ASCE 7 · DESIGN METHODS · LRFD
Ultimate wind loads are strength-level by design
Since ASCE 7-10, the mapped basic wind speed V is already an ultimate (strength-level) value, read straight from the risk-category MRI maps and used directly in LRFD.
THE CONCEPT
What makes a wind speed "ultimate"
Three things define a strength-level speed in modern ASCE 7. None of them involves multiplying a service load up after the fact.
Mapped, not factored
Since ASCE 7-10 the basic wind speed maps publish strength-level speeds directly — V already is the ultimate value.
SINCE 7-10Tied to an MRI map
The risk category selects which return-period map you read: 300, 700, 1,700 or 3,000-year. A rarer storm means a higher V.
300 · 700 · 1700 · 3000-YRUsed directly in LRFD
The resulting pressure enters strength combinations at a wind load factor of 1.0W — no extra amplification step.
1.0WWHERE V COMES FROM
Risk category picks the MRI map
There is no importance-factor multiplier in ASCE 7-10/16/22. Your risk category simply routes you to a different return-period map, and that map's V is already ultimate.
Risk Category I
Low-hazard: minor agricultural and storage structures.
300-YR MRIRisk Category II
Standard occupancy: homes, offices, retail, most buildings.
700-YR MRIRisk Category III
Substantial hazard: large assembly, schools, jails, utilities.
1,700-YR MRIRisk Category IV
Essential facilities: hospitals, fire/police, EOCs, shelters.
3,000-YR MRITHE EQUATION
Velocity pressure from an ultimate speed
Square the strength-level V, scale it by the coefficient and the K-factors, and the velocity pressure qz is already strength-level too.
Because V is ultimate, qz and every pressure built from it land at the strength level that LRFD expects — no conversion needed before applying 1.0W.
THE FACTORS
Every term in qz
One coefficient, four K-factors, and the squared ultimate speed.
0.00256
Unit coefficient pairing mph with psf.
COEFFICIENTKz
Velocity pressure exposure coefficient (height + terrain).
EXPOSUREKzt
Topographic factor for hills, ridges and escarpments.
TOPOGRAPHYKd = 0.85
Wind directionality factor for buildings (MWFRS & C&C).
BUILDINGSKe
Ground elevation factor introduced in ASCE 7-22.
ELEVATIONV
Basic wind speed — already the ultimate, strength-level value.
ULTIMATECROSS-CHECK
Ultimate to ASD / nominal: multiply by 0.6
If you need the service (ASD/nominal) level, scale the ultimate pressure down — never the other way around.
Equivalently, the speed scales by √0.6 (about 0.775). Worked example: an ultimate V of 150 mph drives the strength-level qz, and the matching ASD pressure is that qz × 0.6.
RUN THE NUMBERS
Calculate ultimate pressures to code
Skip the hand arithmetic — generate strength-level qz and design pressures straight from the current ASCE 7 maps.
KEEP READING
Related design-method guides
Nominal Wind Loads
The service-level side of the same pressures.
NOMINALNominal vs Ultimate
How the two levels line up through the 0.6 factor.
COMPARELRFD Method
Where the 1.0W strength combination uses these loads.
LRFDBasic Wind Speed
Reading V from the ASCE 7 maps before you calculate.
SPEED V