×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

100 or 100 + 25?

100 or 100 + 25?

100 or 100 + 25?

(OP)
For years I have been using only 100 psf live load for public assembly areas on a roof, not adding snow (25 psf here).  Professionally, I do not have a problem with this knowing that it is not very likely that people will be standing shoulder to shoulder on the deck in two feet of snow.

Anyone know of an IBC or ASCE 7 code provision where the 25 psf snow load  should be added in addition to the 100 psf assembly load?  I can see just using the snow load and forgetting the assembly load for snow loads in excess of 100 psf too.  In the mountains here, 400 psf is not unheard of at 4000 feet.

Just trying to verify that the codes are logical here :)

Mike McCann
McCann Engineering

RE: 100 or 100 + 25?

The IBC load combinations in section 1605 all use roof live load OR snow, not both together.

RE: 100 or 100 + 25?

(OP)
Nice to verify that the code is logical here.  Thanks.  :)

Mike McCann
McCann Engineering

RE: 100 or 100 + 25?

Except that there is no 100 psf Roof Live Load.

You apparently have 100 psf FLOOR live load and 25 psf snow load.  Floor live and roof snow ARE added together in the combinations.

RE: 100 or 100 + 25?

(OP)
I believe if the area is a public assembly area, as a public roof deck where residents or the public can congregate, then the 100 psf value must be used - see 2006 UBC Table 1607.1, occupancy #28, hotels and multiple family dwellings, specifically "public rooms".  

What constitutes a room would be the only argueable issue here, but you have to consider the intent of the code.  I believe that a space does not have to be enclosed, either partially or completely, to be constituted a "room".  It gets down to the "public" access issue here.  Hence I believe that section 1605 would apply here.

Mike McCann
McCann Engineering

RE: 100 or 100 + 25?

JAE,

IBC 1607.11.2 specifies the live loads to use for roofs and says to use 100 psf for assembly areas.  So the 100 psf is in fact a roof live load and does not need to be combined with snow.

RE: 100 or 100 + 25?

Taro-
I think that there is a subtle but important distinction.
That is in fact a live load that is applied to a roof, but it is not a roof live load.

RE: 100 or 100 + 25?

This is taken from 2003 IBC Commentary, 1602 Definitions

LIVE LOADS. Those loads produced by the use and occupancy
of the building or other structure and do not include construction or environmental loads such as wind load, snow load, rain load, earthquake load, flood load or dead load.
   This definition identifies the scope of the type of loading included in Section 1607. Generally, live loads are not environmental loads or dead loads, but are transient in
nature and will vary in magnitude over the life of a structure.

LIVE LOADS (ROOF). Those loads produced (1) during
maintenance by workers, equipment and materials; and (2) during the life of the structure by movable objects such as planters and by people.

   This definition is needed for the proper application of the load combinations in this chapter. This definition clarifies that roof loads, such as snow loads, are not live
loads.


I would interpret this to mean is the 100 psf live loads are greater than the 25 roof live load, I would use 100 psf.

Don Phillips
http://worthingtonengineering.com

RE: 100 or 100 + 25?

StructuralEIT,

I disagree.  A live load that is applied to the roof is by definition a "roof live load" (as set forth in IBC Section 1607.11).  The purpose of labeling it as a "roof live load" as opposed to any other kind of live load is so that it can be combined appropriately in Section 1605.  That is, the effects of the live and snow loads are considered separately and the worst case is used for design.  The probability/reliability-based load combinations do not require the cumulative effects to be considered.  In other words (as Mike originally said) the roof deck is not going to be packed with people standing shoulder-to-shoulder immediately after the 50-year design snow storm hits.

RE: 100 or 100 + 25?

Taro-
my apologies, I mis-read the posts.  I meant to agree with you and disagree with JAE.

RE: 100 or 100 + 25?

See the ASCE 7-05 commentary:

"Occupancy related loads on roofs are live loads (L) normally associated with the design of floors rather than roof live loads (Lr)"

Although it makes sense to not include snow with such roof occupancy loads, I don't think it is allowed. I am not sure they want you to take a reduction as a floor load and include it in the roof load only combinations.

RE: 100 or 100 + 25?

I'm not looking at the code, but in this example, wouldn't you look  at 0.75 x (100psf LL + 25psf SL) = 93.75psf for concurrent transient loads, but this can't be less than one acting alone, ie 100LL.  So, in effect, you don't need to consider it as 125psf.

RE: 100 or 100 + 25?

Taro - good point.  Also the follow ups.  I would say that "technically" the 100 psf as a live load (an assembly on the roof) would, in my eyes, be more like a floor live load.  

But that said, "practically" I would wonder how you'd ever get a full assembly load on a roof with 12 to 20 inches of snow on it as well.  

I guess this is where engineering judgement comes in...yes?

RE: 100 or 100 + 25?

(OP)
Yes.

Mike McCann
McCann Engineering

RE: 100 or 100 + 25?

msquared;

What have you been doing when it comes to seismic? The roof live (Lr) does not show up in the strength combinations with seismic; 1.2D + 1.0E + L + 0.2S

So if you were consistent with treating it as Lr, then it would not be combined with seismic either. Which doesn't seem correct to me. If there were assemblies on the roof as frequently as the typical floors below, wouldn't that give the roof live load the same chance of occuring simultaneously with an earthquake as any other floor?

Or have you just been disregarding roof assembly live load in combination with snow?

RE: 100 or 100 + 25?

Great point, haynewp.  For the seismic load case, I would probably also use the worst case of the assembly live load or the snow load, but not both.  This is obviously an area where the code needs some clarification.  A former coworker of mine is currently chairing an SEA committee that is trying to sort out this exact issue.  Until they get it sorted out, I guess we have to fall back on engineering judgment (or opinion).

RE: 100 or 100 + 25?

Following ct's post, I would put it in the load combinations and sort it that way.  Those should already include factors to account for two or more transient loads occuring at the same time.  As to whether its a roof or floor load, I suppose I would try to evaluate the frequency this load, or conditions warranting this load, would occur.  If the frequency of occurance is closer to that of a floor load, I would call it a floor load, and vice versa.

RE: 100 or 100 + 25?

Unfortunately, the 0.75 factor used to combine multiple transient loads only occurs in the basic ASD load combo section, not LRFD combos or alternate ASD combos.  The code guys will need to sort it all out.

RE: 100 or 100 + 25?

The effect of transient loads is taken into account in the strength design combinations in the ASCE 7.  Of course, the 0.75 isn't there but some reduction is used.  You don't apply roof live load with snow for instance, and roof live load and floor live load are not both multiplied by 1.6 in the same combination.

RE: 100 or 100 + 25?

"You don't apply roof live load with snow for instance, and roof live load and floor live load are not both multiplied by 1.6 in the same combination."

Exactly, that's what this whole discussion is about...

RE: 100 or 100 + 25?

(OP)
Haynewp:

Sorry for the belated response - just noticed it.  

Consider that according to section 12.7 of ASCE 7, the 100 psf live load to the roof would not need to be included in the seismic calculations.  However, to support the 100 psf load, the dead load of the roof would be substantially increased above the 15 to 25 psf norm, thus adding to the seismic force.

Also consider that for snow loads above 30 psf, 20% of that load is added to the dead load for seismic.

As to the question of whether a portion of the 100 psf load should be added to the roof dead load although it is not directly required, we have to consider the intent of the code.  The reason for the snow load inclusion is the time of duration factor as to how long that load will remain on the roof, essentially part of the weight structure over a longer than normal period of time, relative to the chance of occurrence of a seismic event.  One might consider that if the roof of a building is in full use of the 100 psf loading one to two months over a years time, then maybe adding 25 psf to the roof dead load might be warranted.  Such might be the case of an open air rooftop auditorium for concerts, weddings, graduations, etc.  It becomes a judgement call.  

Note, too, that all or a portion of "permanent equipment", "party walls" and "storage areas" are also included in the seismic dead loads.  Live loads, but having longer duration of use, or loading.

You can factor the equations any way you want to.  I am not really concerned with that as that's just the momentary code requirements and they will change eventually anyway.  The engineering logic with how you approach the problem should not.

Mike McCann
McCann Engineering

RE: 100 or 100 + 25?

Mike,

I wasn't thinking about whether to include the 100psf in the seismic weight, but rather that the 100psf has to be included in the load combination with seismic. The live load effects still have to be added to the seismic even if you don't include the live load in the seismic weight calculation. Agree? (See footnote #1 under the combinations)

 

RE: 100 or 100 + 25?

(OP)
Agree.

Mike McCann
McCann Engineering

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources