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?

 

[Tomfh]

We were discussing a hypothetical dead load-only case with a factor of 1.35G, but if you’d rather consider a 1.2G + 1.5Q scenario, it works similarly and gives the same general overall factor of safety, of around 1.5 or thereabouts. LRFD factors tweak the overall number slightly but the guts of ASD is still there in terms of providing a minimum level of fat to the design.

The LRFD factors were explicitly designed to align with the ASD factors. Those ASD factors are the backbone of the system and are embedded in LRFD, whether acknowledged or not.

If you apply the same factor to both the destabilising force and the stabilising force, you have zero safety margin.

It’s simply an oddity of how LRFD is formulated.

 

[canwesteng]

LFRD is better, but instead of FOS it relies on a reliability index to ensure designs are safe, based on some statistical analysis. If restorative and counteractive dead loads have the same factor, how do you meet the reliability index that is the basis of the code?

 

[human909]

I disagree with the continued assertion that EU code is somehow flawed because it doesn't meet your expectations of a legacy US ASD approach.

The notion that LFRD is forever tied to ASD is fundamentally flawed. Engineering has been an evolving practice for hundred of years. It is continuing to evolve.

 

[Tomfh]

LFRD is better, but instead of FOS it relies on a reliability index to ensure designs are safe, based on some statistical analysis.

The bulk of that 'statistical analysis' was established by ASD. LRFD simply back-calculated the safety factors to ensure the resulting designs matched those produced by ASD, so as to match the overall statistical reliability established by ASD for normal structures.

There's a misconception that LRFD was invented from scratch in some advanced statistical lab. It wasn’t. The LRFD approach deliberately replicated the safety levels provided by ASD, with slight adjustments to account for material differences and specific load scenarios. At its core, the reliability built into LRFD is the same as that of ASD. They copied the overall reliability of ASD, because they knew ASD worked.

 

[TLHS]

 

[Smoulder]

Death patio is in Australian code commentary. Ok not quite but clearly spells out different load factor for overhanging dead load than backspan dead load even though Its a continuous beam.

 

[human909]

There's a misconception that LRFD was invented from scratch in some advanced statistical lab. It wasn’t. The LRFD approach deliberately replicated the safety levels provided by ASD, with slight adjustments to account for material differences and specific load scenarios. At its core, the reliability built into LRFD is the same as that of ASD. They copied the overall reliability of ASD, because they knew ASD worked.

I'm not sure anybody is advocating that. In the same way that I don't think anybody would advocate that ASD was inventor from scratch. What part of the phrase was unclear: "Engineering has been an evolving practice for hundred of years. It is continuing to evolve.".

Either way I wouldn't implement the Eurocode or the AS or the Canadian Building Code code to try to meet an legacy American ASD FOS of 1.5. I'm unsure why you are so desperate to argue that. Personally I would apply the code as written with suitable engineering judgement. (And by suitable judgment that could mean a FOS of 1.2, 1.5 or 2.0. Up to you. The code specifies a minimum, and codes don't specify all combinations of actions that might need to be checked.

 

[Tomfh]

It’s not about designing explicitly to ASD or rigidly adhering to a 1.5 factor—that’s not the argument here. The point is on maintaining the same general level of safety that ASD established, which remains embedded in LRFD today. At its core, it’s about upholding the fundamental principle of separating load and resistance.

When you apply the same factor to both the destabilising and stabilising effects of a load (like dead load), you effectively eliminate the factor of safety, leaving the structure “teetering” from the outset with respect to that particular load. Where dead load is predominant, this could theoretically be a problem. This is why the code commentary explicitly advises factoring up the destabilising effects of a load and factoring down the stabilising effects. That’s all I’m advocating for as well.