Wind load on roof ridge beam
Wind load on roof ridge beam
(OP)
I have a roof ridge beam that spans 27' with tributary width of 10.5'. The house is one-story and the roof height at the ridge is 15'. The wind has exposure C with 85 mph wind. Based on the ASCE tables, the down wind load is 7 psf and the upward wind is 32 psf. I am getting a high uplift at the end connection of the beam due to the uplift wind load combination. Any idea why the uplift wind is so high, 32 psf? I have attached a sample calculation for reference.
WIND LOADS
WIND LOADING ( ASCE 7)
COMPONENTS AND CLADDING , METHOD 1
ENCLOSED BUILDING
EXPOSURE C
BASIC WIND SPEED, V 35, 85 MPH
ROOF ZONE 2, ASCE FIG 6-3 (CORNER ZONE, CONSERVATIVE)
Ps = λ Kzt Iw Pnet30
λ = 1.21 (EXPOSURE C, ROOF MEAN HEIGHT =15', FIG 6-2)
Kzt = 1.0 (AISC SECTION 6.5.7.2, FLAT TERRAIN)
I w= 1.0 (TABLE 6-1, ASCE 7-05, CATEGORY II)
Pnet30 = + + 6.0 AND - 26.0 7 < ANGLE < 27 , ZONE 3, EFFECTIVE AREA 27'X10.5'
Ps Down = 1.21 x 1.0 x 1.0 x 6.0 = 7.3 PSF
Ps Up = 1.21 x 1.0 x 1.0 x 26.0 = 31.5 PSF
NOTE:
THE WIND LOAD IS NORMAL TO THE SURFACE OF THE ROOF. WE ARE USING THE HORIZONTAL
LENGTH FO THE ROOF MEMBER FOR CALCULATION. THEREFORE, WE INCREASED THE WIND
LOAD TO COMPENSATE FOR THE HORIZONTAL LENGTH OF THE MEMBER.
(1/COSINE 18)
Ps Down = 7.3 PSF x 1.05 = 7.6 PSF
Ps Up = 31.5 PSF x 1.05 = 33.0 PSF
WIND LOADS
WIND LOADING ( ASCE 7)
COMPONENTS AND CLADDING , METHOD 1
ENCLOSED BUILDING
EXPOSURE C
BASIC WIND SPEED, V 35, 85 MPH
ROOF ZONE 2, ASCE FIG 6-3 (CORNER ZONE, CONSERVATIVE)
Ps = λ Kzt Iw Pnet30
λ = 1.21 (EXPOSURE C, ROOF MEAN HEIGHT =15', FIG 6-2)
Kzt = 1.0 (AISC SECTION 6.5.7.2, FLAT TERRAIN)
I w= 1.0 (TABLE 6-1, ASCE 7-05, CATEGORY II)
Pnet30 = + + 6.0 AND - 26.0 7 < ANGLE < 27 , ZONE 3, EFFECTIVE AREA 27'X10.5'
Ps Down = 1.21 x 1.0 x 1.0 x 6.0 = 7.3 PSF
Ps Up = 1.21 x 1.0 x 1.0 x 26.0 = 31.5 PSF
NOTE:
THE WIND LOAD IS NORMAL TO THE SURFACE OF THE ROOF. WE ARE USING THE HORIZONTAL
LENGTH FO THE ROOF MEMBER FOR CALCULATION. THEREFORE, WE INCREASED THE WIND
LOAD TO COMPENSATE FOR THE HORIZONTAL LENGTH OF THE MEMBER.
(1/COSINE 18)
Ps Down = 7.3 PSF x 1.05 = 7.6 PSF
Ps Up = 31.5 PSF x 1.05 = 33.0 PSF






RE: Wind load on roof ridge beam
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RE: Wind load on roof ridge beam
I am using ASCE 7-05.
RE: Wind load on roof ridge beam
RE: Wind load on roof ridge beam
RE: Wind load on roof ridge beam
Should you not be using the vertical component for uplift rather than the horizontal? The net uplift should be less than the normal force to the rafters. Subtracting the dead load of the roof, I can see an uplift of 2K at each end. One strap should be able to handle that.
Mike McCann, PE, SE (WA)
RE: Wind load on roof ridge beam
RE: Wind load on roof ridge beam
RE: Wind load on roof ridge beam
RE: Wind load on roof ridge beam
Most of your ridge beam would be in zone 2 with the ends of the ridge beam zone 3.
Your area is more than 100 sf.
This results in 1.21 x 15.2 = 18.4 psf uplift along the interior length of the ridge and
1.21 x 24 = 29 psf uplift on the ends for a distance "a".
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RE: Wind load on roof ridge beam
Just jumped into this one so square me up if I missed something!
RE: Wind load on roof ridge beam
Mike McCann, PE, SE (WA)
RE: Wind load on roof ridge beam
The effective area is all about the beam and not the zones, right?
All that the dimension "a" does is define where 3 ends and 2 begins.
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RE: Wind load on roof ridge beam
Mike McCann, PE, SE (WA)
RE: Wind load on roof ridge beam
RE: Wind load on roof ridge beam
You have a ridge beam that is 27 feet long. It has a tributary area of 10' x 27', but part of that tributary area is in Zone 2 and part is in Zone 3.
The part in zone 2, has a wind loading of x ((27-2a) x 10) x W). The part in zone 3 has a wind loading of y ((a x 10) x W) for each end). Each is treated separately.
RE: Wind load on roof ridge beam
My point is that for the beam, there is ONE effective area to use in determining the wind pressures.
You don't enter that table with a x 10 to find the zone 3 pressure and (27-2a) x 10 to find the zone 2 pressure.
Look at my red box in the table above. The pressures are based on >100 sf because the beam has >100 sf of tributary (effective) area involved.
So the 26 psf that the OP used - not sure where it came from - there is a 26 psf for zone 3 at 50 sf but for >100 sf it should be 15.2 or 24 psf for 2 and 3 respectively.
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RE: Wind load on roof ridge beam
The "yanks" like to relate roof pitch in units of 12, with 12 being the horizontal unit of 1 foot (12") - given the imperial (non metric/SI) system of units. It makes the math easy, I guess.
The first time I heard a term 2:12, or 4:12 I did the same as you...convert it to 1:X.