Title: /
Modification to Wind Design to Accommodate ASCE 7-10 Changes
Date: / Feb 2, 2012Contact: / Name: / Larry Hiner
Company: / Chicago Bridge & Iron Company
Phone: / 815-439-6125
E-mail: /
Purpose: / Provide better compatibility with ASCE 7
Source: / Larry Hiner Nov.16, 2010
Revision: / 0
Impact: / Reduce costs due to misapplication wind speed requirements.
Background: / 1) ASCE 7 –10 Velocity modification
ASCE 7-05 uses wind speeds based on Mean Recurrence Interval (MRI) of 50years. Importance factors are applied based on the function of the structure. No load factor is applied to wind loads in the load combination formulas.
2.4 COMBINING NOMINAL LOADS USING
ALLOWABLE STRESS DESIGN
2.4.1 Basic Combinations
Loads listed herein shall be considered to act in
the following combinations; whichever produces the
most unfavorable effect in the building, foundation, or
structural member being considered. Effects of one or
more loads not acting shall be considered.
1. D + F
2. D + H +F + L + T
3. D + H +F + (Lr or S or R)
4. D + H +F + 0.75(L +T) + 0.75(Lr or S or R)
5. D + H +F + (W or 0.7E)
6.D + H +F + 0.75(W or 0.7E)+0.75L + 0.75(Lr or S or R)
7. 0.6D+ W+ H
8. 0.6D + 0.7E + H
qz = 0.00256KzKztKdV2(lb/ft2)
[In SI: qz = 0.613KzKztKdV2 (N/m2); V in m/s]
ASCE 7 -10 changed philosophy on wind speed requirements. Instead of modifying one wind speed map and providing Importance factors based on occupancy, 3 separate wind speed maps were created for the different structure risk categories. The wind speed maps provided are for 300, 700, and 1700 yearMean Recurrence Interval (MRI). The resulting load is then applied with a load factor of 0.6
2.4 COMBINING NOMINAL LOADS USING
ALLOWABLE STRESS DESIGN
2.4.1 Basic Combinations
Loads listed herein shall be considered to act in
the following combinations; whichever produces the
most unfavorable effect in the building, foundation, or
structural member being considered. Effects of one or
more loads not acting shall be considered.
1. D
2. D + L
3. D + (Lr or S or R)
4. D + 0.75L + 0.75(Lr or S or R)
5. D + (0.6W or 0.7E)
6a. D + 0.75L +0.75(0.6W) + 0.75(Lr or S or R)
6b. D + 0.75L + 0.75(0.7E) + 0.75S
7. 0.6D + 0.6W
8. 0.6D + 0.7E
qz = 0.00256KzKztKdV2(lb/ft2)
[In SI: qz = 0.613KzKztKdV2 (N/m2); V in m/s]
Modification of the ASCE 7 wind speeds for application in API formulas
API uses formulas which are based on 50 yr MRI. ASCE 7-10 modifies the effects of the higher wind speeds by applying a 0.6 factor.
Therefore when obtaining wind speeds from ASCE 7-10 for use with API 650, apply a factor to the wind speed. Wind pressure is a function of Velocity squared andthe load factor. For use with API 650, the reduction factor applied to the ASCE 7-10 wind speed V10would be the =√ 0.6 = 0.775 or just 0.78
Derivation
qz = 0.00256KzKztKdV052 = 0.00256KzKztKdV102 (0.6)
V052= V102 (0.6); or V05= √ (0.6) V10
Where qz = velocity pressure
V05 = wind speed ASCE 7 -05
V10 = wind speed ASCE 7-10
Example Comparison for Chicago, IL
ASCE 7-05 Wind speed is 90 mph
ASCE 7 -10 Risk Cat II Wind speed is 115 mph
115 x 0.78= 89.7 mph
2) Modification to account for Importance factor for ASCE 7 -05 for completeness. It is felt that Importance factors other than 1.0 should be addressed when determining the design wind speed used in API. As with the load factor discussed above, the Importance factor can be accommodated by applying the square root (√I) in wind design speed determination.
3) Note: The current IBCs in effect (2006 and 2009) will require ASCE 7-05. ASCE 7-10 will not be required until the 2012 IBC is issued. Using the latest edition is not automatically allowed in the various jurisdictions. This is provide for information but not included in any proposed changes.
Proposal: / Modify API 650 design wind velocity and wind pressure determination methods.
Specific changes: / 5.2.1 Loads
k. Wind (W): The design wind speed (V) shall be190 km/hr (120 mph), the 3-sec gust design wind speed determined from ASCE 7-05 multipied by √I, Figure 6-1,the 3-sec gust design wind speed determined from ASCE 7-10 for risk category specified by the Purchaser (Figures 26.5-1A, B, or C)multiplied by 0.78 , orthe 3-sec gust design wind speed specified by the Purchaser (this specified wind speed shall be for a 3-sec gust based on a 2% annual probability of being exceeded [50-year mean recurrence interval]). The 3-sec gust wind speed used shall be reported to the Purchaser.
1.Designwind pressure (PWSand PWR)using design wind speed (V):The design windpressureon shell (PWS)shall be 0.86 kPa (V/190)2, ([18 lbf/ft2][V/120]2) on vertical projected areas of cylindrical surfaces. Thedesign wind uplift pressure on roof (PWR) shall be1.44 kPa (V/190)2, ([30 lbf/ft2][V/ 120]2) uplift (see item 2 below) on horizontal projected areas of conical or doubly curved surfaces, where V is the3-sec gust wind speed.These design wind pressures are in accordance with ASCE 7-05for wind exposure Category C. As alternatives, pressures may be determined in accordance with:
a)ASCE 7-05 (exposure category and importance factor provided by Purchaser),
b)ASCE 7-10 (exposure category and risk category provided by Purchaser)with either velocity multiplied by 0.78 or the ASCE 7-10 pressure multiplied by 0.6.
c)A national standard for the specific conditions for the tank being designed.
2. The design uplift pressure on the roof (wind plus internal pressure) need not exceed 1.6 times the design pressure P determined in F.4.1.
3. Windward and leeward horizontal wind loads on the roof are conservatively equal and opposite and therefore they are not included in the above pressures.
4. Fastest mile wind speed times 1.2 is approximately equal to 3-sec gust design wind speed (V).
The old test is:
5.2.1 Loads
k. Wind (W): The design wind speed (V) shall be 190 km/hr (120 mph), the 3-sec gust design wind speed determined from ASCE 7, Figure 6-1, or the 3-sec gust design wind speed specified by the Purchaser (this specified wind speed shall be for a 3-sec gust based on a 2% annual probability of being exceeded [50-year mean recurrence interval]). The design wind pressure shall be 0.86 kPa (V/190)2, ([18 lbf/ft2][V/120]2) on vertical projected areas of cylindrical surfaces and 1.44 kPa (V/190)2, ([30 lbf/ft2][V/120]2) uplift (see item 2 below) on horizontal projected areas of conical or doubly curved surfaces, where V is the 3-sec gust wind speed. The 3-sec gust wind speed used shall be reported to the Purchaser.
1. These design wind pressures are in accordance with ASCE 7 for wind exposure Category C. As an alternative, pressures may be determined in accordance with ASCE 7 (exposure category and importance factor provided by Purchaser) or a national standard for the specific conditions for the tank being designed.
2. The design uplift pressure on the roof (wind plus internal pressure) need not exceed 1.6 times the design pressure P determined in F.4.1.
3. Windward and leeward horizontal wind loads on the roof are conservatively equal and opposite and therefore they are not included in the above pressures.
4. Fastest mile wind speed times 1.2 is approximately equal to 3-sec gust wind speed.
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