This is a super cool article and discussion. As a structural engineer myself, we use a lot of this sort of stuff. Static loads are usually 1.2 factor, ie, weight of the building/materials... live loads... like wind, crowd, snow and other things are 1.5 typically.
But then there is an entire analysis set to get the loadings.... its all probability loading, and working out where the communal risk is worth the extra cost to upgrade and reinforce the loading. We typically use what is misnomer a '1 in 100 year' storm for houses, but maybe a '1 in a thousand years' for a hospital. the 1 in 100 year storm is 1% chance of the design parameters being exceeded in any 1 year.... but in a lot of locations, we do not have accurate wind loading data for 100 years, or even 30 years. So they take the loading data from hundreds of locations nearby, assign factors that say this site is similar to that site, and do huge statistical analysis to work out what those 1% storms are.
But then you have earthquake design, and the strong column, weak slab design... which basically says we acknowledge that there will be the possibility of an earthquake stronger than we can ever possibly economically design for, and design so the failure mechanism mitigates the damage as much as possible... ie, strong column means the slabs fail first... if the columns fail first, all the slabs end 1 inch apart, and noone can survive... strong column means that there is voids left that there is a potential to survive.
And don't let me get started on materials. concrete works, we do know how exactly, only empircally.
This is a famous quote, and it is very accurate:
Structural Engineering is the Art of molding materials we do not wholly understand into shapes we cannot precisely analyze, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance.
It mirrors my efforts at trying to find the basis for safety factors in the Oil and Gas field. The results are similar (SF combination of empirical work / guesstimate), and the outcome is the same: engineers are moving to probabilistic design approaches to help estimate known risks. This is more useful for justifying the use of ageing infrastructure. Instead of telling the regulator that "we used a SF of 2 with a design life of 40 years, and we want to operate for an extra 10 years, so our safety factor is now <x>" you can instead say "if we continue operating for 10 years we have calculated a risk of failure of ~<y> %. Is this acceptable"
Hey Surjan, it's chuie. A fascinating read as usual, and I appreciate all the research you did into this topic. I was surprised you didn't cover the significance of Aluminum to the justification of 1.5 safety factor in aerospace. I had always heard that 1.5 is the difference between yeild and ultimate for aluminum. Anywho, that's neither here nor there. I think there should be more open discussions on the safety factor. because I think everyone is counting on it for different reasons, and that could lead to taking more risks than intended..
As a loads engineer, I try and ignore the safety factor when providing design loads. I love the idea of the probabilsitc approach to design, but it is the road less traveled and pretty expensive.
Good article. Knowing the type of loading and probable failure modes greatly affect the factor of safety. Ductile materials can accommodate higher surface stresses due to bending, and compressive membrane stresses through slender section could result in buckling at stresses below yield. Fatigue and creep failure modes also need to be considered. Composite material designs are highly dependent on manufacturing processes which need to be characterized by extensive testing to determine statistically validated properties. All of these factors go into determining a lower factor of safety that optimizes the structural integrity of a product, which often doesn’t come cheap.
Probabilistic design has been around for a long time.
1963 for reinforced concrete design and 1986 for steel design.
Today's structural engineer will typically use an ultimate strength design, which is based on the load probabilities and an explicit safety factor.
The approach addresses the grouping of multiple types of loads, each having its own load duration, timing and potential for overload. Different factors are incorporated for each type of load. More predictable loads (e.g., dead load) have a lower load factor, while more variable loads (e.g., live, wind or snow) have a higher load factor.
The second modification introduces a “resistance” or “capacity reduction” factor to downgrade the theoretical (nominal) capacity of a structural element to account for variation in material, analysis/design assumptions and equations, fabrication, and erection.
But most structural engineers will then introduce an additional safety factor beyond the explicit safety factors in the relevant design codes, perhaps to satisfy another design code requirement (e.g., ASME over the head lifting device SF of 3) or to simply feel more comfortable with the design.
It depends per sector, and what the performance penalty of weight is. When I was in engineering school, the normal safety factor for general mechanical engineering was 3, while for "heavy" engineering like cranes, 10 was used. The lower you want the safety factor to be, the better you have to understand the loading in your application.
Really nicely written article, thanks Surjan! (Nice stabilizer, too!).
You've articulated a fascinating and expensive blind-spot in engineering; now I'll wait patiently for one of the big CAD providers to create a plugin to calculate some AI-Optimized Probabilistic Safety Factor Calculator, which would take the insane tedium out of the data-gathering you're talking about.
I guess you should also integrate the outcome of a particular failure into account, not just the probability.
For instance, the air conditioning LED of a car does not matter as much as the ABS system; don't lump them both in "electronics failure" and have the same threshold/margin.
Engineering and design are often at odds. Materials used and how they are used muddy up the engineering while design often shunts materials and cost/benefit off onto someone else. Brutalist architecture simply loaded more and more concrete and steel into buildings to make them work with little attention to esthetics. The Eiffel Tower, though subject to the uncertainties of iron casting of it's time and ridiculed for being a waste of space and materials, nevertheless retains an airy elegance due to the harmonies of design and engineering. I think of the brute force engineering that produced cars that could survive high speed (for their time) impacts by loading them with heavy steel frames compared with race cars that use more expensive and more intricate bird nest structures made from smaller tubes of alloy steel and aluminum.... And then there is the Volkswagon "beetle" which combines a thin steel unibody bolted onto a frame that is strengthened by a simple steel tube running the length of the car. This saved weight, increased mileage, cut construction costs and made the light weight cars extremely durable in surviving impacts with much larger and much heavier cars. A simple formula for costs vs. benefits is a good place to start but it is probably impossible to factor in all the possible materials available and the ways in which they can be joined. Those first iron bridges in England were made from sections cast to fit together with tongue and groove and dovetail joints just as wooden bridges had been made.... Because it was what bridge builders understood. They may have been over engineered but they are still standing despite the tremendous changes in the kinds of loads they support now compared to when they were constructed.
This is a super cool article and discussion. As a structural engineer myself, we use a lot of this sort of stuff. Static loads are usually 1.2 factor, ie, weight of the building/materials... live loads... like wind, crowd, snow and other things are 1.5 typically.
But then there is an entire analysis set to get the loadings.... its all probability loading, and working out where the communal risk is worth the extra cost to upgrade and reinforce the loading. We typically use what is misnomer a '1 in 100 year' storm for houses, but maybe a '1 in a thousand years' for a hospital. the 1 in 100 year storm is 1% chance of the design parameters being exceeded in any 1 year.... but in a lot of locations, we do not have accurate wind loading data for 100 years, or even 30 years. So they take the loading data from hundreds of locations nearby, assign factors that say this site is similar to that site, and do huge statistical analysis to work out what those 1% storms are.
But then you have earthquake design, and the strong column, weak slab design... which basically says we acknowledge that there will be the possibility of an earthquake stronger than we can ever possibly economically design for, and design so the failure mechanism mitigates the damage as much as possible... ie, strong column means the slabs fail first... if the columns fail first, all the slabs end 1 inch apart, and noone can survive... strong column means that there is voids left that there is a potential to survive.
And don't let me get started on materials. concrete works, we do know how exactly, only empircally.
This is a famous quote, and it is very accurate:
Structural Engineering is the Art of molding materials we do not wholly understand into shapes we cannot precisely analyze, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance.
A fantastic read!
It mirrors my efforts at trying to find the basis for safety factors in the Oil and Gas field. The results are similar (SF combination of empirical work / guesstimate), and the outcome is the same: engineers are moving to probabilistic design approaches to help estimate known risks. This is more useful for justifying the use of ageing infrastructure. Instead of telling the regulator that "we used a SF of 2 with a design life of 40 years, and we want to operate for an extra 10 years, so our safety factor is now <x>" you can instead say "if we continue operating for 10 years we have calculated a risk of failure of ~<y> %. Is this acceptable"
Hey Surjan, it's chuie. A fascinating read as usual, and I appreciate all the research you did into this topic. I was surprised you didn't cover the significance of Aluminum to the justification of 1.5 safety factor in aerospace. I had always heard that 1.5 is the difference between yeild and ultimate for aluminum. Anywho, that's neither here nor there. I think there should be more open discussions on the safety factor. because I think everyone is counting on it for different reasons, and that could lead to taking more risks than intended..
As a loads engineer, I try and ignore the safety factor when providing design loads. I love the idea of the probabilsitc approach to design, but it is the road less traveled and pretty expensive.
Awesome post, keep writing!
Theme Park overhead safety factor - 20:1.
Good article. Knowing the type of loading and probable failure modes greatly affect the factor of safety. Ductile materials can accommodate higher surface stresses due to bending, and compressive membrane stresses through slender section could result in buckling at stresses below yield. Fatigue and creep failure modes also need to be considered. Composite material designs are highly dependent on manufacturing processes which need to be characterized by extensive testing to determine statistically validated properties. All of these factors go into determining a lower factor of safety that optimizes the structural integrity of a product, which often doesn’t come cheap.
Probabilistic design has been around for a long time.
1963 for reinforced concrete design and 1986 for steel design.
Today's structural engineer will typically use an ultimate strength design, which is based on the load probabilities and an explicit safety factor.
The approach addresses the grouping of multiple types of loads, each having its own load duration, timing and potential for overload. Different factors are incorporated for each type of load. More predictable loads (e.g., dead load) have a lower load factor, while more variable loads (e.g., live, wind or snow) have a higher load factor.
The second modification introduces a “resistance” or “capacity reduction” factor to downgrade the theoretical (nominal) capacity of a structural element to account for variation in material, analysis/design assumptions and equations, fabrication, and erection.
But most structural engineers will then introduce an additional safety factor beyond the explicit safety factors in the relevant design codes, perhaps to satisfy another design code requirement (e.g., ASME over the head lifting device SF of 3) or to simply feel more comfortable with the design.
Loved this, I chuckled a few times while reading it. The resistance to probabilistic design is so strange to me!
It depends per sector, and what the performance penalty of weight is. When I was in engineering school, the normal safety factor for general mechanical engineering was 3, while for "heavy" engineering like cranes, 10 was used. The lower you want the safety factor to be, the better you have to understand the loading in your application.
Really nicely written article, thanks Surjan! (Nice stabilizer, too!).
You've articulated a fascinating and expensive blind-spot in engineering; now I'll wait patiently for one of the big CAD providers to create a plugin to calculate some AI-Optimized Probabilistic Safety Factor Calculator, which would take the insane tedium out of the data-gathering you're talking about.
What are your weather limits? I'm an aviation forecaster and just curious what your criteria are.
I guess you should also integrate the outcome of a particular failure into account, not just the probability.
For instance, the air conditioning LED of a car does not matter as much as the ABS system; don't lump them both in "electronics failure" and have the same threshold/margin.
12345
Engineering and design are often at odds. Materials used and how they are used muddy up the engineering while design often shunts materials and cost/benefit off onto someone else. Brutalist architecture simply loaded more and more concrete and steel into buildings to make them work with little attention to esthetics. The Eiffel Tower, though subject to the uncertainties of iron casting of it's time and ridiculed for being a waste of space and materials, nevertheless retains an airy elegance due to the harmonies of design and engineering. I think of the brute force engineering that produced cars that could survive high speed (for their time) impacts by loading them with heavy steel frames compared with race cars that use more expensive and more intricate bird nest structures made from smaller tubes of alloy steel and aluminum.... And then there is the Volkswagon "beetle" which combines a thin steel unibody bolted onto a frame that is strengthened by a simple steel tube running the length of the car. This saved weight, increased mileage, cut construction costs and made the light weight cars extremely durable in surviving impacts with much larger and much heavier cars. A simple formula for costs vs. benefits is a good place to start but it is probably impossible to factor in all the possible materials available and the ways in which they can be joined. Those first iron bridges in England were made from sections cast to fit together with tongue and groove and dovetail joints just as wooden bridges had been made.... Because it was what bridge builders understood. They may have been over engineered but they are still standing despite the tremendous changes in the kinds of loads they support now compared to when they were constructed.