NBCC Formulations for Wind Accelerations

[slickdeals]

I am in the process of developing a worksheet to calculate the along-wind and across-wind response of tall buildings subject to wind loads.

The NBCC formulation has a gust factor g_p that is calculated based on various parameters, one of which is the fundamental frequency in the along-wind direction.

This gust factor is then used in calculating across and along-wind accelerations a_w and a_d, in conjunction with along-wind and across-wind fundamental periods.

My questions are thus:
1. Do you compute two gust factors, one each for wind along X axis and Y axis, with appropriate building period?

Assuming a building is rectangular in plan 100' x 50', with period of T1 = 5s along short direction and T2=3.5s along long direction, you will have gust factor based on T1 and T2...right?

2. You will then use these gust factors and apply them in the acceleration formulas using the appropriate along-wind and across-wind periods...right?

Appreciate your inputs.

Also, does anyone have a similar worksheet to compute ballpark acceleration values to see if your building is in trouble prior to heading to the tunnel?

 

[SAIL3]

I would use the most conservative value for both X & Y as this would also cover any questions about diagonal wind and simplify the analysis...

 

[slickdeals]

SAIL3,
Are you suggesting that I should use the higher Gust Factor value for both directions, but use the appropriate fundamental frequency for along-wind / across-wind?

 

[SAIL3]

I would use the same wind loading for both X & Y unless time is not a factor in your pracice....

 

[slickdeals]

@SAIL3...I am only doing this to check the serviceability of the building. Either I am not communicating properly or ....

 

[asixth]

From using AS1170.2 Appendix F which I'm guessing is based on the same formulations as NBCC. You treat both along wind and cross wind response separately for each orthogonal direction of your building. So the gust factor can be different based on your X & Y parameters.

 

[slickdeals]

@asixth, hokie,

In Appendix G "Accelerations for Wind Sensitive Structures" AS1170, there is a formula for calculating along-wind accelerations.

It uses
Δz = height of the section of the structure upon which the wind pressure acts

I don't quite understand what Δz is. Is this equal to the height of the structure above ground, or in other words equal to "h"?

 

[slickdeals]

Or should it be calculated for every floor level. Say I have a 200 m tall building, should Δz be taken in 10 m increments?

 

[slickdeals]

Can any of the Aussies in here clarify please?