20 psf minimum roof live load

[PE_JRM]

This a pretty basic question so please forgive me for asking but my area of expertise doesn't include buildings. I am renovating an old building that I own and I would like to check what the contractor proposes for the roof system and its effect on the rest of the structure against code requirements.

I understand that the 20 psf minimum roof load is intended to account for future roof maintenance. In my case I believe the minimum can be reduced from 20 psf to 12 psf according to the code. The code says the minimum roof LL is not required to be applied concurrently. If we design for a 20 psf live load on the roof the foundation must be adequate to carry that load. The foundation must carry roof and floor loads and designing for max roof LL and max floor LL at the same time makes no sense. But designing the roof structure for 20 psf LL and ignoring those effects below the roof also makes less sense ("need not be applied concurrently").

With regard to foundation checks what makes sense to me is some percentage of max floor LL combined with the the loads from the roof. I don't see that anywhere in the code required load combinations. Please help me understand what the code requires.

Background info: I live in a region where snow and ice are pretty rare so the required snow load is 5 psf. The roof DL is only 6 psf (corrugated steel on osb deck with steel Zee purlins 24" OC spanning 20 ft). The foundation in my old building is marginal and showing it "good" is difficult with the 20 psf minimum roof LL requirement. The building is also in an area where code enforcement was non-existant 50 years ago.

 

[phamENG]

Live load reduction is not for the entire roof. It's for certain members that make up the roof structure. So your purlins will need to be designed for 20psf. The girders might be able to be reduced to 12psf. Maybe.

As for applying full roof and floor live loads at the same time, you're right. It doesn't make sense. Which is why the code doesn't require it.

If building structures are not your area of expertise, you should get a structural engineer involved to do the design.

 

[PE_JRM]

Thanks phamENG. I am a structural engineer but I've never done a building. There ARE other sorts of structures you know. I've done a wide variety of structures and worked with at least half a dozen codes and Federal regulations in my 40+ year career as a Professional Engineer. Never a building, Zero experience with UBC. I own this building and it is outside any code jurisdiction so I can do whatever I want with it. I'm just looking for a rational criteria to judge the foundation capability before I start modifying the structure.

Perhaps you should let someone else answer the question if you cant answer without insulting the questioner.

 

[phamENG]

There ARE other sorts of structures you know.

Yep. I am aware of that.

Which is why when somebody calls me to design a bridge for them, or an MSE retaining wall, or a vent stack, or a silo, or one of any number of structures in which I lack the experience and the knowledge to provide adequate engineering services, I turn down the job and, if I can, refer to them to somebody more suited for it. It's also why I hire other engineers to practice their specialties at my property. I'd rather have it done right. I may ask them questions to learn, but I let them do their job.

I'm simply suggesting you do the same.

 

[PE_JRM]

Engineers are not often involved in residential very small building construction. This building is the size of a big garage or workshop. Like I said, I'm doing a sanity check on the contractors proposal. Its not worth the cost spent to me to hire an expert when its not even necessary/required to do the checks. I'm trying to determine if there is enough load applied to the edges of a 4" slab to crack it using recommended design loads and combinations. The contractor says its fine based on his experience. I'm checking up on him - I'm not so sure.

HUD residential construction guidelines (a table footnote) tell me that the 20 psf minimum roof load DOES NOT have to be combined with any other transient loads. ASCE and UBC have similar requirement . I believe the rational is that the roof structure needs be adequate to support temporary construction loads. The reason for the requirement makes sense. But the reason that temporary construction load doesn't have to be combined with a percentage of other live loads does not make sense to me. There is always some live load on a floor. It is never ZERO. Thus, my "confusion" and desire to understand the why of things instead of because the code requires it.

BTW, this roof structure is longitudinal purlins supported by interior walls. No rafters or girders.

 

[JAE]

Not sure UBC is even used anymore. IBC is the generally accepted code in most of the US.

The IBC (which references ASCE 7) has load combinations where floor live and roof live do add together - but at a reduced value due to the small statistical nature of both loads occurring simultaneously. For ASD it's 0.75L + 0.75Lr. For LRFD it's 1.6L + 0.5Lr and also 1.0L + 1.6Lr.

I agree with all phamENG said above and if answers here don't satisfy suggest, for your head check, that you hire a local structural engineer to at least take a quick look at your project - an hour's time for a head check perhaps.

I didn't see anything pham said that was an insult. It's free advice - take it or not - your choice.

 

[PE_JRM]

Not sure UBC is even used anymore. IBC is the generally accepted code in most of the US.

The IBC (which references ASCE 7) has load combinations where floor live and roof live do add together - but at a reduced value due to the small statistical nature of both loads occurring simultaneously. For ASD it's 0.75L + 0.75Lr. For LRFD it's 1.6L + 0.5Lr and also 1.0L + 1.6Lr.

I agree with all phamENG said above and if answers here don't satisfy suggest, for your head check, that you hire a local structural engineer to at least take a quick look at your project - an hour's time for a head check perhaps.

I didn't see anything pham said that was an insult. It's free advice - take it or not - your choice.

Thanks for the ASCE code reference. I'll check it out.

 

[ChorasDen]

I'm real confused how a structural engineer who has practiced for 40+ years is unfamiliar with load combinations? PE_JRM, what kind of projects do you typically work on?

 

[PE_JRM]

I'm real confused how a structural engineer who has practiced for 40+ years is unfamiliar with load combinations? PE_JRM, what kind of projects do you typically work on?

Airplanes since 1992. Water towers, oil, water LPG and LNG gas storage, smokestacks, barges, locks, offshore oil platforms, pressure vessels, nuclear vessels (fission and fusion), etc before that. The largest structure (LNG storage) I ever designed was about 300 feet in diameter. I designed the concrete/steel composite roof (self supporting) and the expansion joint at the floor. I also designed the shell for direct hits from F-16 aircraft (no code for that). The biggest load I ever designed for was about 14,000 ft-K (if I remember right) for a huge oil storage vessel in the Persion Gulf. ACI, ASCE, IBC, etc don't govern any of those structures. I worked in a geotechnical testing lab for a while and in a research facility doing advanced composite materials testing for the Air Force for another few years. I was the forensic investigator for a major airline for aout 5 years. I've worked with Sandia and Oak Ridge National Labs on various aerospace projects. One of them won national recognition for innovation.

I learned the science of fracture mechanics and crack growth and learned all of the NDT methods. Every repair on every commercial service airplane you fly on requires a crack growth analysis (fracture mechanics) for that repair based on the NDT inspection method and inspection frequency chosen and materials used. Not everybody does exactly what they learned in college and not everybody should. I did other things because I could make more money doing them and I found buildings and bridges boring. To each his own, right? That said I can still understand the old stuff I learned in school but it has changed a good bit since 1981 and its been over 40 years since I learned it.

I learned back in the time of hand calculations when things were relatively simple. Now everything requires a $400 code subscription/purchase or a fancy piece of software that costs at least $1000. Trying to understand a code requirement by looking at the snapshot of what you can find online for free takes the meaning completely out of context (easily misinterpreted). Buying expensive code books or computer programs for personal use that will be used once and never be used again is a stupid waste of money. As a retiree, my resources are limited and I had hoped I could suppliment them by asking questions and getting answers here. Unfortunately that hasn't been the case. This project is on my property on land that is outside the area of any code authority - I just want whatever I do to improve it to be as code compliant as possible.

I would urge you to expand your point of view and not judge people. If you are actually curious about my background fine - you asked and now you know. I would be the first to agree that I'm not the average Civil Engineer. If you're happy being a code expert and doing exactly what the code tells you day after day that's great for you but it wasn't for me. After about 8 years of designing structures ruled by codes I went back to school, learned aerospace structures, got a materials certificate and was happy being challenged every day and doing the aerospace thing until retirement.

This supposed to be a collaborative place or at least I thought it was. Unfortunately, I haven't found that to be the case.