Windload EC in API

[KVdA]

As eurocode seem to define two windloadings in §5.1 of EN1991-1-4:
-Wind forces on structures, e.g. for overall wind effects
-Wind pressures, e.g. for cladding, fixings and structural parts

Does anybody has an idea of what windspeed/pressure is required to be used when checking an API tank using the windload of Eurocode?
Does it differ depending on the topic (buckling, anchorage etc)?
Is there any velocity in the EC which could be simmular as the 3second gust design windspeed refered to in API650.

 

[JStephen]

In API-650, in the "loads" section, they define a wind load of 18.6 psf adjusted for wind speed, and this is used for the overall wind effects.
In the section on shell wind stability (and Annex V), they define a wind load of 31 psf adjusted for wind speed, and I believe this is comparable to the components and cladding load, although not spelled out as such.
In those sections, there is some discussion and some options for using different wind speeds, wind pressures, etc.
Note that the main basis for wind loading is service level/allowable stress level wind loading with provision for converting from strength-level loads if needed.
The current API-650 includes wind uplift on roofs which is not covered in ASCE 7-05 and ASCE 7-10. Then ASCE 7-16 includes more involved provisions for wind uplift on roofs, for grouped tanks, etc., that are not covered in API-650. So you may have considerations other than just wind speed to deal with when applying some other code.

 

[KVdA]

I think i have an answer to my question although it gives me more issues:
- When forces are required such as uplift/overturning(API650 §5.11) and tank anchorage (§5.12) i should use the wind force on structure
- When pressure is required such as in wind girder design and external pressure (annex V) checks.

This seems logical the only problem is that i have no guidance on the total force on a domed or cone roof in the EC.
EC only handles domed roofs and not cone roofs.
And it is mentioned in EN1991-1-4 §7.2.8, which is wind pressure and not for the wind force.

So finally to conclude, may I assume all steps in the calculation require the pressure input and not the force input?
Or do i need the use the force on the shell from the EC where possible, and the guidance of API for the roof...???

I'm totally confused and i think i'm not the only one when reading the EC. Don't even get me started on the EN14015.

 

[HTURKAK]

When you check API tank with Eurocode wind loads , you should look to the EN 14015

The relevant excerpt from the same code ,

7.2.10 Wind
The 3 second wind gust velocity to be used in the design shall be not less than 45 m/s. Where wind
gust velocities are expected to exceed 45 m/s, national data shall be consulted to establish an
appropriate value which shall be subject to agreement

So you are required to look for local wind velocity if greater than 45 m/s..

Other codes you are expected to look,

- EN 1993-4-2 ( tanks )

- EN 1991-1-4 ( wind loads )

I would like to remind you wind pressure distribution around shell circumference ( Figure 7.27 of EN 1991-1-4 )

 

[KVdA]

Hturkak, thx for your reply but I did actually look at the EN14015.

There is no guidance what so ever in the EN14015 on what the windspeed should be. The only thing §7.2.10 does, is say that it shall not be less than 45m/s and that the 3second gust wind speed should be used.
Unfortunately there is no such thing in the national applicable document nor in the eurocodes as the 3second wind speed (unless i'm missing something here).
And this for a code which is specifically written to be used in the EU
Hence my earlier statement "don't get me started on the EN14015"...
Or am I missing something here?

Besides what is up with the note in §12 of EN14015
"NOTE When calculating the uplift due to wind, the load should be calculated based on a minimum wind
speed of 45 m/s and a shape factor for the tank shell of 0,7."
Again a new name for possibly something that is used in EC by a different name (probably kw in EN1993-4-2). Or is it the conversion between pressure and force (generally referred as shape factor in other codes).
But than it doesn't seem to comply with the values i obtain when calculating the force for that same velocity.
We can only guess ..... grr....

I have checked EN 1993-4-2 and EN1991-1-4 which lead me to conclude that for buckling the wind pressure should be used (based on the same figure you copied).
But what about anchorage etc...

 

[HTURKAK]

Dear KVdA ,

I will try to reply to your questions based on my EC knowledge and experience..


- The basic wind speed is a local data and you are expected to look Basic wind speed map applicable for your local zone.( I looked to your Internet country domain (BE) . If so, 45m/s basic wind speed should be pretty fine.


- Will you pls provide more info for the subject API tank ? where is it located ?
[quote="NOTE When calculating the uplift due to wind, the load should be calculated based on a minimum wind
speed of 45 m/s and a shape factor for the tank shell of 0,7."
Again a new name for possibly something that is used in EC by a different name (probably kw in EN1993-4-2). Or is it the conversion between pressure and force (generally referred as shape factor in other codes).
But than it doesn't seem to comply with the values i obtain when calculating the force for that same velocity.
We can only guess ..... grr....

I have checked EN 1993-4-2 and EN1991-1-4 which lead me to conclude that for buckling the wind pressure should be used (based on the same figure you copied).
But what about anchorage etc...]
[/quote]

If you like a short answer,

If this an API 650 tank, the design wind speed (V) shall be 190 km/hr ( 52.7 m/sec) which is greater than 45 m/s . Pls check if the local wind velocity more than 52.7 m/sec. If LT 52.7 m/sec , Adopt wind speed (V) 190 km/hr ( 52.7 m/sec) and follow the API calculation procedures.

If you want a long answer,
- Look for local basic wind velocity,
- follow the following chart ( copied from Concise Eurocodes: Loadings on Structures : Eurocode 1
Ian Burgess, Amy Green and Anthony Abu )

- Check the subject tank for EN 14015 , EN 1993-4-2
- Check the subject tank for API 650 with adopting minimum design wind speed (V) 190 km/hr .

Finally you will get a tank complying API 650 and EN 14015.

But you may post more info regarding ( tank, service, location...) to get better responds.

 

[KVdA]

Dear HTURKAK,

1)I don't think the basic windspeed is equal to the 3sec gust factor...
As per API 650 §5.2.1. k) 4)
"The fastest mile wind speed times 1.2 is approximately equal to 3-sec gust wind speed (V)."

If i look at the definition of the basic windspeed it is a windspeed averaged over 10minutes.
I believe that the "fastest mile wind speed" is the velocity which induces the peak velocity pressure.
If that interpretation is correct, the 45m/s in EN14015 is exceeded.
But than again i'm just guessing...

2)My question is not for one single tank. More than often API650 is used here instead of EN14015. And to be honest I understand why.

3)The flowchart seem's to make sense and corresponds with my understanding.

4)So if i understand your reply correct you suggest to use API650.

All good but that still doesn't actually answer my original question...
Where and which values (obtained via EC) do i plug in the code (API650 or EN14015).
Do i need:
-Wind forces on structures, e.g. for overall wind effects
-Wind pressures, e.g. for cladding, fixings and structural parts
Or do i actually only need the windpressure on shell and roof and windspeed and use the shape factor suggested by EN14015...

 

[HTURKAK]

Apparently you digged the relevant Euro codes.. The definition of the basic wind velocity is,

-The basic wind velocity, vb,0, is the characteristic 10 minutes mean wind velocity, irrespective of wind direction and time of year, at 10 m above ground level in open country terrain with low vegetation..

-Not sure how the figure 45 m/sec calculated and suggested at ( EN 14015 ) but this figure is for ( 3 second wind gust velocity min. is 45 m/s )

- I now basic wind speed map for England and the wind speed between 22 to 32 m/sec. Still suggesting you to look for local wind speed map.

- When you get basic wind speed map ( probably characteristic 10 minutes mean wind velocity would be LT 30m/sec for your region ) you can convert to 3-sec gust wind speed using DURST curve as suggested by ASCE 7

.

Just curious :why API650 is used here instead of EN14015 ?

I hope this respond answers to your questions..

GOOD LUCK..

 

[KVdA]

Thx for your reply HTURKAK

I'm still not convinced and the reason is also the answer to your question:

Because there is no reference what so ever to ASCE in the EN14015. As a result the designers are simply left to guessing on what is required here for input.
Unfortunately this not only the case for this specific topic.

The reason why I'm not convinced is the note in API650 I posted earlier
But ok...

I have (26 till 23m/s) => 31m/s average at the top of the tank => Extreme pulspressure at the top 1285Pa => Peak wind velocity at the top 45.3m/s
=> applying factor in API650 = 54.4m/s.

 

[HTURKAK]

Dear KVdA ,
Not sure for the reason that you are not convinced.
I will suggest you , provide the local basic wind velocity, vb, ( 10 minutes mean wind velocity ) together with tank dimensions, elevation, topography etc . to get more specific reply.


If 31 m/s is Vb ( 10 minutes mean wind velocity ) , you can convert this figure to 3 second wind gust velocity for tank design using DURST curve
e.g.

if Vo = 31 m/sec for ten minutes, ( 600 sec). If we use the notation V3 for 3 sec. gust velocity,

V3 /V600=1.5/1.05 =1.43

In this case V3=1.43*31=44.33 m/sec LT 45 M/s and 53 m/sec( API suggests) In this case , use 53 m/sec.

 

[KVdA]

Ok here it goes:
Terrain cat II
OD shell 30m
Height 20m
Air dens 1.25kg/m³
and lets assume orographi, direction, season all are 1

Basic windspeed Vb,0= 26m/s
=>Roughness cr(z)= 1.138 => vm(z)=26*1.138=29.59
Turbulance factor = kl=0.995 (from NAD) => Turbulance intensity = 0.1661

Peak velocity pressure or the extreme pulspressure=1184Pa
=> Peak wind velocity = 43.5m/s

I think it all boils down to, what is this "peak wind velocity"...
My interpretation is that this should be multiplied by 1.2 as suggested by the API.
=> 1.2*43.5=52.2m/s

If we only multiply the Vb = 26m/s by 1.43 than you get 37.18m/s.

Looking at ASCE 7-02 i notice that simular coefficients are added so perhaps that's where i make a mistake...
Recalculating the V in ASCE 7-02 with the pressure obtained by EC might be the only correct thing to do but really... all this because the EN14015 uses the 3second gust value instead of the 10minutes mean...

Perhaps the end result will be +/- the same as you suggest HTURKAK

 

[KVdA]

When calculating it using ASCE 7-02

Cat II = surface roughness C
=> alfa=9.5 and zg=900ft
=> for z=20m or 65.6ft i get Kz=1.158

For I=1, Kd=0.95 (for tanks) and Kzt =1 (flat terrain)
i get V=45m/s from back calculuting it via

qz = 0.00256 Kz KZl Kd V²[ (lb/ft2)

Somebody any comments on this?

 

[FMJalink]

KVdA maybe this helps somewhat. Storage tanks in the EC have to comply with EN 1991-1-4 and EN 1993-4-2 (matter a fact with the full set of Eurocodes). For normal storage tanks, the tank has also to comply with EN 14015. Almost every country in the EC has a National Attachment to the EN 1991-1-4, which can be quite different (history).

A lot of the tank users/owners do not like EN 14015 tanks, as the wall thickness will be less, allowed nozzle loads lower and without by code described sturdy construction details. Often then API 650 is prescribed as construction code.It is than additional.

Personally I add a specification to the requisition describing how the impact of the API 650 should be understood for the tank design. E.g. the API 650 refers to a lot of standards, which are not applicable or have no relevance in the EC.

When the requisition was not accompanied with such a specification, consulting the client could prevent some headaches.
Good luck.