PEB frame design in STAAD.Pro CONNECT Edition (STAAD 2023, 2024 and 2025)

A note on STAAD.Pro versions

STAAD.Pro has been part of Bentley’s CONNECT Edition for several years now. The earlier releases carried version numbers such as V22 with numbered updates; more recent releases are named by year: STAAD.Pro 2023, STAAD.Pro 2024 and STAAD.Pro 2025. The modelling commands we use for PEB frames have been stable across all of them, so the workflow below applies whichever one your office runs. Where something does change between releases, it is usually in the steel design codes available or in the user interface, so keep an eye on the release notes for your version.

The PEB Optimization Tool works with STAAD.Pro CONNECT Edition from V22 Update 10 onwards, including the 2023, 2024 and 2025 releases.

Step 1: geometry and supports

For a single clear-span gable frame (what most of us call an A-type frame), the geometry is simple: two column base nodes, two knee nodes, a ridge node, and intermediate nodes on the rafters wherever you want a splice, a change of section or a load point.

• Split rafters at splices. Each segment between splices becomes its own member, so it can have its own tapered section.
• Add nodes at purlin or flange-brace positions if you want the analysis to report forces there; it also makes unbraced lengths easier to reason about.
• Supports: pinned bases are common for PEB frames without cranes, because they keep foundations small. Fixed bases help with crane buildings and drift-sensitive frames, at the cost of base moments.

For a 2D frame study, model one typical interior frame. For a 3D model, generate the frames along the building length and add the bracing, which matters for the end frames and for longitudinal stability.

Step 2: the TAPERED member property

STAAD.Pro defines a tapered I-section with seven values. In the input file it looks like this:

MEMBER PROPERTY
1 TAPERED 0.30 0.006 0.80 0.20 0.010 0.20 0.010
2 TAPERED 0.80 0.006 0.45 0.20 0.010 0.20 0.010

The seven numbers, in order, are:

1. depth of the section at the start node;
2. thickness of the web;
3. depth of the section at the end node;
4. width of the top flange;
5. thickness of the top flange;
6. width of the bottom flange (defaults to the top flange if left out);
7. thickness of the bottom flange (defaults to the top flange if left out).

The values above are in metres and purely illustrative. Two things catch people out the first time:

• Start and end follow the member incidence. If a column was drawn top-down, its “start” is the knee. Check incidences before assigning depths, or your taper ends up upside down.
• Top and bottom follow the member’s local axes, not “inner” and “outer” of the frame. On a column, “top flange” may be the outer flange or the inner one depending on orientation and beta angle. Display the local axes and confirm.

Step 3: loads

A typical PEB frame carries these primary load cases:

• Dead load: self-weight of the frame (use the self-weight command so it updates as sections change), plus sheeting, purlins and girts as member loads on the rafters and columns.
• Collateral load: services, lights, sprinklers and false ceilings hung from the roof, applied as a separate case so it can be combined sensibly.
• Roof live load: as specified by IS 875 Part 2 or ASCE 7, usually reduced for roof slope and tributary area where the code allows.
• Wind: several cases for wind across and along the building, with internal pressure and suction, from IS 875 Part 3 or ASCE 7. For PEB roofs this is often the case that governs reversal.
• Crane loads where there are cranes: vertical wheel loads at the bracket, surge and the moment from bracket eccentricity, with their own combinations.
• Seismic where required, typically by IS 1893 or ASCE 7.

Step 4: load combinations

The combinations depend on the code: IS 800:2007 Table 4 for limit state design in India, or ASCE 7 combinations for AISC 360 in LRFD or ASD. STAAD.Pro can generate code-based combinations automatically, which saves typing, but always read through what it produced: PEB frames have several wind cases, and the uplift combinations (0.9 DL with wind for IS 800, 0.9D + 1.0W or 0.6D + 0.6W for AISC) are the ones most often missing or mis-signed. Our article on LRFD vs ASD walks through why the factors differ.

Keep serviceability combinations separate from strength combinations. You will want them for the deflection and drift checks at the end.

Step 5: analysis and code check

Run the analysis, then set up the steel design parameters for your code:

• the design code itself (IS 800:2007 limit state, or AISC 360 in LRFD or ASD);
• yield strength of the plates;
• effective lengths for buckling about each axis, and the unbraced lengths of the top and bottom flanges for lateral-torsional buckling, which come from your purlin, girt and flange-brace spacing;
• deflection and ratio limits if you want STAAD to report them.

For AISC 360 frames, also think about the stability analysis Chapter C requires (the direct analysis method or one of its alternatives), which affects whether you run a second-order (P-Delta) analysis and with what stiffness reductions.

Then run the code check. STAAD reports the governing ratio, the clause and the load combination for each member. This is the point where the familiar loop begins.

Where the time really goes: the iteration loop

Members that fail get bigger; members at a ratio of 0.5 get smaller. Change the plates in the TAPERED lines, re-analyse (because the forces change with stiffness), re-check, repeat. On a typical frame this takes five to ten rounds, and every round means waiting for the analysis and reading the output again.

STAAD.Pro’s own member selection is built around sections from its steel tables. It does not search the combination of flange width, flange thickness, web depth at each end and web thickness that defines a tapered built-up I-section. So for tapered members, most offices iterate by hand and stop at the first set of sections where everything passes.

That is fine for safety, and it is how we all learned. It is just slow, and it leaves steel in the frame, as we explain in PEB optimisation explained.

Where optimisation fits in the workflow

An external optimiser replaces the loop in the middle, not the steps around it. With the PEB Optimization Tool the workflow becomes:

1. Steps 1 to 4 exactly as above: geometry, a starting tapered section, loads and combinations, design parameters.
2. The tool reads the model through OpenSTAAD: geometry, loads, combinations and the design settings, so nothing is re-entered.
3. It searches flange width, web depth and taper together for every column and rafter and checks each candidate against IS 800:2007 or AISC 360 (LRFD or ASD), utilisation and slenderness both. It needs an internet connection while you work, for your licence and run count; STAAD itself stays on your computer.
4. It keeps the lightest section that passes, not the first, and writes the result back into the STAAD model in one click.
5. You run your final analysis and design in STAAD as usual, check deflections and drift, and carry on with connections, base plates and foundations, which remain the engineer’s job.

The tutorials show this on real STAAD.Pro models, and the optimizer bench on the home page gives a feel for the search in your browser. When you are ready to try it on your own frame, the free trial gives you 100 optimize runs within 5 days, on models of any size.