Verification › V.02 Frames

V.02.13D Frame: Maximum Forces

All 72 values STAAD.Pro prints match

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Problem

Determine the maximum axial force and bending moments in a space frame with a member inclined in all three directions.

Weaver, W. Jr., Computer Programs for Structural Analysis, p. 146, structure 8.
STAAD.Pro verification problem "3D Frame Max Forces"; its input file is used unedited.

Joints 1 (0, 120, 0), 2 (240, 120, 0), 3 (0, 0, 0), 4 (360, 0, 120) in
Members 1-2, 3-1, 2-4; AX 11 in², IX 83 in⁴, IY = IZ 56 in⁴; E 30 000 ksi, Poisson 0.25; no shear deformation
Joints 3 and 4 fixed
FX 2 kip at joint 1; FY −1 kip and MZ −120 kip-in at joint 2

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

Maximum forces, member 3

Result	Weaver	STAAD.Pro	Sutranet	Difference
Axial force (kip)	1.47	1.47	1.4696	none
Moment MY (kip-in)	84.04	84.04	84.0397	none
Moment MZ (kip-in)	95.319	95.32	95.3189	none

Member end forces, local axes (kip, kip-in)

Member, joint	Source	Axial	Shear y	Shear z	Torsion	MY	MZ
1, joint 1	STAAD.Pro	0.90	−0.43	0.22	22.71	−17.97	−36.37
	Sutranet	0.8959	−0.4322	0.2173	22.7071	−17.9730	−36.3731
1, joint 2	STAAD.Pro	−0.90	0.43	−0.22	−22.71	−34.18	−67.36
	Sutranet	−0.8959	0.4322	−0.2173	−22.7071	−34.1817	−67.3590
2, joint 3	STAAD.Pro	−0.43	1.10	0.22	−17.97	−48.78	96.12
	Sutranet	−0.4322	1.1041	0.2173	−17.9730	−48.7845	96.1216
2, joint 1	STAAD.Pro	0.43	−1.10	−0.22	17.97	22.71	36.37
	Sutranet	0.4322	−1.1041	−0.2173	17.9730	22.7071	36.3731
3, joint 2	STAAD.Pro	1.47	−0.71	−0.48	−37.02	15.69	−53.28
	Sutranet	1.4696	−0.7149	−0.4798	−37.0171	15.6888	−53.2791
3, joint 4	STAAD.Pro	−1.47	0.71	0.48	37.02	84.04	−95.32
	Sutranet	−1.4696	0.7149	0.4798	37.0171	84.0397	−95.3189

The frame is statically indeterminate, so there is no closed-form answer; Weaver's values come from his own program.
Member 3 is inclined in all three directions, so how its moment splits into MY and MZ also checks the member local axes.
STAAD.Pro prints 72 values (joint displacements, support reactions and the end forces above); all match to the last printed digit. The closest is the shear −0.714943, printed −0.71. The largest moment in the frame, 96.12 kip-in, is at the foot of member 2.

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) on load case 1 with P-Delta Analysis unticked.

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_frame_max_forces.rs, run on every change.

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