Seismic Member Slenderness AISC Cl. 13.2a

[StevenPumphrey]

Hey,

Ok, need some help understanding this.

I have a building braced along a number of lines by HSS 6X6X1/2" members as cross bracing. I now need to check if this member complies with the slenderness requirements of Seismic Provisions, Cl.13.2.a. I have quite long lengths for the bracing members, such as 30'+, thus hoping that I can use the exception rule in the clause. I don't quite understand how I go about this check though.

It states "if the available strength of the column is at least equal to the maximum load transferred to the column considering Ry times the nominal strengths of the connecting brace elements of the building", its ok.....now what the hell does that mean????

Any explanation should shed some light on this situation.

Thanks,

Steven

 

[Taro]

Try the commentary, it should shed some light on the situation for you.

Slenderness limits for compression braces have been tightened in order to reduce the tendency of the braces to buckle. But if you don't want to use stocky braces that meet the slenderness limit, you can use the exception. The exception basically says that there is significant overstrength available in the tension brace that can compensate for the buckling of the compression brace. But in order to go this route, you need to design the columns for the expected tensile capacity of the tension braces.

 

[StevenPumphrey]

Ok, let me get this straight........

Now the way I originally interpreted this is that as long as the column does not fail before the brace fails in COMPRESSION, I would be ok. The reason I assumed this was that the upper limit on this clause is a compression limit. But now you say I need to design for tension capacity?

Let's try some numbers. Say the column strength is 360 kip in compression. The Ry for HSS sections is 1.4. The compression resistance of the braces is 90 kip. Tensile resistance is 403 kip (Yield) and 318 (Rupture). So the clause says take the nominal strength and multiply it by 1.4. So for compression this becomes 90/0.9=100 kip and for tension it becomes 403/0.9=447 kip. Therefore, I would have to upsize the column, as 447 kip is greater than 360 kip, correct??

Let me know if this is right.

Thanks,

Steven

 

[StevenPumphrey]

I also forgot...

to multiply 447kip by 1.4 =627 kip.

Therefore the column would have to withstand 627 kip of compression, correct?

 

[StevenPumphrey]

Another question,

lets say I have 2 sets of braces forming in, one set and column midlevel, the other at base level. Does this mean I have to design the column only for max tension force x1? What if I had three sets of bracing, is this max tension force x2?

Let me know.

 

[Taro]

My interpretation is that the columns must have available strength in compression to resist the expected compression strength of the braces and must also have available strength in tension to resist the expected tension strength of the braces when the load reverses. If there are multiple braces attached to the column at different levels, the effect is cumulative.

 

[StevenPumphrey]

Does it make sense though that it would be cumulative? I need somebody to talk to me in terms of an example here. Let's dumb it down. Let's say we have a single braced bay, two stories in height, with cross bracing at both floors of the bay. If we applied a horizontal force to the bay, it would cause two of the bracing members to go into compression, and two into tension, would it not? Let's say compressive resistance of the braces is 40 kip and tensile resistance is 100 kip (both factored). Would the column, according to the rules, if it is cumulative, have to take 1.4(100+100)/0.9 = 311 kips in compression?

What I don't get is why it would be cumulative? Why wouldnt the column just have to withstand the nominal tensile force of a single member?

Help!

 

[Taro]

The X-braced case is a little more complicated because there are both compression and tension braces connecting to a single column. I would suggest drawing a free-body diagram of the column with the expected brace capacities shown as applied loads (tension or compression) at the appropriate locations. You will see that the column force will vary along the height of the column and at the base it will be equal to the cumulative effect of all the bracing loads.