Verification › V.03 Second-order (P-Delta) analysis

V.03.3Fixed Sway Portal

P-Delta within 0.3% of STAAD.Pro

Open in the app and solveNo sign-up needed: it runs in your browser.

Problem

Compare the second-order (P-Delta) response, including the bowed moment along the member, with STAAD.Pro.

STAAD.Pro V8i SS6, the same input file run by Sutranet: linear analysis, PDELTA with SMALLDELTA, and the same model meshed into 10 members per member (for the moment along each member).

Portal 6 m wide and 4 m high, fixed bases
20 kN/m on the beam, 800 kN on each column top, 20 kN lateral at the left corner
Prismatic members: AX 0.01 m², IX 0.0002 m⁴, IY = IZ 0.0001 m⁴ unless stated; E 2 × 10⁸ kN/m²

Front view (X right, Y up). Joints numbered, supports marked ▲. Drawn from the project file.

Bending moment along the member (kN·m)

Result	First-order, STAAD.Pro	First-order, Sutranet	P-Delta, STAAD.Pro meshed	P-Delta, Sutranet	Difference
Left column, moment at 0.9 L (kN·m)	26.22	26.22	25.14	25.11	0.12%
Beam, midspan moment (kN·m)	45.08	45.08	45.44	45.29	0.33%
Right column, moment at 0.1 L (kN·m)	50.19	50.19	52.07	52.04	0.058%

P-Delta support reactions

Result	STAAD.Pro SMALLDELTA	Sutranet	Difference
Left base: FX (kN)	6.93	6.928	none
Left base: FY (kN)	854.06	854.058	none
Left base: MZ (kN·m)	3.28	3.272	0.24%

The beam is the one member the solver leaves unsplit (its axial load is below 0.5% of its Euler load), so its bowed moment is first-order statics on the P-Delta end forces: 45.29 against 45.44 kN·m from the meshed model, within 0.3% of the peak.
First-order displacements, reactions and end forces equal STAAD's linear run to its print precision; P-Delta joint results and end forces are within 0.3% of STAAD's SMALLDELTA run. STAAD reports the moment inside an unmeshed member without the P-δ bow, so the moment along the member is compared with the meshed model.

Open the project in the app (no sign-up needed), or download the file and open it with 📂 Import Job JSON in the File group.
Run Analysis (Static) with P-Delta Analysis ticked (unticked for the first-order result).

The app works in SI units; results in kip and inch are converted (1 kip = 4.448 kN, 1 kip-in = 0.1130 kN·m, 1 in = 25.4 mm).

Automated test: staad_pdelta.rs, run on every change.

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