Concurrent Active and Counteractive Dead Load 2

[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).