Massachusetts Building Code: load combinations

[tweedledee]

The Massachusetts Building Code gives 7 load combinations for ASD.
Does anyone know what the "-" sign is for in combinations 5 and 6.

The combinations are as follows:

1. Dead
2. Dead + floor live + roof live (or snow)
3. Dead + floor live + 0.5 roof live + wind
4. Dead + floor live + roof live + 0.5 wind
5. 0.67 Dead - wind
6. 0.67 Dead - 0.8 seismic
7. Dead + 0.9 floor live + 0.6 snow + 0.8 seismic

 

[SacreBleu]

It looks like checking an uplift condition (such as suction on a flat roof)

 

[SacreBleu]

ps- the suction is due to uplift wind pressure component per Code, not the full horizontal wind pressure.

 

[tweedledee]

Sacrebleu,
thanks.

I want to apply this in a computer program. Would it be corect to say that for this combination I do not input the positive pressure, but instead input the negative pressure due to suction only?.

Thanks for your help

 

[imsengr]

Tweedledee,

Here is my take on the "-" wind situation. When you are designing roof rafters and their connections, for instance, you have to consider all the various ways that wind can act on the roof and the connections holding the roof to the main structure. When wind acts inwardly against a roof (whether flat or sloped), i.e. wind "blowing down" on the roof, the equations with the "+" wind take care of these situations.

However, wind does not just act inwardly against a roof. Wind can also acts outwardly against a roof, i.e. having a "suction" effect on the roof, and trying to blow the roof out of the its connections and away from the main structure. Hence, the "-" wind equations. "-" wind equations provide you the suction effect on the roof. Obviously, the dead load of the roof itself acts in some way to counter the "suction" effect. If you have a heavy roof, the "suction" effect is less, or if you have a light roof, the "suction" effect is more. The Code acknowledges this, and allows you to put in the put in the effect of the dead load, but they only allow you to use 0.67 (or 2/3) of the dead load, not the entire dead load.

There are a few ways you can input the "-" pressure. If you calculating manually, yes, you do input the "-" pressure for eqns. 5 and 6. If you are using a structural analysis modeling software, such as STAAD, STRAP, etc. you invert the direction of the wind so that it is acting "upwards" against the roof in the local axis.

The effect of the "suction" wind is significant on connection design, i.e. you want to make sure that your connections do not fail (either in pullout pullover, etc.) when you have the full effect of the "suction" wind.

On another point, check your Code to see whether it allows for a reduction when you have a combination of two or more loads (other that the dead load).

Hope this helps, and please post more questions if you still don't understand how the "-" wind thing works.

 

[SacreBleu]

Adding further, for a pitched roof, the UBC and IBC Codes that I am familiar have 2 different wind uplift conditions: One is suction on entire roof, using a certain coefficient to determine negative wind pressure. The second condition is uplift on the leeward side of the roof, using the vertical component of the negative wind pressure which exists on the leeward side. That coefficient is different. This is completely defined in ASCE 7-xx (latest is -02 I believe). There are ANSI publications which define this similarly. Since you are East Coast, you don't use the UBC,of which I am very familiar. The UBC called this "Method 1" for determining wind pressures. The IBC has tabkles for all this, but they are rather confusing unless you study a Commentary.

 

[tweedledee]

GAGngr05 & SacreBleu, thanks.

I think I have a good understanding of it now.

 

[LPPE]

Dont get too caught up in the roof situation. What about the "0.67D - 0.8 Seismic"? I dont think I'd want to design my braced frames using THAT combination. Almost seems like a misprint, but I dont know for sure.