Discussion: Notional loads have no major effect on steel design ratio. 2

[Mohamed Mowafy]

As we know, if we want to adopt the direct analysis method in steel design, there are some steps to fulfill the code requirement.

1: reducing stiffness by 0.8
2: Assign the slenderness factors K major and K minor for all members by 1.00.
3: applying the notional load as a percentage of gravity load, say 0.003 w in each floor, to be included with all gravity load combinations, and in some cases it must be included in lateral load combinations.
4: Perform P-delta analysis for all design cases and combinations.

In fact, applying notional load in some software is complicated, so I tried to figure out its effect on members design ratio, and my conclusion is that if your case is designing low-rise buildings such as warehouses, notional load doesn't have a tangible effect on design ratio.

In the case of residential multi-story buildings less than 6 floors and heavy industrial buildings, the effect is around 2-3% increase in design ratio.

In my opinion, considering notional loads in low-rise buildings is time-wasting, but it's effective in high-rise buildings.

So, in my opinion, considering stiffness reduction in the direct analysis method is enough to get a fair result.

I need to see your opinion on this subject, and thanks

 

[phamENG]

You don't get to mix and match. If you want to use the direct analysis method, use it...use all of it. If you want to use an alternative, like the effective length method, then do that. There's a reason that a lot of engineers working in the low and mid-rise arenas still use the older methods. But there are advantages to the new ones, too.

And the notional loads can have an impact on smaller structures, depending on the loading. For instance, patios/decks...especially if they're event spaces and indoors, like theater balcony...can have their lateral design controlled by this. I'm in a low seismic area, so full dead and live load plus notional loads can exceed seismic loading on a lot these sorts of structures. Even outside, some of these have virtually no surface area for the wind to catch, and end up being controlled by notional loads as well.

 

[jayrod12]

Some areas, like mine, don't have seismic activity, therefore for 98% of platform designs, notional loads govern and provide the only designable load for stability. I've been on platforms where people haven't looked at this aspect, and I felt like I was on a boat in high seas everytime.

 

[JoshPlumSE]

I have a similar experience. Notional loads are NOT likely to affect the design of most structures that I've worked on. But, the REASON why they're needed is for the odd cases. Cases where gravity load may be significant, but wind and seismic are not.

In particular, inside mezzanines in low seismic country.

Another case would be a "pressure test" load in industrial structures. Where your equipment and pipes are filled with water and pressure tested. The load is VERY temporary. It's only in place for a matter of hours, so you don't analyze it for wind or seismic. But, the vertical load can be much larger than a normal operating load.

IMO, you can use your engineering judgement to ignore notional loads in most cases. But, you really need to understand it and it's purpose in order to have the proper judgment in ignoring it.

Sometimes, I might calculate the notional loads at each level of my structure. Then if I can demonstrate that it's much lower than wind and seismic then I can ignore it. Honestly, very similar to what I'd do when deciding whether wind needs to be considered in design.

 

[TLHS]

It's not just for lateral force design. It's mostly for making sure things like your dead load only case have a destabilizing force for as a starting point for second order effects.

I do agree, though, that it rarely comes into effect for me. That's likely because I'm working in seismic zones and industrial work, though, where you're going to be fairly laterally robust in general.