IntermediateFramePortalIndeterminateBase fixity

Fixed-base portal frame — what base fixity buys and costs

The same portal as the pinned-base example, with two characters changed. Everything that differs in the answer is caused by base fixity and nothing else — so this is what fixity actually buys, and what it costs.

Figure 1.The 6 m × 4 m portal with fixed bases, carrying 10 kN/m on the rafter.

Given

Span: 6 m
Height: 4 m
: 10 kN/mgravity UDL on the rafter
: 200 GPa
: 100 × 10⁶ mm⁴
Bases: FIXEDthree times indeterminate

Part 1 — Build it, then change one thing

Identical to the pinned portal except for two characters.

Build the same portal as before — four nodes, three members, a 10 kN/m rafter load:

N 0,0 N 0,4 N 6,4 N 6,0 M 1 2 M 2 3 M 4 3 L 2 -10

Now the only difference. Instead of S 1 P, make the bases fixed:

S 1 F S 4 F

Two characters. Everything below follows from them — and because nothing else changed, every difference you are about to see is caused by base fixity alone.

Figure 2.The same 6 m × 4 m portal, now with fixed bases — note the different support symbol.

Part 2, step 1 — The footing now carries moment

The thing a pinned base is defined by not having.

A fixed base holds the column’s rotation, and holding a rotation means resisting a moment. The solver reports a base moment of 11.13 kN·m, where the pinned frame had 0 — zero, by definition.

Vertical equilibrium is untouched, because statics does not care about stiffness:

So fixing the bases did not change what the frame weighs — it changed where the frame carries its bending. Moment has moved out of the rafter and down into the columns and their footings.

Figure 3.The solved fixed-base portal — the same 60 kN of vertical load, now with a moment at each base.

Part 2, step 2 — What you bought: stiffness

The reason anyone pays for a fixed base.

Under this symmetric gravity load neither frame sways much, but the deflections still tell the story — the fixed frame moves 0.02519 mm horizontally at the eaves against 0.01556 mm for the pinned one.

The difference becomes decisive under lateral load, where sway governs the design outright — that is the subject of the wind example. Fixing the bases is the most direct way to stiffen a portal without changing a single section size.

Figure 4.The deflected shape. The columns now curve into their bases rather than rotating freely at them — that curvature IS the base fixity.

✓

Fixed vs pinned, side by side

Same frame, same load, one detail changed.

Verified — hand calculation vs the solver, to round-off

Quantity	By hand	StructureCalcs
Total vertical reaction	60 kN (statics)	60 kN
Base moment — FIXED	indeterminate	11.13 kN·m
Base moment — pinned twin	0 by definition	0 kN·m
Peak member moment — FIXED	indeterminate	22.45 kN·m
Peak member moment — pinned twin	indeterminate	20.75 kN·m

Every value was worked by hand with the classical method, then checked against this site’s solver — the same engine the Try it button opens. This agreement is re-run automatically on every build.

Now make it yours

Open this exact model in the calculator — then change a load, drag a support, and watch every diagram update in real time. The best way to build intuition is to break it and see what happens.

Take it with you

Export this worked example as a PDF, or download it as a .screport and open it in the Report Builder — the model travels inside the file, so you can reconstruct it, re-solve, and build your own report from it.

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