Very Concerned - I may have underestimated load 2

[CTW]

I was designing a second story floor of a two story wood frame building for the intended use as a conference room (2 offices on first floor). Building is quite small, 12' wide by 21' long. I used No. 2 So. Pine 2x10 joists at 16" oc, span = 12' and my LL was 80 psf. I'm quite concerned that maybe this should have been 100 psf. SBC is not very clear for office buildings and I'm beginning to wonder if I was wrong. I used the loading for corridors above first floor under office buildings in the SBC. Should this have been equivalent to a public room in a hotel, 100 psf? Anticipated number of people in the room is 12-15. The floor has already been built.

 

[haynewp]

If it is going to be used as a conference room, my first thought would have been to use the assembly area with movable seats of 100psf.

Now that it is already constructed.
For 100psf you have to have people packed together, standing right beside each other. 80psf is still over 100 people standing in this size room. Document you were notified the expected occupancy of 12-15 people for this room.

The places that worry me are balconies. I'll go above the 100psf for these. This is from personal experience of college, not just because one collapsed the other day. I certainly wouldn't use 60psf as in SBC.

 

[CTW]

Thanks. I too thought about the assembly area with moveable seats, but thought that it did not apply. In the future, I will definitely opt for 100 psf, just to make sure.

 

[Lutfi]

I am confused, if the space to be used as an office, ASCE 7 requires a live load of 50 PSF for offices and 80 PSF for corridors. Use of ASCE is permissible in the SBC. Refer to the start of chapter 16 of SBC. I have not used the code in a while but I would be surprised if this has changes.

I disagree with you regarding the SBC not being clear as far as the office floor loading. Use the classification for the entire building, which is, in this case is office building. I would use heavier loading for storage rooms, or computer rooms to name a few.

What is the problem? Do you think you should have used a loading of 100 PSF for Assembly space? It seems to me that you over designed the floor LL load by 30 PSF unless you have undersized the floor joists. Am I missing something?

Now to make you feel better follow my calculations:

Floor area = 12’X21”=252 ft^2
Assume a person will occupy an area of 2’ by 2’ => 4 ft^2 per person
Total number of people that can stand shoulder to shoulder = 252/4 = 63 people
Assume average person weighs 200 pounds
Total weight = 63X200=12600 pounds
Convert this to a uniform load = 12600/252=50 PSF

You can go to sleep now.
Good luck.

 

[haynewp]

If there was a room in an office building that I was labeled as conference or assembly I would not just go directly by the standard office loading. I worked for a large company once and when we had meetings in the conference room it would get really packed. As much as a corridor during our fire drills. "Everybody try to get in here so you can hear".
I don't think the 80psf is overkill (maybe in this particular case it is), but hey, that's just me. Maybe if you go directly by the standard office you will be covered if something gives.

 

[dlew]

CTW,

The ASCE 7 shows 4 subcategories for OFFICE BUILDINGS:
File and Computer rooms Load based on equipment weight
Lobbies and first floor corridors 100 psf
Offices 50 psf
Corridors above first floor 80 psf

I would consider Conferences rooms as offices and design it for 50 psf. Normally a conference room has a big table and to the most 2 rows of chairs around the table. A office space has at least a table, several file cabinets (very heavy stuff), a couple of chairs, and not much open space. The maximum number of people in an office space is not posted. The load in the conference room will be lighter than any office area unless it is used as storage area.

As Lufti said if the conference room is packed with people the average load is still 50 psf.

 

[haynewp]

I thought you had to use at least an additional 20 for partition (whether they are shown or not) for any part of an office building? What if the conference room is eliminated in the future?

 

[Lutfi]

Guys,

I did not mean to spark up a debate. All I was trying to say that I would consider a conference room located within an office building as an office space unless someone would tell me about some unique and unusual requirements.

At face value, I do not see this design requiring additional load. Of course we are no informed about the full circumstances and uses. As dlew said, less space would be occupied if table space is needed and less people would be in if chairs were to be used. I have checked the MIL-HBK for office floor loadings and I found it to be in concert with the ASCE 7.

If the engineer of record wants to classify, for his comfort, the second floor as an assembly space/building that would be fine. In Florida these types of structural classifications imply that they may hold 300 people or more which makes them Threshold Buildings. Threshold Building status will trigger many special requirements for life safety, inspection, fire protection etc.

I would read the code’s definition of assembly occupancy and go from there. My initial read of assembly is theaters, churches, public gathering arenas.

Having this said and done, I still would design that room for 50 PSF. Structural engineers these days are faced with the vibration problem because most offices are gone paperless. Hence they see less live loads than what they are designed for.

As one of my college instructors used to say, “USE YOUR ENGINEERING JUDGEMENT”

 

[haynewp]

Judgement or not, if you're going to meet code I still think you have to design it for 70 psf. I don't see anything that allows you to forgo the additional 20 because it's called a conference room. It could be a project storage room in 10 years. I don't know how SBC reads on this but 1607.5 of IBC seems to be pretty clear.
I'm not a 100% sure but if I'm wrong on this I would like to know it for my own future reference. What other rooms can I omit the 20 psf partition load?

 

[dlew]

haynewp,

I agree with you that the provision of 20 psf for partitions should be included in addition to the 50 psf live load (per ASCE 7).

 

[Lutfi]

Partition load MUST be included. All codes and common sense requires it. I would include it as part of the dead load (although it does not make any difference except for concrete design or steel LRFD design). SBC requires that actual load be used for partitions if the quantity and location are known. If not, it requires 20 PSF. The point I was discussing is the 50-PSF live load. I think the 80 PSF Live Load is adequate if it included the 20 PSF.

Now I want to argue the engineering judgment for this particular room. The foot print is 12'X21'. I want all reasonable engineers in hereto tell me how many partitions is it going to take to accommodate the 20 PSF. See this is where I think the code does not make sense and I still maintain that the engineer should be entitled to exercise his engineering judgment.

I think this question provoked a healthy debate and exchange of ideas.

Good night

 

[Trussdoc]

Some 20 years ago, the company I worked for was involved in a major lawsuit over an office building floor. The 20 PSF partition loading was a key issue. Even though the architect had indicated the locations of wood/sheetrock partitions, and their weight was accounted for in the floor truss design, the customer added extremely heavy (and expensive) glass partitions that shook and clanked when you walked by them. In addition, lateral file cabinets ran parallel to the trusses. The case settled a week into the trial as it appeared that all parties had some fault.

Additional columns were installed in the basement and the problem was fixed; a structural engineering office is now a tenent there!

The point I'm trying to make is that office loading can be tricky. I wouldn't worry about significant partition loading in the 12 x 21 conference room, but I would check for the mandated concentrated load to allow for future file cabinets, copier, or other heavy object.

 

[Striker]

Well it looks like this particular thread has spawned quite a bit of speculation and conjecture, but based upon the information provided by various participants, I would tend to agree that 80# is sufficient. Now that being said, I suppose I should say that I always design second floor loads of 100# across the board. This gives me the leeway needed when a stupid tennant decides to store heaps of old papers and files in a non-ground floor room.
Lets be a little bit realistic here, if the design load is 50#LL and 12#DL and the room is 250'^2 that is a combined load of 15500. Lets assume the following:
Copier @ 500#
Files @ 4000#
Furniture @ 1500#
45 people @ 200#ea 9000#
Total 15000# < 15500#

Now we have assumed the copier will occupy 10'^2 and the files will take 25'^2 that leaves 215'^2 with a net free area (assuming the other furniture takes up no additional space) of 4.77.. sq ft per person or a section just over 2'^2.

Now that being said...I do find it hard to believe that 2x10X12'@ 16&quot; o/c will carry 80# without some significant deflection. I will check it in the morning, but right now, my brain is a bit fried. I would expect either a 2X12 or 12&quot; o/c, particularly if the framing members have even the slightest chance of being compromised by drilling and boring for electrical, plumbing and HVAC.

 

[Striker]

I have had a chance to review some calculations this morning.

This is my data for 80#LL and 12#DL w/2x10 #1 SYP @ 16&quot; o/c

Reactions Including Self-Weight
R1 R2
Live Load: 640 lb 640 lb
Dead Load: 119 lb 119 lb
Total Load: 759 lb 759 lb

Add'l Detail - Incl. Self Wt.
Max Moment: 2,278 ft-lb
Member Design Shear: 662 lb
Total Deflection: 0.351 in
Live Deflection: 0.296 in
Req'd EI, no self-weight added
(in^2-lb): 1.244E+08
Approx. Self Weight: 3.90 plf
Min. Calc'd Bearing Length: 0.90 in

Efficiency of Member:
Bending Overdesign: 23.7%
Shear Overdesign: 25.8%
Deflection Overdesign: 35.2%

In you calculate the floor load at 100# then based on these calculations if #1 lumber was used, then 2X10 @ 16&quot; o/c will still work by 2% with bending being the controlling criteria. Unfortunately 2X10 #2 will not work at 16&quot; o/c due to deflection underdesign.

All is not lost however. If we include the IBC floor load of 50#LL and additional 20#DL for future partitioning along with a typical 15#DL, then we still make the design threshold by 4.2% with 2X10 SYP #2 @ 16&quot; o/c.

You would still need to consider the load of the corridor along with any supporting columns or walls to determine if these loads are going to work for you.