Are ASCE 7-02 wind loads ultimate or service level?
Are ASCE 7-02 wind loads ultimate or service level?
(OP)
Are the wind loads in ASCE 7-02 at ultimate or service level. IBC 1605.3.2 seems to want you to multiply ASCE 7 wind forces by 1.3 and this requirement is in the "allowable stress" subsection of IBC 1605.3. But ASCE 7 indicates ultimate strength level forces for seismic load. This is confusing and I can't find anywhere in IBC or ASCE that says specifically that the ASCE 7-02 wind loads are allowable stress level.
Am I correct in this assessment, that they are in fact allowable stress level loads?
Am I correct in this assessment, that they are in fact allowable stress level loads?






RE: Are ASCE 7-02 wind loads ultimate or service level?
RE: Are ASCE 7-02 wind loads ultimate or service level?
RE: Are ASCE 7-02 wind loads ultimate or service level?
RE: Are ASCE 7-02 wind loads ultimate or service level?
This doesn't seem right. If the ASCE wind loads are too low already and you take them down another 25% (per 1605.3.1.1), aren't you lower than the 1.6 wind stress increase factor used when using 1605.3.2? (1.33 * 1.3 = 1.73)
RE: Are ASCE 7-02 wind loads ultimate or service level?
Section 2.4.1 also states "Increases in allowable stress shall not be used with the loads or load combinations given in this standard unless it can be demonstrated that such an increase is justified by structural behavior cause by rate or duration of load." It sounds like the intent is that you use the wind loads as calculated with the reduction factors shown in the allowable stress design load factors. Or alternatively, if you have some other design code that allows stress increases for wind, you'd use the calculated loads without the reduction factor.
RE: Are ASCE 7-02 wind loads ultimate or service level?
RE: Are ASCE 7-02 wind loads ultimate or service level?
RE: Are ASCE 7-02 wind loads ultimate or service level?
RE: Are ASCE 7-02 wind loads ultimate or service level?
RE: Are ASCE 7-02 wind loads ultimate or service level?
Essentially, I do use the Kd = 0.85 factor, I do use 3/4 of transient loads (when both wind and live load act simultaneously), and I don't use a stress increase with any material (except I still use the 1.6 duration of load factor with wood design).
DaveAtkins
RE: Are ASCE 7-02 wind loads ultimate or service level?
After all - they say you can use 3/4 of the transient loads when two or more are combined in the same load case...seismic and wind are never combined so there must be other transient loads.
So is Live load a transient load?
RE: Are ASCE 7-02 wind loads ultimate or service level?
RE: Are ASCE 7-02 wind loads ultimate or service level?
Back on the wind load issue, I believe it is helpful to think of the wind loads provided by ASCE in terms of their recurrence intervals rather than stamping them with a "service" or "ultimate" level. The VELOCITY levels provided by the code maps are in terms of a 50 year recurrence interval. This can be verified using the velocity recurrence interval multiplier in the commentary: Fc = 0.36 + 0.1 ln(12T). With T=50yrs, Fc = 1.0. Keep in mind this is a velocity multiplier and force levels are equal to velocity levels squared. In this case, 1.0 squared = 1.0.
Therefore if you used a 1.0 load factor you would have a 50 year wind. As demonstrated in the commentary, you can back calculate the return period for a certain load factor by plugging in the square root of the load factor as Fc and solving for T. Keep in mind that the return intervals are referring to wind VELOCITY and that force levels are based on velocity squared. So by removing the directionality factor from the typical 1.3 load factor = 1.3/0.85 = 1.53. And taking the square root of this to make it in terms of force: Fc = 1.23. Back solving for T results in a return period of approximately 500 years. Therefore using a 1.3 load factor with the directionality factor results in a 500 year "ultimate" return period which is in line with seismic design.
As an end note, the code recommends using 0.7W for "service" level deflection checks. This is based on bringing the 50 year map speeds down to 10 year speeds. By plugging T=10 into the Fc equation you get Fc = 0.838. Fc is the wind speed multiplier so to get the force multiplier you must square 0.838 = 0.7.
Sorry for the novel, hope it helps.
RE: Are ASCE 7-02 wind loads ultimate or service level?
Also, I'm not sure I'd say:
"Essentially all loads with the exception of dead load and superimposed dead load can be classified as "transient"
Earth loads just can't be transient as most times they are quite permanent conditions on a structure.
How about snow loading? In some cases snow sits on a roof for weeks.
RE: Are ASCE 7-02 wind loads ultimate or service level?
I agree that the statement was a bit broad - certainly permanent earth pressures etc. would be considered permanent loading conditions and not "transient". User judgement is certainly in order.
The assertation that live load is a transient load is based upon two things. First, code wise, is section 1605.3.1.1 of the IBC 2000 which states "it is permitted to multiply the combined effect of two or more transient loads by 0.75 and add to the effect of dead load." By specifically splitting the loads into two camps - "dead load" and "transient loads", the code directly implies live loading is transient. Also, if this did not apply to live load, then you would rarely if ever get to use the 0.75 reduction in that normally you only have either wind or earthquake conditions which is clearly not the code intent. Second, common sense wise, is that live load is not always there and is therefore transient. Also the majority of "canned" engineering software out there such as RAM follows this logic and applies the 0.75 factor to conditions with Live+Wind, Live+EQ etc.
RE: Are ASCE 7-02 wind loads ultimate or service level?
RE: Are ASCE 7-02 wind loads ultimate or service level?
So I don't think transient is the best term to apply to snow load if a reduction is implied to be used by the code. What is the limit on time that can pass? What is brief?
I would still hesitate to apply the 0.75 reduction to a column that supports live load from a single floor and snow load from a roof in a place like Minnesota. I don't have the IBC with me now, maybe there is a provision that excludes this "floor live + snow" reduction, or at least for a certain amount of snow?
RE: Are ASCE 7-02 wind loads ultimate or service level?
D + H + F + 0.75(L + T) + 0.75(Lr or S or R)
where S is the snow load.
This suggests that the snow load is transient in terms of the logic in the code.
-Mike