Units and sign conventions
Units #
The model is stored and solved in kip, inch, second, ksi and radian. What you see and type follows the unit system chosen in View ▸ Units (or the status bar, or Settings):
| Quantity | kip, in | kip, ft | kN, m (SI) |
|---|---|---|---|
| Coordinates, lengths | in | ft | m |
| Section dimensions, thickness | in | in | mm |
| Force | kip | kip | kN |
| Moment | kip·in | kip·ft | kN·m |
| Line load | kip/in | kip/ft | kN/m |
| Pressure | psf | psf | kPa |
| Stress, E, Fy | ksi | ksi | MPa |
| Displacement | in | in | mm |
| Rotation | rad | rad | rad |
| Roll angle | ° | ° | ° |
Number fields accept arithmetic (12*3.5, (20+4)/2) and, in US units, feet and inches (12'6", 12' 6-1/2"). Each model remembers its unit system; new models use your default (Settings).
Axes #
- Global: right-handed X, Y, Z with Z up; gravity acts in −Z. Nodal loads, displacements and reactions are global.
- Member local: x runs from end i to end j; z is the strong axis (Iz = AISC Ix) and y the weak axis. With roll 0:
Member local z local y Not vertical x × Z, horizontal z × x, points upward Vertical global X (made orthogonal to x) z × x So a beam along +X has y = +Z (up) and z = −Y; a column drawn upward has z = +X and y = −Y. The roll angle turns y and z about x by the right-hand rule. View ▸ Local axes draws them.
- Plates: the normal (the "top" face) follows the right-hand rule on the node order; counter-clockwise seen from above points up. Reporting axis 1 is global X projected onto the plate (global Y when the plate is nearly normal to X).
Member end forces #
(N, Vy, Vz, T, My, Mz) at each end in local axes. N is tension positive. Vy, Vz, My, Mz at an end are the force and moment the joint applies to the member, positive along / about +y and +z; so a simply supported beam under gravity has Vy = +wL/2 at both ends.
Diagrams, sagging and hogging #
The diagrams give the internal forces at each station (0 at end i to L at end j); dMz/dx = −Vy and dMy/dx = +Vz.
- A positive Mz puts the +y fibre in tension; a positive My puts the −z fibre in tension.
- On a horizontal beam (local y up), a sagging moment (bottom in tension) is negative Mz: a simple span under w has Mz(L/2) = −wL²/8. A hogging moment is positive: a fixed-end beam has Mz = +wL²/12 at the ends.
- A downward deflection of such a beam is a negative local uy.
Reactions and loads #
- Reactions
(Fx, Fy, Fz, Mx, My, Mz)are global: the force the support applies to the structure. A gravity load gives Fz > 0. - Loads act along the chosen axis: a gravity load is negative Z (global) or negative y (local, on a horizontal beam).
Plate and solid results #
- Membrane forces Nxx, Nyy, Nxy (kip/in): tension positive.
- Plate moments Mxx, Myy, Mxy (kip·in/in): positive = tension on the bottom face, so an upward-facing slab shows sagging as positive.
- Surface stresses σtop = N/t − 6M/t², σbot = N/t + 6M/t²; von Mises and principal stresses (σ1 ≥ σ2).
- Solid stresses are global, tension positive.
Design ratios #
Ratio = required strength / design strength (ϕRn). Up to 1.00 passes; above 1.00 does not. Not certified means the engine could not check that limit state conservatively: it is not a pass.
Codes #
AISC 360-16 (member checks labelled with their clauses, LRFD and ASD; the direct analysis method of Ch. C and the B1-B2 method of App. 8 as in AISC 360-22), AISC 341-22 and AISC 358-22, ACI 318-19, NDS 2018 (ASD), TMS 402-16, ASCE 7-22 (loads, combinations, drift) and ASCE 41-23 (pushover hinges). Help ▸ About lists the same editions.