Concurrent Active and Counteractive Dead Load 3

[infinitely_rigid]

Hello,

This is a Canadian Building Code LSD (NBCC/OBC) related question, but I imagine there might be similar confusion in ASCE, IBC etc.

When you have a scenario when there are both active and counteracting dead loads, does your load combination become 1.25Dactive + 0.90Dcounter + 1.5L etc.

I don't think the building code explicitly states this, but I assumed it was implied.

An example of where this happens all the time is Metal Building System (PEMB) foundation design. Gravity loads drive horizontal thrust from the frames, i.e. active Dead Load. Footing Self-Weight, Soil Overburden would represent counteractive Dead Load. The active load in this scenario is theoretically both driving and counteracting, however one can't exist without the other so, I don't feel particularly obliged to split out the horizontal and vertical load into counteracting and active. The Foundation self-weight on the other hand is purely counteractive from a OT/Sliding standpoint, but for all the other limit states, would likely be considered active as it would increase bearing on the underside of footing etc.

Any opinions on this?

 

[HTURKAK]

I do not know Canadian Building Codes and i have experience with Eurocodes . Regarding the terms active and counteracting dead loads, EC uses favorable and unfavorable dead loads. The favorable and unfavorable dead loads are not additive , and shall be used for different load combinations .
Some common EC combinations listed below Just to give an insight ;

- ULS Strength design favorable Dead Load combination ; 1.0Gk +1.5Qk
- ULS Strength design UNfavorable Dead Load combination ; 1.35Gk +1.5Qk
- ULS Static equilibrium, overall stability design favorable Dead Load combination ; 0.9Gk +1.5Qk
- ULS Static equilibrium, overall stability design favorable Dead Load combination ; 1.1Gk +1.5Qk
- Geotechnical design favorable/unfavorable Dead Load combination ; 1.0Gk +1.3Qk

Common practice for (PEMB) geotech . foundation design combinations at least in my zone ; 1.0Gk +1.0Qk and 1.0Gk +1.0Qk + 1.0W

EDIT; Sorry for this. I have corrected the ULS Static equilibrium, overall stability design favorable and unfavorable Dead Load combinations. Simplifying with copy and paste then forget to do the necessary corrections is a common problem for my age group .

 

[hardbutmild]

It makes no sense to split the loads from the same source. That is also why in HTURKAK's post you don't see it being split - this is specifically stated in eurocodes and I guess it might be somewhere in every code (somewhere at the beginning where general stuff is).
I would define active and counteracting for every limit state independently, but inside a single limit state check one source can either completely be active or completely counteracting.

 

[Tomfh]

The question of how to factor a load that simultaneously destabilises and stabilises has long been a topic of heated debate.

In my view, the correct approach is to factor the load up for its destabilising effects (LOAD) and down for its stabilising effects (RESISTANCE), as you appear to be proposing.

Critics of this method often argue that load factors are simply meant to address statistical uncertainty in loads and resistance. Since the same load is involved, they claim there’s no statistical uncertainty—it’s a known quantity—and therefore, it shouldn’t be factored up and down simultaneously.

My objection to that is it overlooks a critical purpose of these factors. These factors (e.g., 1.25, 0.9 etc) are not solely intended to account for statistical variance; they also ensure a margin of safety by separating the load and resistance. If you don’t factor the load up for its destabilising effect and down for its stabilising effects, that separation disappears. As a result, the structure is at the point of instability at the outset, at least as far as that particular load is concerned.

 

[Smoulder]

Stability limit state you increase destabilising part of load and decrease stabilising part.

Strength limit state you use the single source principle. Either factor it all up or all down.

 

[Cpw628]

Interesting discussion, I have not heard about this idea before. If anyone has done it with IBC or ASCE 7 load cases I would be interested in how you did it

 

[Smoulder]

Eurocode splits load for both stability and strength. EN1990 6.4.3.1(4)

(4)P Where the results of a verification are very sensitive to variations of the magnitude of
a permanent action from place to place in the structure, the unfavourable and the favourable
parts of this action shall be considered as individual actions.
NOTE This applies in particular to the verification of static equilibrium and analogous limit states, see 6.4.2(2).

 

[infinitely_rigid]

Thanks for the replies everyone!

Thanks for the Eurocode reference @HTURKAK and @Smoulder .

It appears the Eurocode addresses this in a manner similar to what I'm describing.

EN 1990-6.4.3.1(4):

Where the results of a verification are very sensitive to variations of the magnitude of a permanent action from place to place in the structure, the unfavourable and the favourable parts of this action shall be considered as individual actions.

Aha, so, the Eurocode tells us that we can split a load up based favourable and unfavourable actions. Which are actually defined by the Gamma G notations under Eurocode.

EN 1990-6.4.3.1(5):

Where several of one action (e.g. bending moment and normal force due to selfweight) are not fully correlated, the partial factor applied to any favourable component may be reduced.

Wow, this is even better, now the Eurocode is telling us if the permanent actions are uncorrelated, we can reduce the effects of the favourable actions. This seems to apply directly the the PEMB situation. Dead Load base reactions with both stabilizing and destabilizing effects are correlated, therefore factored together and we do not treat the favourable and unfavourable actions separately. The Self-Weight of a Footing for example would be an uncorrelated favourable action which we can reduce separately.

But wait, there's more, and my previous interpretation may be invalid.

EN 1990-Table A1.2(B), NOTE 3:

The characteristic values of all permanent actions from one source are multiplied by γG,sup if the total resulting action effect is unfavourable and γG,inf if the total resulting action effect is favourable. For example, all actions originating from the self-weight of the structure may be considered as coming from one source; this also applies if different materials are involved. See however A2.3.1(2).

Is this to say, all self-weight in a structure is the same source, including that of the footings? Therefore those actions are not split out, regardless of whether they are favourable or unfavourable while arguably being uncorelated? Meaning the only time permanent favourable and unfavourable actions are split out are when distinctions are made between structure self-weight, superimposed dead loads, collateral dead loads etc.

Going back to the PEMB example, does this mean the following sources can have γG,sup & γG,inf applied individually to each as required under Eurocode (if you're designing the footing for example)?:

• Source A: Building self-weight, footing self-weight;
• Source B: Building collateral load (M/E, finishes etc.); and,
• source C: Soil self-weight.

Or, have I completely read the Eurocode wrong?

I should also clarify that this Note 3 is under Table A1.2(B), which are combinations applied to STR/GEO, or member resistance and geotechnical limit states. I imagine Overturning/Sliding limit state belongs to Table A1.2(A) EQU combinations, which has no Note 3 with this clarification. Thus, simultaneous favourable and unfavourable permanent action is allowed for the EQU limit states.

This also seems to depend on the approach taken under A1.3.1(5).

 

[human909]

Going back to the PEMB example, does this mean the following sources can have γG,sup & γG,inf applied individually to each as required under Eurocode (if you're designing the footing for example)?:

• Source A: Building self-weight, footing self-weight;
• Source B: Building collateral load (M/E, finishes etc.); and,
• source C: Soil self-weight.

Or, have I completely read the Eurocode wrong?

Pretty much. Though I don't see the pressing need to separate A and B in this case. The Eurocode makes a common sense approach of considering favourable and unfavourable loads explicit other codes are less explicit. You still just need to use good engineering judgment on what is favourable and what is unfavourable and also any additional combinations that may be rationally required.

The AS code quite clearly says this though leaves it for the engineer to figure out:

 

[canwesteng]

The best analogy to using eurocode is like cutting a load of bread with a scalpel. It gives the illusion of precision at the cost of great pains. If you were to go back to the old ASD ways, you might factor all loads with 1 and need a factor of safety of 1.5. How do you achieve that if the stabilizing and destabilizing component of the PEMB dead is factored the same if you use LFRD (in some fictional case where PEMB dead is enough for overturning)?

 

[human909]

The best analogy to using eurocode is like cutting a load of bread with a scalpel. It gives the illusion of precision at the cost of great pains. If you were to go back to the old ASD ways, you might factor all loads with 1 and need a factor of safety of 1.5. How do you achieve that if the stabilizing and destabilizing component of the PEMB dead is factored the same if you use LFRD (in some fictional case where PEMB dead is enough for overturning)?

I don't understand this criticism, this reads heavily of "back in my day".

It really isn't much effort to suitably factor you loads in an appropriate LFRD fashion. I don't see how the Eurocode is "is like cutting a load [sic] of bread with a scalpel".

How do you achieve that if the stabilizing and destabilizing component of the PEMB dead is factored the same if you use LFRD (in some fictional case where PEMB dead is enough for overturning)?

Easily.

 

[canwesteng]

Well I'd say EC are quite good at making simple things complex, and not always in the spirit of making them more accurate.

In the hypothetical case we pretend the footing is weightless, I'm not sure how you easily achieve a factor of safety of 1.5. The only loads on it are the dead load (vertical and thrust) from the PEMB. If the resisting moment and overturning moment are exactly equal, and you apply the same factor to them, then your factor of safety is 1 (old way), or your reliability index (LFDR) is below 1. In this case, some variation in frame/joint stiffness, building alignment, actual location of dead loads, etc... will create variability in the ratio of thrust to vertical load.

This is taken to the extreme and it's not likely that the building dead is such a large component of the resisting moment in reality, and if you wanted to write a thesis on it, you could probably justify meeting the code mandated reliability without factoring dead from the PEMB down, and I'm sure most people would just accept it as ok anyway.

 

[Tomfh]

If the resisting moment and overturning moment are exactly equal, and you apply the same factor to them, then your factor of safety is 1 (old way), or your reliability index (LFDR) is below 1.

This is why I agree with increasing it for overturning and decreasing it for resisting. That way you maintain the same reliability index that ASD established and that the LRFD factors were designed to match. Using the same factor for both overturning and resisting cancels all that out, reducing the reliability index.

But you say, in reality it’s unlikely to be a critical thing.

 

[human909]

Well I'd say EC are quite good at making simple things complex, and not always in the spirit of making them more accurate.

In the hypothetical case we pretend the footing is weightless, I'm not sure how you easily achieve a factor of safety of 1.5.

You seem to be making this simple factoring of loads complex.

And I'm not sure why you are chasing this hypothetical ASD FOS of 1.5 in a LRFD world.

I would comment more about your hypothetical but I can't understand you description. I does seems though that you aren't considering correlated loads. You don't reduce one favourable load and increase another unfavourable if they are the same load. That is illogical and EC doesn't advocate that.

 

[HTURKAK]

The original thread was for Concurrent Active and Counteractive Dead Load and now turned to the discussion of Eurocodes specially EN 1990.

Is this to say, all self-weight in a structure is the same source, including that of the footings? Therefore those actions are not split out, regardless of whether they are favourable or unfavourable while arguably being uncorelated? Meaning the only time permanent favourable and unfavourable actions are split out are when distinctions are made between structure self-weight, superimposed dead loads, collateral dead loads etc.

No..not really ..In this case , the dead weight of the footing favorable for the OT and sliding stability check so factored by 1.0. Sometimes it is better to provide a worked example to see the case, ( Copy and paste from STEEL DESIGNERS’ MANUAL, The Steel Construction Institute, by Buick Davison)

If you want more info, i will suggest you to look (DESIGNERS’ GUIDE TO EUROCODE: BASIS OF STRUCTURAL DESIGN EN 1990 by GULVANESSIAN ).

 

[Smoulder]

Think I understand @CanWestEngs idea. Its fiction but should ignore that because simplifying things can show basic issues clearer. Fictions are 1) only dead load applies to building and 2) footing is weightless. Case is vector of column base reaction goes through corner of footing base. OK for LRFD if loads are factored but not for ASD because FOS=1.0<1.5. If all dead load is factored same then vector direction doesn't change and passes LRFD check. Doesn't matter if factor up or factor down. But LRFD should factor up load near ridge which is destabilising and factor down load near column which is stabilising. Bit of a pain. I would do the ASD check for convenience even though not exactly code wording. Don't think many checkers would have a problem.

 

[human909]

Think I understand @CanWestEngs idea. Its fiction but should ignore that because simplifying things can show basic issues clearer. Fictions are 1) only dead load applies to building and 2) footing is weightless. Case is vector of column base reaction goes through corner of footing base. OK for LRFD if loads are factored but not for ASD because FOS=1.0<1.5. If all dead load is factored same then vector direction doesn't change and passes LRFD check. Doesn't matter if factor up or factor down. But LRFD should factor up load near ridge which is destabilising and factor down load near column which is stabilising. Bit of a pain. I would do the ASD check for convenience even though not exactly code wording. Don't think many checkers would have a problem.

Thanks for explaining the scenario in a clear fashion. That makes sense but it wouldn't be a common situation. Many structural codes have moved on and don't have two systems side by side, so the desire to meet some arbitrary 1.5 FOS isn't particularly applicable.

However if you did encounter a circumstance similar to this fictitious situation then good engineering judgement should be applied. Structural codes are a minimum set of guidelines and will never cover everything.

 

[canwesteng]

Sure, you don't need to meet a FOS anymore, but you should have a sufficient reliability index to resist overturning. That is the fundamental of LFRD design, but can be harder to grasp, so FOS is a good illustration. If you want to apply the same factor to the horizontal and vertical load, show how it could achieve the code desired reliability.

 

[Tomfh]

Canwestent is right in principle: when you factor dead load up and the resistance it provides down, you’re applying the same safety margin that ASD relied on, which LRFD was specifically calibrated to match. ASD is essentially baked into LRFD. For example, a dead load factor of 1.35 and a resistance factor of 0.9 gives 1.35/0.9 = 1.5. Similarly, 1.2G + 1.5Q reflects the same logic. This isn’t a coincidence—it’s rooted in ASD principles.

Ignoring this logic, such as using a flat factor of 1.0 for both a load and the resistance it provides, undermines the intended safety margin.

 

[human909]

Canwestent is right in principle: when you factor dead load up and the resistance it provides down, you’re applying the same safety margin that ASD relied on, which LRFD was specifically calibrated to match. ASD is essentially baked into LRFD. For example, a dead load factor of 1.35 and a resistance factor of 0.9 gives 1.35/0.9 = 1.5. Similarly, 1.2G + 1.5Q reflects the same logic. This isn’t a coincidence—it’s rooted in ASD principles.

There seems to be a prevalent insistence that ASD to "RIGHT and CORRECT" and LRFD is a fudge to match ASD. While in some case in the US that might be the case, in others and other standards that may not be. Time moves on and as does advances in codes. Various tweaks are made which increase or decrease capacity as new understanding comes to light. Eventually LRFD become rooted in LRFD principles and the continued call back to ASD is a misnomer.

In the code that I use for example the dead load factors are 1.2G & 0.9G. None of this magical FOS of 1.5 that keeps getting referred to.

Ignoring this logic, such as using a flat factor of 1.0 for both a load and the resistance it provides, undermines the intended safety margin.

The intended safety margin is the safety margin given by the relevant code. Claiming that the Eurocode undermines the intended safety margin by referring to a US ASD code is absurd.