Frame Inputs
Representative slab frame data
Enter metric values for one slab frame in the analysis direction.
Formula Used
Elastic screening relations
The page idealizes one uniformly loaded slab frame with equal rotational restraint at both supports. It is a transparent preliminary model, not a complete code-based equivalent-frame analysis.
qD = γch + qSDL; qu = γDqD + γLqL; wu = quB
Is = Bh³ / 12; Ic = bcdc³ / 12; EIs = αEcIs
Ks = 6EIs / L; Kc = n(4EcIc / Hc); r = Kc / (Kc + Ks)
Mu− = r wuL² / 12; Mu+ = wuL² / 8 − Mu−; Vu = wuL / 2
The service deflection blends fixed-end and simple-span elastic values according to the restraint ratio. Use verified cracked stiffness and code-prescribed service load treatment for final checks.
How to Use This Calculator
Input and review sequence
- Choose one slab frame along a column line.
- Enter the clear span and tributary frame breadth.
- Enter the slab and column dimensions in the bending direction.
- Use project concrete properties and gravity loads.
- Set the load factors required by the governing design standard.
- Select a justified slab stiffness modifier for the service estimate.
- Set strip moment shares only when they match your approved design basis.
- Review demand values, then complete flexure, shear, punching, detailing, and deflection checks separately.
Design Context
Practical notes for concrete slab frames
Why the frame representation matters
Equivalent frame analysis converts a two-way slab system into separate frames. Each frame follows a column line. The frame includes a slab band and supporting columns. This makes a three-dimensional floor easier to study. The method is useful when span lengths, columns, and loads differ across a plan.
Load and stiffness behaviour
The selected frame breadth represents the tributary slab width. Gravity load on that width becomes a line load. The slab band acts like a horizontal member. Column segments provide rotational restraint at the supports. Greater restraint raises the support hogging moment. It also lowers the positive span moment.
This calculator begins with slab self-weight. It adds superimposed dead load and live load. User-selected load factors create the factored frame load. The slab inertia uses the full frame breadth and thickness. A stiffness modifier reduces gross stiffness for a practical service estimate. The column inertia uses the selected bending direction. Two column segments commonly represent columns above and below an interior joint.
What the calculated moments mean
The program uses a symmetric, single-span screening model. It converts column restraint into a rotational stiffness ratio. That ratio ranges from zero to one. A low ratio behaves closer to pinned supports. A high ratio behaves closer to fixed supports. The reported negative moments apply at both supports under symmetric assumptions. The positive moment applies at midspan.
Items outside this preliminary model
Actual equivalent-frame design needs more modelling. It considers adjacent spans, pattern loading, member stiffness changes, torsional members, edge conditions, openings, drops, beams, and lateral effects. It also requires code-specific moment distribution and strength checks. Flat slabs need special attention at columns. Punching shear can control thickness before flexural strength does.
Using the results responsibly
Use the output to compare framing options. Test a wider frame, thicker slab, or stiffer column. Observe changes in line load, restraint, and moments. Keep units consistent. Review the design strip allocations before using them for reinforcement. The allocation percentages are user controls, not automatic code provisions.
A qualified structural engineer should confirm the final analysis model. Check load combinations under the governing local code. Check flexure, one-way shear, punching shear, deflection, reinforcement development, openings, seismic actions, and construction stages. Do not use a preliminary result as a construction design.
Document assumptions for every revision. Save the calculation with drawings, material data, load sources, and engineer approvals. Clear records reduce later coordination errors significantly.
Frequently Asked Questions
Common review questions
What does frame breadth represent?
It is the tributary slab width assigned to the selected column-line frame. Area loads are multiplied by this breadth to obtain the frame line load.
Are the results ready for reinforcement design?
No. They are preliminary demand estimates. Final reinforcement needs a code-compliant analysis, strength design, detailing, development, and serviceability verification.
Why is there a slab stiffness modifier?
Cracking lowers flexural stiffness under service conditions. The modifier lets you study an assumed effective stiffness. Select its value using your approved design standard and engineering judgment.
Can this model represent an exterior panel?
It can provide a comparison only. Exterior panels have unequal restraint and special edge effects. Use a complete frame model with the correct boundary conditions for final design.
How does column height affect the result?
Shorter column segments are rotationally stiffer. Their higher stiffness increases support restraint, raises negative moment, and reduces the calculated positive span moment.
Does the calculator check punching shear?
No. Punching shear near columns is a separate critical check. Verify the governing perimeter, openings, transfer actions, reinforcement, and applicable strength limits.
What are column-strip moment shares?
They are user-entered percentages for allocating the calculated frame moments. They are not automatic code values. Set them only from an approved strip-design method.
Must I use metric inputs?
Yes. Enter metres, kilonewtons, kilonewtons per square metre, and gigapascals. Mixing units will invalidate the stiffness and demand results.
Which gravity loads are included?
The calculation includes slab self-weight, superimposed dead load, and live load. Add finishes, partitions, services, and other permanent actions within the superimposed dead load.
Why is a service deflection estimate shown?
It helps compare framing options early. It is not a final long-term deflection calculation. Include creep, shrinkage, cracking, reinforcement, and code requirements later.
What should be reviewed before construction?
Confirm the structural model, load combinations, flexure, shear, punching, serviceability, reinforcement detailing, openings, construction loads, and local code requirements.