Should I model my structure as a beam, a truss or a frame?

Ask two questions: do the joints carry moment, and does load land between them. A single member spanning between supports and loaded across it is a beam. A triangulated framework with pinned joints, loaded only at those joints, is a truss. Members that bend and also share moment at their connections — a portal frame, a multi-story rigid frame, anything that sways — need a frame model. A truss loaded between its joints is no longer a truss, and a beam whose supports resist rotation is really part of a frame.

What is the difference between structural analysis and structural design?

Analysis computes what forces and displacements a given structure develops under given loads: reactions, shear, bending moment, axial force, deflection. It is arithmetic on a model and the answer is the same for everyone. Design then asks whether those forces are acceptable for the chosen material, section and connection under a specific code, which is where judgement and safety factors live. Every StructureCalcs tool does analysis; none performs a code capacity check.

How do I check that an analysis result is right?

Start with equilibrium. Add up the vertical reactions the tool reports and compare them against the total load you applied; they must match. It takes seconds, needs no theory, and catches the two most common modeling mistakes there are — a load entered with the wrong sign, and a load entered twice. After that, check that the moment is zero at any pin or roller, and that the shear crosses zero where the moment peaks.

What is the difference between ULS and SLS?

They ask different questions. The ultimate limit state asks whether the structure will fail: strength, stability, buckling, overturning, checked under factored loads. The serviceability limit state asks whether it will be usable: deflection, vibration, cracking, checked under unfactored loads. A member can pass one and fail the other, and on long spans serviceability usually governs.

Structural analysis basics: loads, load paths, and choosing the right model

The orientation page. What analysis is actually doing, how a load gets from where it lands to the ground, and — the question the other guides cannot answer because it spans them — which of the five tools to reach for.

What analysis is, and what it is not

Structural analysis answers one question: given this structure and these loads, what force does every part carry, and how far does everything move? It is arithmetic on a model — objective, checkable, and the same for everyone.

Structural design is what follows: is that force acceptable, for this material, this section, this connection, under this code? Design is where judgement, standards and safety factors live. Every tool on this site does analysis. None of them does a code check, and none of them pretends to — what they give you is the honest input a check needs.

The gap between the two is where most real mistakes happen. An analysis can be perfectly correct and still useless, because the model was the wrong shape for the structure. Which is what the rest of this page is about.

Loads: what you are analyzing for

Dead (G) — permanent and known: self-weight, screeds, finishes, services. It never leaves, so it is the one load you can be confident about.
Live / imposed (Q) — occupancy: people, furniture, stock, vehicles. Codes tabulate it by use, and because it moves, finding its worst position is part of the problem.
Wind (W) — pressure and suction, both. Suction lifts roofs, and a light roof under uplift can put its members into the reverse of their gravity force.
Snow (S) and rain (R) — including drift against parapets, and ponding, where deflection lets water collect and the extra water causes more deflection.
Earthquake (E) — inertia from ground motion, driven by mass and by how stiff the structure is.
Thermal (T) and imposed deformation — restrained expansion, support settlement, a member fabricated to the wrong length. These produce force with no applied load at all, and only in structures that are indeterminate.

The calculators tag each load with its case and will build combinations and an envelope for you, so you get the worst effect at every position rather than the worst of any single case. The factors are yours to set: they belong to your code, not to a website.

Equilibrium, and the check that costs nothing

A structure that is not moving is in equilibrium: forces balance in every direction, and moments balance about every point. In a plane that is three equations — ΣFx = 0, ΣFy = 0, ΣM = 0 — and everything else in analysis is built on top of them.

When the unknown reactions number three or fewer, those equations are enough and the structure is determinate. More than three and it is indeterminate: the extra reactions depend on relative stiffness, which is why every tool here runs the direct stiffness method rather than equilibrium alone.

Whatever the method, check the total. Add up the vertical reactions the tool reports and compare against the load you applied. It takes five seconds, needs no theory, and catches the two most common modeling errors there are — a load entered with the wrong sign, and a load entered twice.

Load paths — and how much the detail changes them

Every load takes a path to the ground: slab → secondary beam → primary beam → column → footing → soil. Tracing it is the first thing to do with any structure, because each step in that chain is one analysis model, and the reaction from one step is the load on the next.

The path is not settled by the shape alone. Here is one portal frame — 6 m span, 4 m columns, 10 kN/m on the rafter — with two different base details:

Figure 1.Pinned bases. Total vertical reaction 60.0 kN, matching the 60 kN applied, and a horizontal thrust of 5.19 kN at each foot. The bases carry no moment at all — by definition — so the whole of the frame’s bending is resisted in the steelwork: 20.8 kN·m hogging at the eaves, and 24.2 kN·m of sagging at mid-rafter, which is the moment this frame is sized for.

Figure 2.The identical frame with FIXED bases. The same 60.0 kN goes down, the thrust rises to 8.40 kN, and each base now carries 11.1 kN·m of moment as well. The eaves hogging RISES to 22.5 kN·m — fixity pulls moment toward the ends — and it is now the governing moment in the frame, the mid-span sagging having dropped below it. Net effect on the peak the steel is sized for: 24.2 → 22.5 kN·m.

Nothing about the geometry or the load changed, and yet almost every number did. Fixing the feet redistributes rather than simply reduces: the eaves work harder, the mid-span works less, the governing moment drops a little, the thrust grows by half again, and 11.1 kN·m appears in a footing that previously had none. Slightly lighter steel, a noticeably more expensive foundation, and a frame that sways far less under wind.

That is a real engineering trade, and it is completely invisible unless the frame is modeled as a frame. It is also a reminder that “stiffer” is not a synonym for “better” — it moves the problem, and you have to look where it went.

Which is also the argument for the Frame Calculator over analyzing that rafter as an isolated beam: an isolated beam cannot know its supports resist rotation, so it can find neither the base moment nor the horizontal thrust that comes with it.

Which model? A decision, not a preference

The most consequential choice in an analysis is which idealization to use. The rule of thumb: ask whether the joints carry moment, and whether load lands between them.

Tool	Reach for it when	It gives you
Beam	One member spanning between supports, loaded across it. Floor beams, lintels, purlins, girders, cantilevers, continuous runs over several supports.	Reactions, and shear, bending moment, axial force and deflection along the span.
Truss	A triangulated framework, pin-jointed, loaded only at the joints. Roof trusses, bridge trusses, transmission towers.	Axial force in every member, tension or compression, plus joint displacements.
Frame	Members that bend AND share moment at their joints. Portal frames, multi-story rigid frames, anything that sways.	Reactions, and axial, shear and moment diagrams on each member of the frame.
Section	The cross-section is not a catalog section. Plate girders, cover-plated beams, composite decks, rails, sections with holes.	A, I, Z, S, r, the principal axes and the plastic neutral axis.
Report Builder	The analysis has to leave your screen — a submission, a check set, something a reviewer signs.	A Word-class document with the live diagrams and result tables inserted from the calculators.

Two boundary cases are worth naming. A truss loaded between its joints is not a truss any more — the chord bends, and that needs a frame model or a separate local beam check. And a beam whose supports resist rotation is really part of a frame: modeling it alone is safe for the beam, because the moments it sees come out larger, but it silently hides the moment the columns are taking.

How a job flows through the five tools

Trace the load path and decide where each analysis starts and stops.
Analyze each element — beam, truss or frame. The reactions from one become the loads on the next.
Get the section properties — pick a catalog section from the built-in library, or draw the real one in the Section Calculator and feed its I straight back into the analysis.
Check strength and serviceability against your code, using the peak actions and deflections the analysis reports.
Iterate. Changing the section changes the self-weight and therefore the answer; the calculators re-solve as you type, so this is cheap.
Write it up — insert the diagrams and result tables into the Report Builder, so the figures in the report are the ones the solver produced rather than screenshots that will go stale.

There is also an API, if that loop is something you would rather script than click: same engine, same answers, over HTTP.

Safety factors, briefly

Modern codes use limit state design: loads are factored up, material strengths factored down, and the check is that factored capacity exceeds factored action. The two families of limit state ask different questions —

Ultimate (ULS) — will it fail? Strength, stability, buckling, overturning.
Serviceability (SLS) — will it be usable? Deflection, vibration, cracking. Checked under unfactored loads.

The factors themselves belong to your governing code — AS/NZS, AISC 360, the Eurocodes, IS 800 — and they differ between them. The analysis output here is unfactored and honest; applying the right factors is the engineer’s call, which is where it belongs.

Where to go next

How to analyze a beam — supports, loads and the four diagrams, all drawn.
How to analyze a truss — joints, sections, and what the stiffness method adds.
Understanding steel sections — what I, Z, S and r are actually for.
The 35 worked examples — beam, truss, frame and cross-section, each solved by hand and checked against the solver.

Frequently asked questions

Should I model my structure as a beam, a truss or a frame?: Ask two questions: do the joints carry moment, and does load land between them. A single member spanning between supports and loaded across it is a beam. A triangulated framework with pinned joints, loaded only at those joints, is a truss. Members that bend and also share moment at their connections — a portal frame, a multi-story rigid frame, anything that sways — need a frame model. A truss loaded between its joints is no longer a truss, and a beam whose supports resist rotation is really part of a frame.
What is the difference between structural analysis and structural design?: Analysis computes what forces and displacements a given structure develops under given loads: reactions, shear, bending moment, axial force, deflection. It is arithmetic on a model and the answer is the same for everyone. Design then asks whether those forces are acceptable for the chosen material, section and connection under a specific code, which is where judgement and safety factors live. Every StructureCalcs tool does analysis; none performs a code capacity check.
What is a load path?: The route a load takes to reach the ground: slab to secondary beam to primary beam to column to footing to soil. Tracing it is the first thing to do with any structure, because each step is one analysis model and the reaction from one step becomes the load on the next. The path is not fixed by shape alone — a detail as small as whether a column base is pinned or fixed moves bending between the steelwork and the foundation.
How do I check that an analysis result is right?: Start with equilibrium. Add up the vertical reactions the tool reports and compare them against the total load you applied; they must match. It takes seconds, needs no theory, and catches the two most common modeling mistakes there are — a load entered with the wrong sign, and a load entered twice. After that, check that the moment is zero at any pin or roller, and that the shear crosses zero where the moment peaks.
What is the difference between ULS and SLS?: They ask different questions. The ultimate limit state asks whether the structure will fail: strength, stability, buckling, overturning, checked under factored loads. The serviceability limit state asks whether it will be usable: deflection, vibration, cracking, checked under unfactored loads. A member can pass one and fail the other, and on long spans serviceability usually governs.