Dynamic Properties - Wind Tunnel

[slickdeals]

Folks,
My understanding so far has been that when providing dynamic properties (time periods, mode shapes) for wind tunnel testing, the properties are based on service level loads, meaning that the cracking in columns, walls, slabs, beams etc. can be based on a 1.43 * stiffness modifier specified in ACI Chapter 10.

I was recently told to provide dynamic properties based on stiffness values associated with factored loads, meaning much higher periods due to higher cracking at strength (factored) level forces.

ACI mentions that the stiffness EI used in elastic analysis for strength design should represent stiffness immediately prior to failure (meaning crack factors associated with strength level forces). The confusing portion is that in the same commentary, it is mentioned that "....service level loads are to be used for wind tunnel testing".

Can anyone shed some light and explain the nuances of structural properties for wind tunnel testing (for strength and serviceability).

 

[slickdeals]

Any takers for an explanation?
Thanks

 

[kslee1000]

Your question is unclear at large. Please simply state your goal, and the question associated with it. I think you will getting better responses.

 

[slickdeals]

Let me try to make it clearer.

Do you provide building dynamics (periods, mode shapes) for wind tunnel analysis based on stiffness (cracking) at factored loads or service loads?

 

[kslee1000]

It is a serviceability concern, I would use service load level properties regardless of load levels. The testing conducted using factored load is to safe guard the implication of imminent (ultimate) failure will not occur. Unless you want to gather responses from the concrete at ultimate.

By concept of strength design, at factor load level around strategic locations of members, the concrete should have cracked to the point that the only concrete region remains intact is the compression zone. In reality, it is not likely to occur, and shouldn't be allowed to occur. Check me on this.

 

[slickdeals]

Thanks kslee,
This has been my interpretation also, but our logic is being questioned by a big name firm and I wanted to gather input from other practicing engineers.

 

[kslee1000]

Wish it helps. Good luck.
Where are our wind/concrete/building experts?

 

[swivel63]

service loads. that's what i've always supplied wind tunnel companies.

 

[WillisV]

service.

 

[slickdeals]

Thanks for confirming my thought process. Are there any specific circumstances when properties would be provided based on cracked stiffness under factored loads?

 

[kslee1000]

slick:

I guess this question was raised during a peer review process. The reviewer shall state clearly theirs objectives, and list all relevant references. If they did not, politely asking them to do so before renders response.

 

[civilperson]

It doesn't matter, the wind does not produce periodic loading that might match the period of the building.

 

[swivel63]

^^^^^ huh? are you meaning resonance or am i reading that wrong?

 

[civilperson]

Flutter or aeolian vibration occur at moderate velocities much less than the design speeds. Dynamic properties are not applicable to the wind forces used in ASCE 7.

 

[swivel63]

when i've calculated the gust factor by hand, i've always needed the building period.

 

[slickdeals]

A building's flexibility will affect the dynamic component of the wind response. More flexible the building, more the wind loads on it due to gusting. Please correct me if I am wrong in stating that.

 

[swivel63]

i don't think you are. that's what i learned when i was a green engineer.

(a few years ago, LOL)

 

[slickdeals]

I am a green engineer []

 

[kslee1000]

"More flexible the building, more the wind (loads) force on it due to gusting."

Hope we all agree.

Evergreen

 

[rb1957]

i hope you guys are focusing to the building per se, and remember the function of the wind tunnel ? talk of gusts makes me think you're thinking about the external effects, and maybe there's some internal effects to be considered ?