Reading analysis results: deflection, axial, shear and bending moment diagrams

An analysis produces thousands of numbers. The skill is to read them as an engineer: to see the load path, spot what governs, and notice when something is wrong. This tutorial explains each kind of result Struxt3D shows, the conventions it uses to draw them, and a few quick checks that catch most modelling errors.

#Start with the deflected shape

Always look at Displacement first, animated. It tells you, at a glance, whether the model behaves like the structure you imagine:

• Floors sag between columns, cantilevers droop, the frame sways downwind.
• Nothing flies off on its own — a member moving far more than its neighbours usually means it is not connected, or its supports are missing.
• Under symmetric loads on a symmetric structure, the shape is symmetric.

The deformed shape is exaggerated so you can see it. Displacements are shown in millimetres; by default the service combination (tagged SLS, D + L (service)) is shown because that is what deflection limits are checked against.

#The member forces

Inside each member, at every point, the analysis gives six internal forces in the member's local axes (x along the member, y and z across it):

Result	Symbol	What it does to the member	Shown as
Axial force	N	Stretches (tension, positive) or squashes (compression, negative).	Axial, kN
Shear force	Vy, Vz	Slides one part across the other.	Shear, kN (the larger of Vy and Vz, signed)
Bending moment, major axis	My	Bends the member in its strong direction — a floor beam sagging.	Moment, kN·m
Bending moment, minor axis	Mz	Bends it in its weak direction — wind on the side of a column.	Minor moment, kN·m
Torsion	T	Twists it.	Member forces table

#How the diagrams are drawn

For Axial, Shear, Moment and Minor moment, Struxt3D draws a filled diagram off each member and colours the members by the value. Moments are drawn on the tension side: a sagging beam has its moment diagram hanging below it, where the bottom fibres stretch; over a support of a continuous beam, the diagram flips above the beam, where the top is in tension. This is the convention used in most of Europe and Asia, and it tells you directly where the reinforcement in a concrete beam goes.

Each member gets labels at its peaks. On large models (more than 60 members), only the twelve largest peaks are labelled to keep the view readable — hover or click a member to see its own values.

#Relations worth remembering

• Shear is the slope of the moment: where the shear crosses zero, the moment is at a peak.
• Under a uniform load, shear is a straight line and moment a parabola; under point loads, shear is stepped and moment is made of straight lines.
• At a pinned end or a free end, the moment is zero.
• In a column of a frame, axial force grows storey by storey towards the base.

#Worked check: is the moment right?

A floor beam spans 7.5 m between columns, at 6 m spacing, on an office floor (Eurocode). The envelope moment diagram in a 3D frame shows −142 kN·m at the supports and 98 kN·m at midspan. Is that plausible?

1. Load per metre: ULS floor load 13.1 kN/m² (see loads and combinations) × 6 m = 78.6 kN/m — if this beam carries a 6 m strip.
2. The "free" moment — the moment as if it were simply supported — is wL²/8 = 78.6 × 7.5² / 8 ≈ 553 kN·m.
3. The sum of the average support moment and the midspan moment must equal the free moment: 142 + 98 = 240 kN·m. That is far less than 553.

So either the beam carries a much narrower strip (secondary beams take part of the floor) or load is missing. In a frame with secondary beams at 2 m centres, a main beam picks up point loads instead, and the check is made with those. This free-moment check — support average plus span moment equals wL²/8 — works for any span of any continuous beam, and is the fastest way to confirm a diagram.

#Load cases, combinations and the envelope

The Load case list in the results lets you choose what you look at:

• Load cases (LC1, LC2…) — each load on its own: Dead, Live, Wind +X… Use these to understand the structure.
• Combinations (C1, C2…) — the factored sums from your code.
• Envelope (ENV) — the largest and smallest values over all strength combinations. Diagrams show two curves, the max and the min. Use this for design.
• Vibration modes (M1, M2…), when earthquake is on — each mode's shape, period and mass participation.

#Reactions

Support reactions (Fx, Fy, Fz, Mx, My, Mz at each supported node) are the best global check. For a gravity case, the sum of the vertical reactions equals the total weight of the load case. For a wind case, the horizontal reactions add up to the total wind force. If they don't, load is going somewhere it shouldn't — or not arriving at all.

#Slab results

For slabs and walls, Slab moment shows the design moment per metre width (kN·m/m) — at each point, the larger of |M_x| + |M_xy| and |M_y| + |M_xy|, which includes the twisting moment as reinforcement design does. After design, Slab reinforcement shows the steel area needed (mm²/m). See slabs and walls as plates.

#Utilization: the design at a glance

After you run the design, Utilization colours every member, slab and wall by the ratio of demand to capacity from the governing code check. Below 1.00 passes; anything over 1.00 is shown in red. Members are coloured segment by segment along their length, so you can see where along a beam the check governs. A well-sized structure is mostly in the 0.6–0.9 range.

#Try it in Struxt3D

1. Open the Multi-storey frame template and press Run analysis (F5) from the loads step.
2. Under Show, choose Displacement and press Animate at the left of the colour legend.
3. Choose Moment. In Load case, pick Dead, then a wind case, then Envelope, and watch how the diagrams change.
4. Click a beam to see its forces in the panel. Do the free-moment check on one span.
5. In the Full interface, open Results › Reactions and add up Fz for the Dead case.
6. Run the design (Run design, F6) and choose Utilization.

#Summary

• Look at the animated deflected shape first.
• Moments are drawn on the tension side; shear crossing zero marks a moment peak.
• Check diagrams with the free moment wL²/8 and totals with the reactions.
• Design from the envelope; read the design from the utilization colours.