Duration of Loading

[dik]

I don't do a lot of wood design, but occasionally need to connect guards or handrails to wood structure. I've been using values based on CSA O86, the Canadian Wood Code and treating the load duration as Normal. Since it is unlikely that any loading for these elements would be sustained. Should I be using short term duration for design of these fasteners?

thanks...

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Pmtottawa]

I always use short term duration for these items.

 

[jayrod12]

I agree, short term for guardrail loading. And wind loading. In the case of guardrails you'd likely have a hard time getting anything "typical" to work otherwise. In my experience, mounting guardrails to wood requires an extremely sharp pencil and some assumptions of load sharing.

 

[phamENG]

A lot of people use short term loading for this. I do NOT use short term. The reasoning is this: it's considered a live load. NDS recommends live loads get 1.0. That's the justification I use when questioned by non-engineers who know enough to be dangerous. For engineers, I use this reasoning: It's 200lbs at any point or 50plf. What does it take to get to 50plf? Not a whole lot. Also, it's not practical to assume you're going to have a 50plf impact load along the entire length of the guardrail. It is, however, reasonable to assume you'll have a crowd of people leaning on a rail - especially if there's a view (and consider that what is a view to you may be different for somebody else). Using 1.6 as a duration factor under the NDS assumes that you'll have 10 minutes of that load over the entire life of the structure. So if you're certain that for the 30-50 years that guardrail will be in service you'll have a TOTAL of 10 minutes of people leaning on it, be my guest - design for 1.6. I won't.

 

[driftLimiter]

I agree with PhamENG that at least for ASCE7/NDS provisions, the load is a Live Load, the NDS is clear about duration factor for that. However, I can see why there would be some debate about the actual duration of these loads. The floor live loading duration factor of 1.0 equates to 10 years duration at the maximum load. Hard for me to believe that if you add up all of the times someone almost falls over a handrail in a buildings lifespan that it will equal 10 years.

 

[dik]

Thanks... just as confused. I'll think about this for a bit. Reason for not reducing is that it's a safety issue. I'm not sure of the NDS logic... even live loads can be short duration. I'll talk to my timber guy and see what he thinks... Thanks.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[phamENG]

dik - the the duration is just that: the total cumulative time over the life of the structure/member that it sees the full design load. So the idea is that, for a live load, it will see full loading for up to 10 years of it's life. So if it's a 50 year structure, it can only see full dead and live load for up to 10 cumulative years over that span before you may start seeing long term loading issues.

drift - I agree that it's unlikely to see full load for a full 10 years, but given the shape of that function, I'm hesitant to reduce it. At 1.15, you're already down to 2 months.

 

[dik]

pham... I suspect strongly that it will never see the design load in it's lifetime... I'm not sure why it's cumulative; I've understood that damage to wood in a slightly overload condition is somewhat time dependent, at the time of application... this damage could be cumulative. I don't know if loading would be cumulative. Problem is we don't know.

I haven't heard back from Vic... until I do, I'll treat it as normal duration, as I usually do, just to be safe.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[driftLimiter]

This cumulative duration thing seems counterintuitive to me as well. How does the wood know how long a load was applied in the past? I always thought the material was better against shorter duration of applied load, not cumulative though.

I suppose if you come form the other side of things start at permanent then work your way up it makes a bit more sense.

I think we've talked of this before in the past but does anyone have access to or know any of the research that led NDS to use a duration factor in the first place? Would be interesting to see how this plot was developed.

End of the day I hate doing handrails because they have the trifecta; small components, high life safety risk, perceived as simple/easy/cheap by public.

 

[dik]

I do maybe 20 or more handrails a month... and have numerous SMath programs to deal with them... from mounted wall plates to connections to an HSS or pipe post... maybe 20 programs. They are a piece of cake once you get into them. Problems with using Lag type fasteners and usually suggest other fasteners with flat head. Lag screws are OK for the base... but, still try to get away from that.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[phamENG]

It brings about a failure called 'creep rupture'. If you think about it, creep in wood is the result of long term, time dependent elongation of the wood fibers. Under load, wood will deflect elastically and then start to creep. If the load is large enough and applied long enough, those fibers will eventually break. So you can load it a lot for a short time without creep and the resulting elongation and rupture, but if a load sits on it for a long time it will eventually fail. The longer the load is applied, the lower the allowable stress to prevent it. That's why a dead load (applied in perpetuity) is 0.9, while an impact duration is 2.0. A true impact is going to be much less than a second (usually) and is going to be rare...so you may get several impacts before you reach your cumulative 2 seconds.

For some reading, check out the USDA/US Forest Service's Wood Handbook: Wood as an Engineering Material (Chapter 5 has information on Creep, Duration of Load, and Creep-Rupture). Breyer's Design of Wood Structures also has some information in Chapter 4, but mostly defers to the Wood Handbook for detailed explanation.

 

[driftLimiter]

Understood thanks for the additional reading. Its interesting that they were able to break it out in terms of different duration loading events. I suppose its fancy statistics that gives a good result out of this. I was thinking that the creep deflection is constantly accumulating on wood, regardless of the source of loading. It is interesting that the result of using Cd can account for that. Like an a major wind event occurring at the beginning of the structures life, wouldn't expect there to be much accumulated creep.

However the same event occurs near the end of the structure's life, we would expect more accumulated creep, yet we still have the same duration factor. Seems counterintuitive to me but interesting indeed.

 

[phamENG]

No problem. It's all about time spent at peak load. That's why we apply load duration factors based on the largest Cd for a given load combination.

D Cd=0.9
D+L Cd=1.0
D+.75(L+Lr) Cd=1.25
D+.75(L+S) Cd=1.15
D+0.6W Cd=1.6

For each load combination, our design load is our "peak load"...so we're adjusting the max allowable stress based on the cumulative time we expect it to be at that load. So even though we check downward wind loading with full dead load, we only expect that full downward wind load to exist for 10 minutes or less over the life of the structure. So dead load goes along for the ride in that check. But when we're just looking at dead load, it's always going to be there so it gets the 0.9 to reduce the allowable from the 'normalized' allowable stress.

I'm sure the sequence of various load levels does play a role in the actual behavior, but as with all things in wood design, this covers it well enough - especially within the typical lifespan of a structure.

 

[driftLimiter]

Yes that make since. If the LC with DL controls we penalize the allowable stress, and from there we go up the curve.

 

[Mike Mike]

Thanks for the lesson phamENG. I didn't realize it was cumulative, despite it being clearly explained right in the main NDS, 2.3.2.

Lori Koch at AWC said use 1.6 for railings in a webinar a couple years back.

There are a couple other factors to consider (just my opinion here, not necessarily worth a squirt of piss):
Is this in a crowded museum? 1.0
Is this in a restricted facility with almost no foot traffic? 1.6
Is this a rail for tourists on top of the grand canyon preventing a mile drop? 1.0
Is this just to denote where people are supposed to stand in line at the airport, all flat floor with no potential life safety issues? 1.6

Does that make it all clear Dik? If it helps, just think of it as a singularity of death. Feel better now?

 

[dik]

Thanks, gentlemen...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik