I have a client that would like to put a new roof on a pre-engineered building. It appears that the building was desinged for a 40 psf snow load in addition to the dead load of the structure. In a past project approximately 3 to 4 psf was added to the roof (ceiling, lights, HVAC, sprinklers). The new roof design would add another 4 psf to the roof. Does anyone have experience adding supports to a pre-engineered building?
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Pre-Engineered Building Capacity
- Thread starter Rick33
- Start date
JedClampett
Structural
Not exactly like your case, but we've added monorails to existing metal buildings. And my advice is to not do it. The original pre-engineered metal building supplier will be no help. They never can find their calculations. They use a bunch of propreitory sections that have no published section properties. And besides, they design to the gnat's butt, with no spare capacity.
Sorry to be so grim. Maybe someone else had a better experience (hard to have a worse one). We ended up adding new frames to take the entire monorail load.
Sorry to be so grim. Maybe someone else had a better experience (hard to have a worse one). We ended up adding new frames to take the entire monorail load.
Rick33 -
Check with your local snow load requirements.
In years past, at least in the midwest US, many, if not most, building codes required heavier snow loading than what is required by the current codes.
In my area, for example, the snow load back in the 1960's through about 1980 was 30 psf. Today, the IBC requires only 21 psf depending on roof slope, etc.
This then offers you an additional capacity to work with when adding roofing or other loads.
Check with your local snow load requirements.
In years past, at least in the midwest US, many, if not most, building codes required heavier snow loading than what is required by the current codes.
In my area, for example, the snow load back in the 1960's through about 1980 was 30 psf. Today, the IBC requires only 21 psf depending on roof slope, etc.
This then offers you an additional capacity to work with when adding roofing or other loads.
Note however, depending on the State and the code being used in the past, under the IBC the snow loads have went up in many cases. For example out state Minnesota.
I would agree with JedCampett. Unless I had complete design information on the orginal building, I would be hesitant to recommend increasing the roof dead load. Also if 4 psf was added in the past for misc and mech, only part of that capacity might be available. After all the building probably still has lights, mech ect.
Agreeing with JC the orginal building was probably designed to 100% of the allowable design stress. In addition without the orginal design calculations, you don't know if they reduced the snow load due to sliding. For example you could have a building that required the use of a 40 psf design load. If that building had flat roof areas, and sloping roof areas, the flat roof areas would be designed for 40 psf while the sloping roofs might reduce the 40 psf due to sliding snow.
I would agree with JedCampett. Unless I had complete design information on the orginal building, I would be hesitant to recommend increasing the roof dead load. Also if 4 psf was added in the past for misc and mech, only part of that capacity might be available. After all the building probably still has lights, mech ect.
Agreeing with JC the orginal building was probably designed to 100% of the allowable design stress. In addition without the orginal design calculations, you don't know if they reduced the snow load due to sliding. For example you could have a building that required the use of a 40 psf design load. If that building had flat roof areas, and sloping roof areas, the flat roof areas would be designed for 40 psf while the sloping roofs might reduce the 40 psf due to sliding snow.
SteveNapper
Structural
HI, UK based so maybe not too much help. Do have access to spreadsheets to calculate to British Standards (BS5950-5) capacaties for cold rolled sections - particularly axial compression loads.
Deflection NORMALLY limits design here in the UK, so axial capacity might be available. Let me know if I can help. Might be slow to respond for rest of day though, baby due and wife thinks having contractions, but not sure!!!
Deflection NORMALLY limits design here in the UK, so axial capacity might be available. Let me know if I can help. Might be slow to respond for rest of day though, baby due and wife thinks having contractions, but not sure!!!
I would like to input a few comments here:
JAE is correct in the snow load capacities, but this will only hinder your success if you are designing this building in the South (the biggest region of PEMB suppliers).
1)I would like to advise you to model the typical interior rigid frame in RISA 2D/3D, STAAD, or some other similar program. This can be done easily if you are familiar with the member size tables offered in the rigid frame profile page of the PEMB drawings.
2)Ask for the shop drawings from the PEMB, and if they aren't available primarily because of "Murphy's Law", then visit the site and visually inspect the welds and the endplates. It is very rare for a PEMB company to design moment connections that are not shopweld/field-bolted connections, so it may be necessary to upgrade the connections too.
3)If the steps of 1 and 2 don't help, it may be better to pass the buck to the PEMB. Unfortunately, because I've worked for a couple of PEMB's (no longer), they aren't always the most honest folks. They will tell you they can't find the calc's and then they will bend over backwards for you to sign a contract to retrofit their own building.
Regards,
OUe
JAE is correct in the snow load capacities, but this will only hinder your success if you are designing this building in the South (the biggest region of PEMB suppliers).
1)I would like to advise you to model the typical interior rigid frame in RISA 2D/3D, STAAD, or some other similar program. This can be done easily if you are familiar with the member size tables offered in the rigid frame profile page of the PEMB drawings.
2)Ask for the shop drawings from the PEMB, and if they aren't available primarily because of "Murphy's Law", then visit the site and visually inspect the welds and the endplates. It is very rare for a PEMB company to design moment connections that are not shopweld/field-bolted connections, so it may be necessary to upgrade the connections too.
3)If the steps of 1 and 2 don't help, it may be better to pass the buck to the PEMB. Unfortunately, because I've worked for a couple of PEMB's (no longer), they aren't always the most honest folks. They will tell you they can't find the calc's and then they will bend over backwards for you to sign a contract to retrofit their own building.
Regards,
OUe
Hi Rick33
I dont think if it will cost you that much if it involves adding an additional uniform load(actually it depends on the span).Reinforcements due to additional load at roof will only involve reinforcing the existing rafters - by welding additional plates at the bottom flanges.
If an additional concentrated loads are applied, reinforcing both the flanges & the web will help.
I would advice you to consult a PEMB.I dont think if it will harm you by simply negotiating.
Regards,
SEM8
I dont think if it will cost you that much if it involves adding an additional uniform load(actually it depends on the span).Reinforcements due to additional load at roof will only involve reinforcing the existing rafters - by welding additional plates at the bottom flanges.
If an additional concentrated loads are applied, reinforcing both the flanges & the web will help.
I would advice you to consult a PEMB.I dont think if it will harm you by simply negotiating.
Regards,
SEM8
msquared48
Structural
Usually the design dead load for these pre-manufactured metal building structures is only in the range of 5 to 10 psf, and, unless you have the design drawings and/or calculations, you probably will not be able to find out. Buildings obviously have to be re-roofed periodically, and considering the addition of the mechanical loads, it would be most prudent to strip off the old roofing before applying the new.
We had a 28 psf snow/ice here in Mid-Missouri on Dec 1st, 2006. I investigated a lot of damage throughout the state--full collapses, large deflections, plastic deformation, buckling columns, sustained rotation (I wish I could attach pictures!). 90% of the observed damage was in pre-engineered buildings.
We have modeled most all of the building systems we investigated (in RISA) and found the buildings to be way under designed. We have seen calcs that show the stresses work when clearly by analysis the section fails. We have seen snow loads reduced similar to that of a live load!
The materials and plates are spliced and corners are cut everywhere in these buildings. I have found several round columns that are not even standard pipe sections (some range from 0.13" thickness).
If you decide the building needs retrofit, I recommend doing a thorough as-built survey regardless of the existing drawings. Remain cautious and assume nothing. I have seen these cut-rate designs materialize into huge liability and life safety disasters!
cldea8
We have modeled most all of the building systems we investigated (in RISA) and found the buildings to be way under designed. We have seen calcs that show the stresses work when clearly by analysis the section fails. We have seen snow loads reduced similar to that of a live load!
The materials and plates are spliced and corners are cut everywhere in these buildings. I have found several round columns that are not even standard pipe sections (some range from 0.13" thickness).
If you decide the building needs retrofit, I recommend doing a thorough as-built survey regardless of the existing drawings. Remain cautious and assume nothing. I have seen these cut-rate designs materialize into huge liability and life safety disasters!
cldea8
I recently retrofitted a pre-engineered building by adding another frame line (and breaking the span of the purlins). The new frame lines were then carried to the existing columns. This eliminated strengthening the frames. Only the columns needed retrofit.
cldea8
cldea8
darkwing888
Mining
cldea8 - bien trabajo
If you are going to stamp anything, you better do a site visit and some analysis.
"Clever of me to use my spine to break my fall like that."
If you are going to stamp anything, you better do a site visit and some analysis.
"Clever of me to use my spine to break my fall like that."
msquared48
Structural
I heartily agree with darkwing888.
Nice guys finish last - don't try to save anyone money when the risk is all yours. CYA and do your due dilligence. If they don't want to pay for what is needed, don't do the project. If asked by the client, I would not recommend the project to another engineer either. Knowing what had just transpired, I would protect my friends and associates.
Mike McCann
McCann Engineering
Nice guys finish last - don't try to save anyone money when the risk is all yours. CYA and do your due dilligence. If they don't want to pay for what is needed, don't do the project. If asked by the client, I would not recommend the project to another engineer either. Knowing what had just transpired, I would protect my friends and associates.
Mike McCann
McCann Engineering
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