NBCC automatic load combinations

[STpipe]

This might come off as a bit of a rant, but I'm wondering if there is a good guide on how to properly setup the automatically generated load combinations for steel design based on the National Building Code of Canada?

The first frustration I found was in setting up the notional load. You can easily setup the auto feature and assign it to the Dead load pattern, however somehow snow is not part of the dropdown option? I found a workaround where if the snow load is defined as a "roof live load", then that load pattern can be chosen to automatically generate the notional loads. When defining the load combinations however, you then need to re-assign it as a Snow load or else it won't generate any load combinations with snow because there is no such thing as a roof live load in NBCC unlike ASCE 7.

Next, it generates the pure gravity loads, alternating the notional loads correctly with the right load factors - great. But then when it comes to the other load combinations where you have wind either as a primary load, or as a companion load, the notional load is not included. So am I missing something here, or do you need to go in and manually add them afterwards?

 

[BulbTheBuilder]

I am not a SAP2000 guru but is it not possible to set a notional load combination and include them in other load combination?

 

[STpipe]

Hi Bulb,

You can set any notional load pattern and include them in the load combinations manually, which is what I'm trying to find a work around. The advantage with the approach I described is that the notional loads are automatically calculated by the software which saves time, especially if there are situations during the design where the magnitude of the gravity loads needs to change.

Similarly with the load combinations, you can create any load combination manually, but the selling feature of using software is that some of these tasks can be automated.

In previous companies I had access to software where this was done automatically, which saves a lot of time.

 

[BulbTheBuilder]

Help me understand how SAP2000 works. Say I define a notional load N1= 0.002(D+S). Won't the N1 be a 0.2% of what you define D and S to be? In that case, if you update your D, shouldn't that also reflect in the N1 load?

I get that, we pay too much for application for them to be giving unnecessary updates rather than what actual users need. I can't help much with SAP2000...but I like this forum. Hopefully, JOSH can see this and help.

 

[STpipe]

From what I can see, you can only define a notional load for individual load cases. So in effect, N1 = 0.002D, N2 = 0.002S etc. Since the notional loads are only defined at the individual load case level, there is no way to define them as a group. The big issue is that the program doesn't allow you to define N2 = 0.002S, because snow is not one of the options in the dropdown menu.

So I see two options based on how the program works:

1- manually defining the notional load for the snow load case based on the axial load in the columns.

2- The work around by defining the snow load as a roof live load (with all the additional steps I mentioned in my original post)

Either options require some manual work in defining the load combinations which is frustrating.

 

[BulbTheBuilder]

This might be me overthinking. How about "tricking" the program? You define two snow loads. One as snow for your load combination generation and other snow load as "dead-snow" categorized as dead so that can be used solely for notional load.
if that doesn't work, then I am sorry

 

[STpipe]

That's in effect what I ended up doing. I tricked the program by classifying the snow load as a roof live load, and then in the load combination generation options, telling thr program to treat it as a snow load because roof live loads do not exist as a category in NBCC.

The other issue is not understanding why the program doesn't add the notional load to combinations with lateral loads.