials?
(b) How about the deformations DU, DX, DR, and DL for inelastic member?
Ans
An important clarification first: DU, DX, DR, and DL are not universal standard notation for material strains or inelastic-member deformations. Their exact meaning is defined by the software, so the definitions given in the GEAR / Column Designer interface or documentation should be relied upon rather than a generic interpretation. In particular, DU/DX/DR/DL should not be mapped to performance levels such as Immediate Occupancy (IO), Life Safety (LS), or Collapse Prevention (CP) unless the software explicitly defines them that way.
With that caveat, here is how material strains and member deformations are generally defined and used:
(a) Material strains (concrete and rebar):
Material behaviour is described with clearly defined, physically meaningful strain quantities:
- Concrete: cracking strain (εcr), strain at peak compressive stress, and ultimate crushing strain (εcu).
- Reinforcing steel: yield strain (εy), strain-hardening strain (εsh), and ultimate tensile strain (εsu).
In Column Designer / GEAR these are entered directly in the material stress-strain definitions (for example, the Mander confined-concrete model, or steel with strain hardening), and they set the strain limits used in the nonlinear section analysis. Whatever labels the interface uses should be matched to these underlying strain parameters via the program’s own definitions.
(b) Inelastic member deformations:
Member behaviour is expressed through curvature, rotation, and deformation together with code- or guideline-specific acceptance criteria, derived from the section response as follows:
- Generate the M-φ curve for the given cross-section and axial force.
- Extract the key curvature limits: yield curvature (φy), ultimate curvature (φu), and any post-peak limit curvature.
- Convert to plastic rotation: plastic curvature (φp = φu − φy) multiplied by the equivalent plastic hinge length (Lp) gives the plastic rotation capacity (θp = φp · Lp).
These curvature and rotation quantities are then checked against the acceptance criteria in the relevant code or guideline (for example ASCE 41) to establish member deformation limits.
QNo 14
Can we get the moment curvature curve for the entire building with different loading and combinations?
Ans
Same as Q12: no. Moment-curvature is inherently a section-level result, specific to one axial load and one bending direction, so it cannot represent an entire building. Building-level nonlinear performance is instead expressed through the global load-deformation (base shear vs. roof displacement, or drift ratio) curve, built up from section (M-φ) to member (moment-rotation) to full structural pushover or time-history analysis.
QNo 15
I have CSiCol v12 perpetual license, how is it more advantage?
Ans
CSiCol and Column Designer are built on the same underlying analysis methodology, so the core technical results (section capacity, P-M-M interaction, moment-curvature, and slenderness checks) are equivalent. A CSiCol v12 perpetual licence therefore already provides the full analytical capability.
What Column Designer adds is mainly workflow and accessibility:
- Purpose-specific templates: pre-configured templates (Building Frame Column, Bridge Pier, Shear Wall, and Retrofit Assessment) that speed up section setup.
- Direct ETABS import and batch processing: multiple columns can be imported directly from ETABS and evaluated or designed together in a single workflow.
- Web-based accessibility: runs entirely in a web browser with no desktop installation, so it is accessible from any device.
- Licensing model: Column Designer is offered as a web-based subscription, whereas CSiCol uses a standalone desktop perpetual licence.
In summary: an existing CSiCol v12 licence covers the analysis itself; Column Designer adds the web-based workflow, ETABS batch integration, and quick-start templates.
QNo 16
Follow up?
If two columns produce identical moment curvature curves but have different detailing, confinement, splice locations, and transverse reinforcement, would you expect them to exhibit the same seismic performance? If not, what critical parameters are missing from moment-curvature analysis?
Ans
No. Identical monotonic moment-curvature curves do not guarantee identical seismic performance.
M-φ captures only the section’s monotonic strength-deformation relationship. It does not by itself capture:
(1) Cyclic degradation: strength and stiffness loss under load reversals, which depends heavily on confinement detailing, hoop spacing, and cross-tie configuration even if the peak M-φ values match.
(2) Bar buckling: governed by tie spacing and leg configuration, which can trigger degradation earlier than the monotonic curve suggests.
(3) Splice performance: a lap splice located in a plastic hinge zone can fail well before the section reaches its theoretical ultimate curvature.
(4) Shear-flexure interaction: M-φ says nothing about whether a shear failure precedes the flexural ductility being realised.
(5) Bond-slip / strain-penetration effects at the member base.
Two columns with matching M-φ curves but different detailing, confinement, splice location, and transverse reinforcement can therefore have very different real seismic performance. The missing parameters are the cyclic/hysteretic degradation model, splice adequacy, shear capacity, and the confinement detailing quality that determines how closely the real section tracks the theoretical confined curve before bar buckling or hoop fracture.
QNo 17
Can the Beam Section Designer be used directly to model and design a prestressed concrete girder, or is it intended only for defining the cross-sectional geometry and conventional reinforcement?
Ans
No, not as a complete prestressed-girder design tool. The beam-focused application on the Eagle Eye platform is the GEAR RC Beam Moment-Curvature app which, like Column Designer for columns, is built for general cross-section geometry and conventional (mild-steel) reinforcement.
The same approximation described for Q5 can be applied where an axial force and custom material curves are supported: represent the prestress as an equivalent axial force, and model the tendons as high-strength ‘rebar’ with the prestressing steel’s stress-strain properties. This gives a reasonable estimate of ultimate flexural capacity and moment-curvature.
Full prestressed-girder design, including service stresses, staged construction, and time-dependent losses (creep, shrinkage, relaxation), requires a dedicated prestressed-concrete workflow, for example in CSiBridge.