How many load combinations

[mrMikee]

When I design a four column structure like an elevated bin I end up using about 26 load combinations due to orthogonal seismic loads and wind loads per ASCE Figure 6-9. An engineer I want to do some of this work for me doesn't want to do this, and claims that he never saw it done that way at the two design offices he worked at. To me it is clearly called for in the IBC codes and besides that, it actually makes sense. Calculations confirm this too.

Here's how I get the 26 combinations (simplified form) IBC 1605.3.1

1 D
1 D+L
8 D+L+0.7E
8 D+L+W
8 0.6D+L
---
26 = total number of load combinations

Any comments would really be appreciated.

Best Regards,
-Mike

 

[UcfSE]

I would imagine that is 0.6D+W, not L. Either way, I think you are right on, I do the same thing but I don't have as many combinations because I work in a non-seismic area. I check wind in each horizontal direction as well as positive and negative roof pressure where applicable. I should note that I am probably the only one in my office that does this. I don't care for the argument that it doesn't need to be done because no one else is doing it or because "we have been doing it that way for [xx] years". That's what lemmings do, or Gomer Pyle Engineering.

 

[mrMikee]

Oops.

UcfSE, you are right... Should be 0.6D+W

I had the IBC right here, and was looking, but not seeing.

The compromise I made with the engineer I mentioned above was that if he didn't look at orthogonal loads he couldn't use the 1.33 factor for columns. In all fairness, I don't thick you can consider all the load combinations if you do your calcs by hand.

Thanks,
-Mike

 

[dbuzz]

If the structure and the loads are symmetrical surely you can reduce the number of load combinations.

 

[Tomfh]

Do you guys work out different wind speeds for each direction, or do you just take the worst speed and use it everywhere?

 

[JAE]

Tomfh - if you are referring to the fact that ASCE 7 has different wind forces depending on the "long" side and "short" side lengths of the building, then we usually generate the wind loads for both (with a spreadsheet) and then if they are not too far apart, simply use the higher one. If there is a larger difference, then we reconsider that, but usually we just use the larger.

We also have begun generating the larger number of load combinations referred to above. However, there are many times in simpler structures where you can observe the ones that control and the ones that logically will not control. This helps reduce them somewhat. But technically, the way the codes are written, you do have a large number that are required.

This also reminds me of a conversation I had with a professor (who I think sat on the ACI column committee once) - I was complaining about the Chapter 10 delta (moment magnification) factors and how they were getting quite cumbersome. He said that it was just a couple of delta factors to calculate, what was the problem? I answered that a pair of deltas would have to be generated with each and every load combination, which could amount to over 50 deltas depending on the number of LC required. I was surprised to see that he hadn't fully considered that each delta depends on the Pu which varies with each LC.

 

[mrMikee]

Tomfh:

The worst wind speed is the highest wind speed, isn't it?

I use the same wind speed to compute pressures and the resulting force on each face of the structure. Them worst column load is when the wind comes across the corners of the structure and one way to handle this effect in the US is shown in Figure 6-9 of the ASCE.

For example, you would check wind loads from the north at 100%, from the east at 100%, and then from the north-east by using 75% of the north and east combined.

-Mike

 

[mrMikee]

dbuzz and JAE:

On symmetric structures I use 11 load combinations and design for critical members. Other members are by inspection.

Another consideration I didn't mention is that the orthogonal effects for seismic aren't required in the lower seismic categories. So where I have 8 loads in the D+L+0.7E equation there would only be 4.

Thanks,
-Mike

 

[Tomfh]

I clearly should have provided more information!

I am in Australia and maybe our code is different.

In our code there are different wind speed multipliers for each of the 8 cardinal directions, i.e. N, NE, E, SE etc. The worst multiplier is 1, which you are free to use for all directions if you wish, since they are less than 1.

It is complicated to work out all the different directions, especially since you also have to take the worst value from +/- 45 degrees of whatever direction you happen to be interested in. E.g. If I am interested in NNE, I have to find the worst value in between ENE and NNW, based on an envelope defined by the original 8 cardinal direction multipliers.

Its a real nag. They always made us do it this was at uni, but now that I am working I find most people dont have any time for this system. Most people just use 1.0, unless they are in trouble and need to justify a lower pressure. Typically you might reduce pressures by 10% or so in a few directions if you go through this system.

I was asking if you guys bothered doing this, but perhaps your code doesnt have such a system...

Tom

 

[mrMikee]

Tomfh:

Thanks for the input. Your code looks like a real headache. Most of the US uses the IBC (International Building Code) which in the case of wind loads pretty much defers to the ASCE 7 Standard (American Society of Civil Engineers.) I'm not familiar with anything in our codes that is like what you are talking about. I appears that your code is very explicit about this topic.

The older codes here used to say something like "design for winds from any horizontal direction" which to me meant from the worst direction. I think that many engineers would calculate wind forces on each face of the structure and leave it at that. The ASCE spec now has figure 6-9 that clearly shows how to determine the orthogonal (to borrow a word from seismic) wind loads. But still I think there is a lot of "we don't do it like that here" attitudes.

It ends up with a lot of load cases however, which is what prompted me to ask the question in this forum, to see if others have come to the same conclusion or not.

Thanks,
-Mike

 

[UcfSE]

Maybe what Tomfh describes is like the wind directionality factor Kd the ASCE 7 uses (0.85). It sounds like it to me. The ASCE 7 also allows us to check only 4 cardinal directions if the building is under a certain height restriction. I think that's been mentioned above. That's a few less combinations .

 

[JStephen]

In some circumstances, it will be fairly obvious what load cases are going to control the design, and you don't have to actually check every load case. Or you may be able to check every load combination, but only report those that govern design. In any case, don't be afraid to check umpteen load cases if that's what it takes.

 

[mrMikee]

I think my concern is more about designing for the proper load case than the extra work. The time it takes a computer to calculate P/A once or 26 times is not too significant. The real problem is the customer. If he is working with two engineers and finds out after a while that one of them is consistently designing 10% to 20% heavier than the other, he is probably going to give all his future work to the engineer with the lighter designs. After all, they both are PEs and must be qualified.

-Mike

 

[SWComposites]

This thread is rather amusing. 26 or 11 or whatever loadcases is nothing - just set up a spreadsheet or feed them into a FE model. In the aircraft industry we routinely have many, many hundreds of loadcases to deal with, and are very happy when we can boil them down to 20-30 critical cases for preliminary sizing.

 

[mrMikee]

Thanks all,

I appreciate your posts. It is reassuring to know that other engineers think about these issues too.

Regards,
-Mike

 

[J1D]

mrMikee,
I don't get why you have 8 combinations from each of last three types. For example, 0.6D+W (DL with wind), you have wind in two directions and plus/minus in each direction, it's 2x2=4

From my riew, after safe and reasonable estimate of loads, load combination is the most important step in structural analysis. It is not necessary to limit the number of combinations in the computer age. But for certain structural type, once we have experience about what is for what, the total number is not too many. For most civil/commercial/industrail buildings 10 to 30 should be good enough.

Unfortunately, our company doesn't unify the combinations, every engineer has his/her own interpretation and number of the combinations. You can find the nubmer is from a few to 70+.

 

[mrMikee]

J1D,

Like you mention in your post the wind could come from 4 directions. Let's assume these are the principle axes based on the plan view layout and are N, S, E or W. Since the wind could come from "any horizontal direction" so you also need to check NE, NW, SW, and SE for a total of 8 directions for each load combination. Figure 6-9 in ASCE 7 shows a way to do this. The same logic holds true for seismic loads too and are called orthogonal loads because you are adding the effects of forces from two orthogonal directions. In the lower seismic categories you are allowed to neglect these effects.

As indicated above I came up with a general case of 26 load combinations. If you have a symmetric structure with a symmetric layout out of applied loads you can reduce some of these cases. I think 11 load combinations would work out well in some situations. On the other hand special conditions such as counteracting loads, alternate span loading, etc. could significantly increase load combinations. I am currently looking at a project that will have two bins supported on a single structure and anticipate that there will be about 64 load combinations.

I agree with you that the consideration of load combinations is very important. My work mostly involves design of elevated bins, structures that are relatively high compared to base dimensions. The column forces due to the overturning effect of wind and seismic forces are considerable and underestimating these by 30% is a significant oversight. This is not a trivial thing.

My complaint is not the number of load cases now that computers are widely used.

My concern is the safety of structures and orthogonal loads from wind and seismic are important.

Regards,
-Mike