Are ASCE 7-02 wind loads ultimate or service level? 1

[JSA2]

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?

 

[UcfSE]

Wind loads in ASCE 7 are service level. Seismic loads are ultimate.

 

[JStephen]

I believe ASCE-7 also gives recommended load factors in the first part of the book- check if that helps any.

 

[aggman]

I believe the load combinations in 1605.3.2 are based on utilizing allowable stress increases. ASCE wind loads are not written to take this into effect which is why you are required to increase the wind load by 1.3. So you can either use the equations from 1605.3.1 utilizing the load reductions from 1605.3.1.1 or you can use the alternative combinations, but increasing the wind load to account for the allowable stress increase.

 

[JSA2]

So you increase 1605.3.2 wind loads by 1.3 and take allowable stress increases OR reduce combined loads by 25% and not take the increases?

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)

 

[JStephen]

ASCE-7-02, Section 2.3 gives load factors used for "Strength Design". Section 2.4 gives load factors used for "Allowable Stress Design".

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.

 

[UcfSE]

The 1.6 factor I believe is increased from 1.3 to take into account the fact that the ASCE 7 uses a directionality factor Kd = 0.85 to reduce wind loads. 1.3/.85=1.53 --> rounded to 1.6.

 

[aggman]

It has to do with the calibration of the load combinations. The load combinations of 1605.3.1 are calibrated to provide maximum response by either the worst case of any two loads applied simultaneously or the combination of multiple loads reduced by 25%. The combination equations of 1605.3.2 are an alternate, which I would assume when applied correctly will give similar results to the equations of 1605.3.1. Note that the ASCE wind loads utilize a wind directionality factor (Kd) which are not to be utilized without using there load combinations. You can calculate wind loads per ASCE without the (Kd) factor and use whatever load combinations you think are appropriate.

 

[UcfSE]

Check out the ASCE 7 commentary C2.3 and C2.4.

 

[JSA2]

The ASCE commentary and your responses have been very helpful. Thank you all for your contributions.

 

[DaveAtkins]

I'm glad to see that the way I interpret the Code is essentially in line with most of you.
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

 

[JAE]

Hey everybody - launching off of what Dave just said - what do you consider "transient" loads in terms of the IBC? I know we've had these discussions before in Eng-tips - the IBC doesn't seem to define what the word transient means - I know its wind and seismic, but the context of the code seems to imply that there are "other" transient loads that may be combined with wind or seismic.

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?

 

[aggman]

The only other transient loads would be soil, flood, etc. I personally would never allow them to be reduced and the equations in ASCE 7-02 Section 2.4 don't allow this either. They only reduce wind or earthquake when applied with other loads. Also in my IBC 2003 1605.3.1.1 they say "It is permitted to multiply the combined effect of two or more variable loads by 0.75 and add to the effect of dead load." I wouldn't reduce any earth pressure or flood loads just due to the fact that they really aren't defined that well and they really are not variable, except maybe surcharge loads from traffic, etc.

 

[WillisV]

Yes live load is a transient load. Essentially all loads with the exception of dead load and superimposed dead load can be classified as "transient" and therefore come under the umbrella of the 0.75 reduction. The 0.75 reduction is not intended to account for the transient nature of the loads (or you would be able to use it with only one of them instead of two or more). Rather it is intended to account for the reduced probability that the transient load will occur simultaneously - it is doubtful you would have a full live load during a hurricane for instance.

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.

 

[JAE]

WillisV - appreciate your comments - but in the first paragraph you assert that live load is a transient load per the IBC...can you provide any backup evidence for this other than your opinion? I would really like to know what the ICC intent was so if I'm challenged in the future, I have a source. I'm not trying to diminish your statement, its just that I can't say "WillisV told me so".

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.

 

[WillisV]

JAE,

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.

 

[JAE]

WillisV - sounds right to me. thanks.

 

[haynewp]

On your question about snow, I asked somebody at the Southern Building Code a few years ago about it and he said that snow is a live load and therfore a transient load. Transient by dictionary definition is "passing with time" or "remaining in a place only a brief time".

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?

 

[mrMikee]

In section 2.4.1 of ASCE 7-02 for basic combinations and equation 4...

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