Do open Pinic Shelters qualify as Category I buildings? 1

[RARWOOD]

Open pinic shelters present an interesting design problem, when they are carrying their largest snow load, they are rarely occupied.

When calculating snow load, can open shelters, be classified as Category I cstructures. This would result in the use of a 0.8 importance factor.

 

[structuralaggie]

I wouldn't do it. There is that "low hazard to human life" clause I guess that you are looking at. I think it would be the same line of thinking to design beach houses as a category I structure because everyone should have evacuated when the hurricane is coming.

 

[MiketheEngineer]

I would stick with Cat II --- one could even make the argument - that large enough - it could hold over 300 people - say in a thunderstorm.... Think about that one.

 

[DaveVikingPE]

Bravo, MiketheEngineer! That's an excellent line of reasoning that, my opinion, couldn't be argued against.

That's the kind of justification that should be written in your design calcs next to your reference to choosing the occupancy category from ASCE 7.

I'm sure going to steal that logic for any similar type projects I do!

 

[csd72]

Not only the people inside the building, but if in a severe storm the building collapsed and parts of the roof started flying around, could you guarantee that it wouldnt go flying into peoples cars/houses.

I was called out to a site once where a piece of roof sheeting had ripped off in a wind storm and knocked over the neighbors brick chimney.

Good risk management to stay with category 2.

 

[RARWOOD]

Note: I posed the question regarding the use of a importance factor of 0.8 for snow load design. Wind load design is a very different situation. High winds are very common in the summer when pinic shelters are heavily used.

I am trying to develop some discussion about snow loads for pinic shelters with roofs only and no side walls. The vast majority of these shelters are from 500 sq. ft. to 1500 sq. ft. As the shelter size grows so that you are dealing with a much higher occupant load the design needs change.

Almost all of these shelters are never used in the winter for several reasons: Many parks are closed during the winter. It is very common for parks to remove the pinic tables from the shelters during the winter. Unenclosed shelters are of very little value for shelter during the winter because they offer no protection from the wind.

In the past this type of problem was solved by the use of engineering judgement. An old manual, used by my company defines engineering judgement "as the ability of an engineer to view his entire problem as a whole and to weigh the importance of the various limitations and conditions that must be considered in the design.

Today with ever changing design codes which attempt to more accurately describe the actual design loads and material behaviours, designs which have been successfully used for decades no longer will meet the design and building codes.

At the same time an engineers judgement is becoming greatly limited by liability and code officials who will not allow any deviation from the building code. It is not uncommon for engineers to design to the exact letter of the code, even when such designs are impractical, in order to avoid liability in the future.

 

[MiketheEngineer]

RARSWC -

Many of your points are well thought out....many old designs are hard to justify...but that is why they are "old".

However - the CODE is the CODE and we must live or die by it....people (crazy??) do picnic in the middle of winter... AND to try to defend yourself by not designing to the CODE --- GOOD LUCK!!!

25 years ago - hardly anything in this area needed a seal by a PE. NOW - almost everything does... that's why I send Xmas cards to all our local building officials... they enhance my income tremendously....

Unless you want to post a sign on your structures saying something like: "Do NOT use if there is snow on the roof"
... what are you going to do??

Good Luck!!

 

[structuralaggie]

Keep in mind too that when you design a structure for the loads as defined by the governing building code, you are doing precisely that, and not guaranteeing that the building won't fall down. There is always the possibility that these loads could be exceeded (although highly unlikely). Building code loads are historically based on loads that have occurred before. As time goes by, the code loading requirements will change as they have in the past. Our only recourse is to design with the governing building code’s loading as a minimum.

 

[csd72]

RERSWC,

I can see where you are coming from on your last comment. Engineers in the US have become a bunch of code watchers. Reminds me of lawyers always looking up the code book, but without the corresponding salary.

My experience in Australia and the UK is that designers are more willing/able to use engineering judgement to assess if the code is applicable in a particular case. The only time I have heard the term 'code violation' is in the US, elsewhere it is more acceptable to use alternative means to justify your work.

Miketheengineer:

how do people get to the picnic shelter if there is 4' of snow on the ground? - assuming the snow load is 4' or so.

 

[MiketheEngineer]

csd72 -

Snow load around here is only 20 psf (Missouri) -- a good wet snow of 12'' can excced this. In about 1982 - like the the second of April!! -- if you can believe that - we had about 14-16'' of wet snow in the southern part of the state - there were a number (100's) of roof failures.....

As for getting there in 4' of snow --- Weren't you ever a teenager... NOTHING ever stopped us from doing stupid stuff!!

As an engineer --- I often disagree with the code - esp when I can prove otherwise --- but like you point out - we are tied to the "lawyers"..... If you have ever had to stand before a judge and prove your point.... you get the idea!!!

And the CODE is a minimum -- I often exceed it when I feel it is justified

Good Luck

 

[structuralaggie]

RARWAC - I understand your reasoning, but why would you want to do this? I don't think you would be talking about that much more expense, unless you were building hundreds of these things.

 

[RARWOOD]

I want to thank every one for their comments. I have put some thought into this topic trying to solve one particular problem. The problem is the economic selection of solid wood roof decking for pinic shelters.

The company I work for sells anywhere from two to three pinic shelters a year to a dozen or more, mainly to cities and counties in Minnesota. Due to changes in the State building code and the NDS, shelter designs which have been used successfully for years, no longer comply with the code.

Code changes have not had a significant impact on the main structural elements of the shelter, in part because of the need for stiffnes in the elements to provide stability to the shelters. In addition to not wanting the shelters to blow over in a wind storm, you also do not want people trying to rock the shelters to see how much sway they can induce.

Code changes however have had a big impact on deck design. With deck the only ecconomical option generally is to change the layup pattern or reduce the span. The cost of 3" deck over 2" deck is so high that going to thicker deck is generally not ecconomically feasible.

The frustrating part is that wood decking spans are almost always controlled by deflection not strength. Right now there are hundreds of picnic shelters in out state Minnesota with 2 x 6 wood decking spanning 8'-0", which in the past were designed for 30 psf snow load.

Under the current code those same shelters must be designed with a basic snow load of 50 psf. And for shelter with a gable roof, an unbalanced snow load of 75 psf. The 75 psf load us required for the decking design.

The dilemma is that all those shelters out there which in the past have seen 75 psf of snow, didn't have deck that was overstressed, just deck that was deflecting a lot more than L/180.

 

[MiketheEngineer]

See IRC Table 1604.3

Roofs not supporting a celing is 1/120 using D+L. I assume your dead load can be shown to be about 2 or 3 psf. Also see notes D & G as shown in the table. This MAY help.

You definitely have the strength -- but deflection is a problem. I do pole barn designs and run into the same problem with purlins and girts.

You could also supply ALL the calcs to the local building official and he MIGHT waive the requirement -- but I doubt it.

A quick check using 50 psf, 2x6 flat, 1.6E - which is DF#1 on a simple 8' span shows a deflection of .85; - 1/113. I can live with that. Knock off about 3'' each side for bearing and now it is 1/137. ALSO - use the two span defelction equation if you are using 16' boards. That will help a lot.

Good Luck...

 

[structuralaggie]

I understand now. I remeber a conversation I had with a glulam manufacturer that had done a project that had glulam trusses in Dallas (I think). The city had just changed to the IBC from the UBC. Apparently one of the load combinations in the IBC caused his trusses to have uplift, where with the UBC they did not. He had bid the job with out bottom chord bracing, and now he had to provide it because of the code change.

I guess it is easy to say don't do it when its not your pocket book.

 

[RARWOOD]

I've been in similair situations. EOR's are not too happy when suppliers who are only providing material to execute the EOR's design, point out that change orders will need to be issued to bring the engineers design up to current code. And sale departments aren't happy when inside engineers designing to current codes, require the addition of material not included in the salesman's bid.

With decking one of the problems I face, is trying to justify the use of 2x6 decking, laid up using a two span layup as specified by the EOR, when the building has an odd number of bays.

An odd number of bays, requires the use of a combination single span two span layup. This type of layup will not meet code where the capacity of two span is required. In cases like this I have to use judgement. I know that the deck will not be over stressed its just that some areas will deflect more under the design snow load.

 

[structuralaggie]

I know sometimes you have to make a judgment call, especially if you are backed into a corner. If it is just deflection, then I guess that means someone has to actually see it when there is 6 feet of snow. Maybe they won't notice? I'm not trying to encourage under designing. These are just thoughts I would have after I realized that the code changed, a mistake was made, loads changed, or something that is going to cause a change order.

If it is up front and you are trying to explain to the client why the deck is going to have to the thicker, just blame the code. You can't help that the loads have gotten larger.

 

[civilperson]

The allowable deflection due to snow is L/180, the L/120 refers to D + L, (both cases not supporting a ceiling). 2000 IBC,Table 1604.3