Condition 1 and Condition 2 ~ Table 1609.6.2(2), 2009 IBC

[codewonk]

Does anyone have an idea as to what the terms; "Condition 1" and "Condition 2" are referring to in Table 1609.6.2(2)? I've word searched the 2009 IBC to no avail. I've also been poking around in ASCE 7-05 and not finding anything.

I would appreciate any help. Thanks!!!

 

[Phil]

1609.6.4.3.2 says to use the most severe value. This is similar to the tables in ASCE 7-05 that often times have two values for the same condition. The importance of these is more apparent when one coefficient is negative (uplift) and the other condition is positive (downward force).

 

[Architect1281]

The Conditions listed as 1 & 2 relate to designs for partially enclosed structures ( Rather than as protected opening diaphram structures)

Condition 1 the total area of openings in a wall that recieve positive external pressure exceed the balance of openings wall and roof by 10 percent.

Condition 2 the total area of openings in a wall that receives positive external pressure exceeds 4 sqft or 1% of that wall area whichever is less and the percentage of oprnings in the balance of the building envelope does not exceed 20 percent

Does that make it any clearer?

 

[codewonk]

After poking around some more, there is a direct reference to these “conditions” in Figure 6-6 of ASCE 7 (Main Wind Force Resisting System – Method 2). They correspond to the two values of Cp listed in the table and Note 3 at the bottom of that figure also refers to “both conditions.”

 

[codewonk]

And as if this subject wasn't titillating enough ... I just received this explanation from ICC staff engineers:

Q: Table 1609.6.2(2) refers to Conditions 1 and 2 for Windward roof slopes, how do I find out what Conditions 1 and 2 mean?

A: This is not a simple question to answer. First of all the conditions 1 and 2 has nothing to do with how the building is constructed or how strong the wind is blowing.

To understand how the two wind load conditions are derived in Table 1609.6.2(2), one must understand how and where the values in the table come from. The basis is Cnet = Kd [(G) (Cp) – (GCpi)] (see Section 1609.6.2) and the values obtained for Cp.

When considering the possible wind loads for the “windward roof slope,” please refer to the illustrations of Figure 6-6 of ASCE 7-05, where you will notice that on windward side of the sloping roof of the “Gable, Hip Roof”, there are two wind load conditions, one “upward” and the other “downward” as illustrated by the two arrows. Both conditions on the windward roof slope will provide two values of Cp and thereby providing two values of Cnet per Table 1609.6.2(2), see example below:

For an enclosed building with a simple gable roof (slope < 2:12 or 10 degrees), Table 1609.6.2(2) shows, reading across Windward roof slopes: Slope < 2:12 (10 degrees), we have:

Condition 1, Cnet = -1.09 for + Internal Pressure of Enclosed Building

Condition 1, Cnet = -0.79 for – Internal Pressure of Enclosed Building

Condition 2, Cnet = -0.28 for + Internal Pressure of Enclosed Building

Condition 2, Cnet = +0.02 for – Internal Pressure of Enclosed Building

How are these value obtained in the table? These values are obtained by solving:

Cnet = Kd [(G) (Cp) – (GCpi)], where:

Kd = 0.85 per ASCE 7, Table 6-4

GCpi = plus or minus 0.18 for enclosed building per ASCE 7, Figure 6-5

G = 0.85 per ASCE 7, Section 6.5.8.1

Cp = -1.30 (wind load condition 1) and -0.18 (wind load condition 2) per ASCE 7, Figure 6-6 (bottom table for “Roof Pressure Coefficients, Cp” going down the column for 10 (degrees) for h/L > 1.0.

By plugging in these values, we get:

Condition 1, Cnet = 0.85 [(0.85) (-1.30) – (+0.18)] = -1.09 for + Internal Pressure of Enclosed Building

Condition 1, Cnet = 0.85 [(0.85) (-1.30) – (– 0.18)] = -0.79 for – Internal Pressure of Enclosed Building

Condition 2, Cnet = 0.85 [(0.85) (-0.18) – (+0.18)] = -0.28 for + Internal Pressure of Enclosed Building

Condition 2, Cnet = 0.85 [(0.85) (-0.18) – (– 0.18)] = +0.02 for – Internal Pressure of Enclosed Building

 

[High Desert]

Thanks codewonk. If an engineer asks me about this I'll send them to you and you can 'splain it to 'em.

 

[codewonk]

Ya cut me deep this time High Desert. Cut me deep.

 

[High Desert]

All in fun codewonk. It was a feeble attempt to say I didn't understand any of it.

 

[codewonk]

My entry was in jest as well.