STAAD Pro Solar Structure Design: Loads, Load Combinations and Member Checks

A solar structure’s load combinations should be built from the governing standard and the project specification, not copied from a previous model. The process is to list every credible load case, define factors for strength and serviceability checks, take care with uplift where dead load helps resist wind, check sign conventions, and then review utilization ratios, deflection and reactions against the governing combinations.

Step 1: List the load cases

Start with a load case register. For most ground-mount solar structures the cases are:

•             Self-weight of steel members, applied automatically by the software.

•             Module and rail loads, including clamps, rails and wiring trays, applied to the supporting members.

•             Equipment loads, where inverters, combiner boxes or cable trays are attached to the structure.

•             Wind pressure and uplift, for each wind direction and each pressure case.

•             Maintenance or live load, where the code or specification requires a load for access during cleaning or inspection.

•             Seismic load, where the project location and code require it.

•             Temperature effects, where long rails or restrained members could develop thermal forces.

•             Construction and temporary conditions, such as wind on a partly erected table or loads from lifting.

•             Snow or other climatic loads, where relevant to the site.

Give each case a clear name and record its source, so a reviewer can trace the numbers.

Step 2: Understand which cases are likely to govern

In most cases wind governs because the structure is light and exposed. Seismic effects are often smaller because the mass is low, but seismic design rules depend on the location and code. Do not skip the check because “wind governs”. Document the check or the reasoning, and follow the specification. Maintenance load matters for members like purlins that could be walked on, but only where the specification says so.

Step 3: Ultimate and serviceability combinations

Strength (ultimate limit state) combinations use factored loads to check member capacity and stability. Serviceability combinations use unfactored or lightly factored loads to check deflection and, where relevant, vibration and module integrity. They are different sets, and a model is not complete unless both are defined.

The governing standard for steel in India is typically IS 800 (limit state method), with load definition from the IS 875 series. Factors, combinations and allowed deflections come from those standards and the project specification. This article does not quote factors because they depend on the code edition and project.

Step 4: Pay attention to uplift

Uplift combinations are the most commonly mishandled. Because dead load helps resist wind uplift, standards typically reduce the dead load factor when it acts favourably. Check:

•             That uplift wind is combined with the reduced dead load as required by the standard, not with full dead load.

•             That module and rail weights are included in the permanent load only to the extent they are reliably present, such as before modules are installed or after they are removed.

•             That foundation uplift and anchor design use the correct combinations.

Step 5: Sign conventions

STAAD uses a global axis system where, by default, the vertical axis is Y. A wind uplift load must have the correct sign with respect to global axes and the member’s local axes. Mistakes happen when:

•             Pressure is applied along local member axes instead of global axes.

•             Wind direction is reversed for back wind.

•             Moments are read with the opposite sign to the reaction table.

After running the analysis, plot reactions and deformed shapes for individual cases and check that a downward load causes downward displacement and wind uplift causes upward displacement.

Step 6: Build and organise combinations

A clean combination set follows naming that tells the reader what it contains. For example: strength with wind from the front, strength with wind from the back, uplift with wind from the front, serviceability with wind, strength with seismic, and so on. Avoid auto-generated combinations that nobody has reviewed. Keep the generated list and a combination matrix (which case, with what factor, in which combination) in the calculation report.

Do not copy combinations from another project

Combinations depend on the code and edition, the type of structure, the location and the specification. A set that was valid for another project may use different factors, omit seismic or temperature cases, or apply a different treatment of uplift. Reviewing a past project’s set as a starting point is fine. Adopting it without checking the standard and specification is a risk.

Step 7: Review member utilization

Utilisation (or capacity) ratio shows how much of a member’s capacity is used under the governing combination. Review it carefully:

•             Look at which combination governs each member, not just the highest ratio.

•             Check that no member is over-stressed in a case you did not expect (for instance, a purlin governed by uplift in a bay at the table edge).

•             Treat ratios close to the limit with attention to section properties, effective lengths and design parameters, since small input changes can flip the result.

•             Investigate members with very low ratios, because they may be unloaded by a modelling error rather than being efficient.

Step 8: Review deflection and reactions

Check deflection against the project limits, which should reflect module manufacturer requirements as well as the structural code. Extract support reactions by combination, and check that the governing compression, uplift, lateral and moment values are passed to the foundation designer with their combinations and sign convention.

Step 9: Check the model against a hand calculation

Use a simple tributary calculation on a typical purlin or rafter and compare bending moment and reaction values. The two will not match exactly because the software includes continuity and frame action, but the order of magnitude should agree. If it does not, find the cause before issuing results.

Verification checklist

•             ☐ Governing code, edition and project specification named in the report.

•             ☐ Load case register with sources for each load.

•             ☐ Ultimate and serviceability combinations defined and reviewed.

•             ☐ Uplift combinations use reduced dead load where required.

•             ☐ Wind applied for all required directions, with correct signs.

•             ☐ Seismic and temperature cases checked or excluded with documented reasoning.

•             ☐ Reaction balance checked for key combinations.

•             ☐ Deformed shape checked for each major case.

•             ☐ Governing combination listed for each critical member.

•             ☐ Deflection checked against project limits.

•             ☐ Reaction table issued with combination, sign convention and factored or unfactored status.

Common mistakes

•             Reusing combinations without checking the standard.

•             Forgetting the uplift case with reduced dead load.

•             Using the same factors for strength and serviceability.

•             Leaving auto-generated combinations in the model without review.

•             Reporting only utilisation without the governing combination.

When to involve a specialist

Bring in a structural engineer when the site has unusual wind exposure, when tracker systems or complex geometry are involved, or when there is no clear code basis. Solyug Energy offers STAAD Pro wind analysis for solar structures and general solar design consultancy. More guides are in the resource center.

Standards and references

•             IS 875 series (India): design loads for buildings and structures, including wind loads (Part 3) and special loads and load combinations (Part 5).

•             IS 800 (India): general construction in steel, limit state design.

•             IS 1893 (India): earthquake-resistant design, where applicable.

•             Project specification and module manufacturer mounting guidelines.

Frequently Asked Questions

What load combinations are used for solar structures?

They are built from the governing code and the project specification, covering strength and serviceability. There is no single universal set, so do not take one from another project.

Does seismic load matter for solar structures?

It depends on location and code. The structure is light, so wind often governs, but the seismic check should still be done or its omission justified.

What is an uplift combination?

A combination where wind uplift acts with reduced permanent load, because dead load helps resist uplift. It is critical for connections and foundations.

What does utilisation ratio mean?

It is the ratio of design action to design capacity for a member under a given combination. A value above 1.0 means the member fails the check.

How do I check STAAD Pro results are reliable?

Check load totals and reaction balance, review deformed shapes, compare with hand calculations and examine governing combinations.

What deflection limit should I use?

Use the limit specified by the project and module manufacturer, which may be stricter than the structural code limit.