Charging Canopy Structure under Wind and Seismic Loads

The problem

Charging hubs on highways and in cities use large canopies with lighting, signs and sometimes PV panels. These structures face strong wind, including cyclones on coastal routes, and earthquakes in many zones. Failures have occurred on canopies in high winds. A thorough wind and seismic study shows how loads are derived, combined and checked, and how far a CAD-integrated tool can support it.

Objectives

• Model an example canopy for four charging bays as a beam structure with realistic member sections.
• Derive wind loads for downward pressure and uplift and seismic loads by an equivalent static method.
• Run static, frequency and buckling studies and check members and connections.
• Compare simulation with hand calculations and discuss limitations.
• Optimise member sizes for mass while meeting all checks.

What you need

Key concepts

Design wind speed

A basic wind speed for the site, modified by factors for terrain, height, topography and importance, as set out in IS 875 Part 3. Values must be taken from the current standard.

Equivalent static seismic method

A method in IS 1893 that converts earthquake effects into a horizontal base shear based on zone, importance, response reduction and the structure's period.

Load combinations

Factored sums of dead, live, wind and seismic loads specified by codes. Wind and seismic are not usually combined at full value together.

Linear buckling load factor

The factor by which the applied loads would have to be multiplied to reach elastic buckling. Imperfections reduce real capacity, so a margin is needed.

Step-by-step guide

1. 1

   Define the canopy and site

   Define an example canopy covering four charging bays, about 12 × 8 m, with a clear height of about 5 m, a flat or slightly sloped roof, lighting and a sign fascia. Choose two example sites, one coastal and one inland in a higher seismic zone, to compare governing loads. Record what the structure carries: roof sheeting, optional PV, fascia and lights.

   Checkpoint: A canopy layout and two site definitions with their assumed zones and terrain.

2. 2

   Build the beam model

   Model columns, primary beams, secondary beams, purlins and bracing with Structural Members using standard hot-rolled or hollow sections. Keep connections at real node positions. Add an alternative bracing layout as a configuration. Check the cut list and mass. Prepare the model for a beam study by checking joint groups and member end conditions. Add the fascia and sign frame as members too, since they collect a large share of the wind load and can govern column design.

   Checkpoint: A weldment that meshes as beams with correct joints and a cut list mass within an expected range.

3. 3

   Calculate wind loads

   Using IS 875 Part 3, compute the design wind speed and pressure for each site, stating every factor and that values are from your reading of the current standard. Determine net pressure coefficients for a free-standing canopy for downward and uplift cases and for the fascia. Convert to line loads on purlins and point loads on the fascia supports. Consider wind from at least two directions.

   Checkpoint: A wind load table per site and direction, with all factors and coefficients documented.

4. 4

   Calculate seismic loads

   Using IS 1893, compute seismic weight from dead load and a portion of live load, estimate the natural period by the code formula and later by your frequency study, and calculate the design base shear with zone, importance and response reduction factors for the site. Distribute the base shear to the roof level as horizontal loads in both directions. Carry out the calculation for both example sites and note which site has the higher base shear and why.

   Checkpoint: A seismic load calculation with base shear for each direction and the period assumption stated.

5. 5

   Run frequency and update period

   Run a frequency study with mass from dead load as distributed or remote mass. Compare the first lateral period with the code estimate. If it differs significantly, discuss which to use, as the code may limit how far computed periods can be used. Look at mode shapes to see if the fascia or a single column dominates. Also check the frequency of the fascia and light poles separately, since slender attachments can have low local modes.

   Checkpoint: First lateral modes and periods reported and compared with the code formula.

6. 6

   Run static load cases

   Set up static studies for dead, live, wind (pressure and uplift, each direction) and seismic (each direction). Apply loads as distributed beam loads and point loads. Fix or pin the column bases. Check reactions balance applied loads in each case. Then form code load combinations with factors, either as separate factored studies or with the Load Case Manager if your licence has it.

   Checkpoint: Each load case balances, and combination results are tabulated for all members.

7. 7

   Check members and buckling

   For each combination, check axial plus bending stresses against allowable or design values, and deflections against serviceability limits. Run linear buckling on the governing combinations and record load factors. Remember that linear buckling ignores imperfections, so compare with a hand column check using an effective length and a code buckling curve. Prepare a utilisation table for every member group in the governing combination so the most and least stressed members stand out.

   Checkpoint: All members pass with stated margins, and buckling factors exceed the target with the hand check agreeing in trend.

8. 8

   Compare with hand methods

   Hand-check a typical frame as a 2D portal under wind and seismic loads using simple frame formulas, and compare column moments, beam moments and drift with the simulation. Explain differences due to bracing, 3D load sharing or fixity. Optionally, run a response spectrum study in Simulation Premium and compare its base shear with the equivalent static result. Present the comparison as a short table with percentage differences, and explain each difference of more than about 15 per cent.

   Checkpoint: Hand and simulation values within a reasonable band, with differences explained.

9. 9

   Check connections and bases

   From beam end forces, design a bolted moment connection and a column base plate with anchor bolts for the governing uplift and moment. Model each as a solid assembly with bolt connectors and contact, apply the end forces and check plate stresses and bolt forces. Note that anchor and foundation design needs a structural engineer. Use mesh control at bolt holes and plate edges, and check that peak plate stress converges as the mesh is refined.

   Checkpoint: Connection and base plate studies show factor of safety above the target and bolt forces within rated values.

10. 10

    Optimise and report

    Reduce member sizes where utilisation is low and strengthen critical members, re-running the governing combinations. Compare steel mass of the original and optimised designs, and of the two bracing layouts. Write a report with layouts, load derivations, results, hand checks, connection studies, optimisation and limitations, stating that final design must be checked and approved by a qualified structural engineer. Keep standard section sizes only, so the optimised design can be bought and fabricated without special profiles.

    Checkpoint: An optimised design meeting all checks with reduced mass, and a report that clearly states its limits.

Testing & validation

• Reactions balance applied loads in every case.
• Frequency results are compared with the code period estimate.
• Hand portal frame checks agree in trend with simulation.
• Buckling factors exceed targets with a hand column check.
• Connections are checked with local solid models.

Expected results

A canopy structural study for two example sites with documented wind and seismic loads, static, frequency and buckling results, hand checks, connection designs and an optimised, lighter structure.

Take it further

• Add PV panels and compare wind uplift with and without them.
• Run a nonlinear buckling study with initial imperfections in Simulation Premium.
• Study fatigue of the sign fascia supports under fluctuating wind.

Viva and review questions

Learn it at DIYguru

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