Wind Simulation vs Eurocode Loads Comparison

[Anonymous]

I have recently carried out a basic validation exercise to compare the wind loads generated using wind simulation in Robot 2017 against those calculated manually from the Eurocode. I thought it worth sharing on this forum for the benefit of other users.

I undertook this exercise for a simple, steel-framed rectangular box building, 10m x 10m x 3m high with full cladding surround. I’m only really interested in the global structural design (ULS and SLS for beams and columns), not the more localised forces which should be considered for cladding design.

For simplicity, I only compared the loads in the windward/leeward direction and the vertical wind uplift.

The rough process for deriving wind loads to Eurocode 1 (+ UK National Annex) is as follows:

• Calculate the basic wind velocity (vb) using the wind map, and incorporating coefficients for direction, season, probability, and altitude.
• Calculate the basic velocity pressure (qb) by multiplying the basic wind velocity by 0.613 (0.5 x air density)
• Calculate the peak velocity pressure (qp(z)) by multiplying the basic velocity pressure to incorporate exposure, orography etc.
• Calculate the external and internal pressures on each face of the building by multiplying the peak velocity pressure (qp(z)) by external and internal pressure coefficients cpe and cpi for that face. I generally derive two sets of loads, one for positive internal pressure (cpi) and one for negative internal pressure (cpi)

For this building, my approximate loads calculated from Eurocode were as follows:

Total horizontal (windward + leeward): 10.2kN – 12kN

Total vertical (roof): 23.2kN (uplift) or 7.5kN (downward)

In Robot wind simulation there are two options for inputting the wind parameters: wind velocity (m/s) or wind pressure (kPa).

At first I ran the simulation with the basic wind velocity (vb) in m/s that I had calculated from Eurocode. The wind pressure maps were as expected but the resulting reactions underestimated the wind loads by a factor of approximately 2:

Total horizontal (windward + leeward): 7.6kN

Total vertical (roof): 11.6kN (uplift)

I suspect this underestimation is because, when the peak velocity pressure qp(z) is derived in the Eurocode, the pressure is multiplied by an exposure factor ce(z) which is always greater than 1.5 and typically around 1.8. Robot will not include this factor when simulating from the basic wind velocity vb.

I repeated the wind simulation by instead inputting the peak velocity pressure qp(z) in kN/m2 that I had calculated from Eurocode. These reactions were much closer to what I expected:

Total horizontal (windward + leeward): 11.2kN

Total vertical (roof): 19.3kN (uplift)

From this I have concluded that the Robot wind simulation seems reasonable for mimicking the Eurocode wind surface loads by inputting the peak velocity pressure qp(z), derived using hand calculations. This appears to be suitable for global member design, although I haven’t undertaken detailed checks for localised forces.

Some follow-up thoughts:

• Has anyone carried out a similar exercise. If so, did you find similar results?
• Are there any pitfalls to be aware of where the wind simulation may underestimate loads?
• For cladding panels, the wind simulation generates area loads (kN/m2) and line loads on the edges of the cladding (kN/m). If the steel members are included within the selection, the wind simulation generates line loads on the beams/columns, resulting in slightly higher overall reactions. Why is this? Is the simulation considering the geometry of those members e.g. flanges sticking out? Is it best practice to include members within the simulation, or just the panels?

[Artur.Kosakowski · Accepted solution]

Hi @Anonymous

Thank you for sharing this comparison.

[Artur.Kosakowski]

Hi @Anonymous

• For cladding panels, the wind simulation generates area loads (kN/m2) and line loads on the edges of the cladding (kN/m). If the steel members are included within the selection, the wind simulation generates line loads on the beams/columns, resulting in slightly higher overall reactions. Why is this? Is the simulation considering the geometry of those members e.g. flanges sticking out? Is it best practice to include members within the simulation, or just the panels?

According to my knowledge the wind generator sees the profiles as solid boxes of sizes corresponding to their outer corners so if the profile width is larger than the thickness of the panel the generated load would be larger too.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Hello!

I made also some simpel test to find out, how the wind load simulation wors in Autodesk Robot Structural Analysis Professional 2018. The model is a cube 20x20x20m. I used bars and claddings.

I changed various parameters and I got very different results. Very interesting is situation with loads deviation factor [dev].

I used dev=0,5%, 1% and 2%.

And comparable reactions are:

In the model, there are the others tests values also (with wind speed and profile, with basic wind pressure and wind profile).

Why the dev factor affects the results so much?

[Anonymous]

The model did not appear in the previous post

[Anonymous]

@Anonymous

i did hand calculation for a building and i m getting a difference of almost x2

my hand calculation answer

Windward pressure=1.04kpa

and software answer

windward pressure = 0.45kpa

Now what i didn't understand from the above is that how did you check validity of your answer by putting the wind pressue

what reactions did u see to compare

can i know in detail about it because i am new user and i am not that much fimilar with software 

Your help would be appreciated 

[Anonymous]

In Robot, are you applying the peak velocity pressure qp(z) in kN/m2, or a wind velocity in m/s?

The former works best when trying to apply the Eurocode as it will include the exposure factors etc.

When Robot undertakes the wind simulation, it effectively generates lots of forces and applies them as panel loads. To compare against your hand calculations, I suggest isolating some representative panels to see what panel forces have been generated.

[Anonymous]

@Anonymous

yes i have used wind velocity (m/s) as an input, and got the above results (in kpa) on the cladding. But i was wondering, which results (KN) are you taking from robot to compare with your answer?

I have attached a screenshot of my results and marked the top left part in black, which is pressure on cladding in kn/m2 

now my question is that which forces are u comparing to check your hand calculation?

[Anonymous]

i have same question ..

calculating by eurocode formuals, resuult is 2x greater than robot results.  why so?

i use wind speed 21ms/s

[Anonymous]

@Anonymous 

Can u please help me?

its important and urgent for me to know the results 

[Anonymous]

It's explained in my text above. If you input with velocity vb in Eurocode notation then you have not included for the exposure factor ce(z) and Robot will not include for this either.

I suggest that you calculate the peak velocity pressure qp(z) in kN/m2 and input this into Robot rather than a velocity.

[Anonymous]

@Anonymous

thank you for your reply

yes i got what You have explained above 

i am just confuse in the part

"total horizontal force= windward +leeward"

from where can i see this result  (total horizontal force) in robot structural analysis software?

[Anonymous]

By summing the horizontal reactions of the support nodes in the direction of the wind

[Anonymous]

@Anonymous

okayy

i have to check the support reaction on the column and add them   

Thank you man, appreciate your help👍🏻

[thomasYS8V3]

Hi Ben,

Ive been working on a project and finding similar results to yourself. One thing that confuses me though is that I had assumed that the 'wind profile' tab would account for the exposure factor (screenshot below). Have you any thoughts on this since 2017? 

Thanks

Tom