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  • 1.  Factor of Safety for Net Uplift Tension in Buildings

    Posted 01-31-2023 10:31 AM
    In foundations, we design for overturning by balancing the acting moment and the resisting moment to a minimum factor of safety.  However, there are also cases in superstructure where we rely on bearing to transfer force and need to prevent net tension, using structural weight as ballast.  I'm working on an existing building with slab on metal deck over bar joists for the roof, where the original designer balanced out uplift forces by providing just enough dead load at ~0.6D to counteract the ASD wind.  Overturning from wind acting on the side of some new RTU's creates a small net uplift; my task was to check whether it would unzip the whole bay, assuming a bearing-only connection to the joists.  As it turns out, the roof could maintain net downwards force on the joists at ASD wind (0.6D+0.6W), but it failed at LRFD 0.9D+1.0W.  I was surprised to realize that the ASD uplift combination is less conservative on dead load resisting uplift (0.6:0.6 = 1:1 vs. 1:0.9).  Strange.

    When the net uplift is caused by dead load and resisted by dead load, it gets even stranger.  It's not uncommon to have long cantilevers (three or four times the backspan) in contemporary construction, resulting in permanent uplift on the back column, under dead load alone.  The back column may be stabilized by the dead load within its trib area: dead load against dead load.  It got me wondering about design for uplift when dead load is both the bad guy and the good guy.


    Say we model the above frame in analysis software with auto combinations--dangerous, I know.  The software would create a demand envelope considering many combinations, using 1.2D for downward effects and 0.9D for uplift.  If the wind and snow loads are very small by comparison (let's say it's a decorative canopy inside a shopping mall that experiences virtually no environmental hazards), there is a scenario where the dead load could be almost perfectly balanced; say 10 kips uplift due to dead load on the cantilever against 10.5 kips downwards due to the structural weight within the trib area of the column.  In real life, the detrimental dead load could easily have been underestimated by a small margin, lifting the column under dead load alone.  However, the software would show no net uplift at target reliability.

    I'm curious as to whether anybody's come across this, and what method you would use to meet the standard of care.  Is it appropriate to design to 0.9D_resisting + 1.2D_acting (equivalent FS = ~1.3)?  Perhaps 0.9D_resisting + 1.4D_acting (FS = ~1.5)?  Better to use a foundations approach with acting vs resisting forces at FS=2?  I imagine this FS is intended to consider variability in soil weights, which are much higher than variability of dead load.  Is it possible to perform such a check in a single software model?

    Excited to hear some thoughts on this.  Feel free to comment on just part of this very dense post.

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    Christian Parker P.E., M.ASCE
    Structural Project Engineer
    Washington DC
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  • 2.  RE: Factor of Safety for Net Uplift Tension in Buildings

    Posted 01-31-2023 01:34 PM
    Correct the program conside 0.9D for uplift load combination because of the load effect in etabs itsself ,as its for substructure while it is 1.2D for superstructure

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    [Khalid ] [Zuiater] [Civil Engineer ]
    [Civil Engineer ]
    [Chief Contracting, Altorath International]
    [Abu Dhabi] [United Arab Emirates ]
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  • 3.  RE: Factor of Safety for Net Uplift Tension in Buildings

    Posted 02-01-2023 12:08 PM
    Right, the issue is that there's no distinction in any one load combination between beneficial dead load which you'd want to underestimate, and simultaneous detrimental dead load which you'd want to overestimate.  It's akin to designing for an expected service load with a phi factor of 1.0 and a load factor of 1.0.  You'd be designing that component to fail, FS=1.0.

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    Christian Parker P.E., M.ASCE
    Structural Project Engineer
    Washington DC
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  • 4.  RE: Factor of Safety for Net Uplift Tension in Buildings

    Posted 02-01-2023 08:16 AM
    Based on my experience it is better to not design too close to the allowed code for loads that cannot be easily or exactly calculated.  I have had the privilege of working on project where the wind load was given by wind tunnel models and the wind load on the roof was surprisingly different from what would have been calculated based on the code.

    Additionally, I really don't think that we should be using connections that are not resistant to uplift.  And we should be thinking about redundancy.  And so on.

    The code is a minimum standard and engineering judgment determines where we should be more conservative and where we can follow the code more closely.

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    Sarah Halsey P.E., M.ASCE
    Civil Engineer III
    New York NY
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  • 5.  RE: Factor of Safety for Net Uplift Tension in Buildings

    Posted 02-01-2023 10:15 AM
    The ETABS Provide load combinations for Substructure and superstructure based on iteration methods it governs the current load combinations for uplift the soil pressure is an issue for the current requirment

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    [Khalid ] [Zuiater] [Civil Engineer ]
    [Civil Engineer ]
    [Chief Contracting, Altorath International]
    [Abu Dhabi] [United Arab Emirates ]
    ------------------------------



  • 6.  RE: Factor of Safety for Net Uplift Tension in Buildings

    Posted 02-01-2023 11:20 AM
    Sarah,

    Thanks for the response.  These are all good points, and for new construction we would always consider alternative load paths and detail for all plausible demands, even if there's ambiguity as to how to determine those demands.  It was a presentation about a new stadium that first got me thinking about this, and in that case all column splices were tension splices and the foundations were engineered to negate the net uplift.  The paired columns that supported each cantilever roof girder and raker for the stands were quite slender and closely spaced, especially considering the large trib width of those frames.  I consider my firm quite open and collaborative with architects, but I'm not sure we would have accepted the design concept of that project without a fight.  I chose not to bring this up in the post because the answer is too simple: provide tension splices for the tension members and design the foundations the way you design foundations.

    For an existing building like my project with the new RTU's, it is a little different.  Regardless of what we would have preferred to see in the original detailing, this was a small additional load on an existing roof which was in flawless condition, had been in service for 50 years, and was intended to remain so through the renovation.  If we determined the additional uplift was unacceptable, the structural work would go from light gauge curbs and a few L2x2 twigs at joist panel points, to extensive field welding in an existing plenum space and/or reframing of large portions of the existing roof.  Needless to say, we're responsible for providing a safe design either way; but I'd much sooner add 2% to the structural cost of a new building than add 500% to the structural cost of an MEP retrofit in the name of belt and suspenders.  The real difference is how much design effort and precision I'm willing to do to convince myself (and my boss) that a leaner design works.

    I certainly agree about engineering judgment determining degree of conservatism; I generally try to calibrate my target DCR based on level of confidence in the accuracy of assumptions and analysis methods.  In this case, being more or less conservative would depend on the method of demand/capacity comparison: ASD vs LRFD vs 1.2v0.9 vs 1.4v0.9 vs FS=2.  What approach would you take?

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    Christian Parker P.E., M.ASCE
    Structural Project Engineer
    Washington DC
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  • 7.  RE: Factor of Safety for Net Uplift Tension in Buildings

    Posted 01-17-2024 08:14 PM

    Intriguing update to this discussion: I stumbled upon a recent controversy over ballasted solar panel uplift design that has resulted in SEAOC issuing a formal statement to clarify proper design practices.  The magazine article elaborating on SEAOC's statement reads, in part:

    "The 0.6 factor applied to D [in the load combination 0.6W + 0.6D] represents the factor of safety against uplift, equal to 1/0.6=1.67. Using 1.0D instead of 0.6D would eliminate the factor of safety, which is an integral part of ASD. Just because the dead load is well known does not mean that a coefficient higher than 0.6 can be used for dead load."

    This doesn't necessarily address the original questions, but it does provide some interesting insights into the thinking behind uplift load combinations, and further reinforces the idea that the code intent for uplift and counteracting loads is more similar to foundation FS thinking than typical member design based on low annual probability of exceedance.

    Here's the brief SEAOC statement, and the article with some more detail.



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    Christian Parker P.E., M.ASCE
    Structural Project Engineer
    Chicago IL
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  • 8.  RE: Factor of Safety for Net Uplift Tension in Buildings

    Posted 01-22-2024 08:08 AM

    The difference between ASD and LFRD has been an on going debate since LRFD has first been introduced. This became quite apparent. when LRFD was introduced to geotechnical engineers.  The geotechnical engineer had great difficulty coping with the load combinations.  The LRFD combinations provide an envelop of reactions  ie 1.2D + W or .9D + W based on the statistical variations of self-weight of permanent loads.   

    Using  D +.6W or .6 D + .6 W give you similar results  in ASD  

    When you referred to ballasted loads   using the .D +.6W only accounts for the load affects you then also need to account the variation of the resistance and I believe that is what the SEAOC is referring to as missing.



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    David Thompson P.E., M.ASCE
    Principal
    KTA Structural Engineers Ltd.
    Calgary AB
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  • 9.  RE: Factor of Safety for Net Uplift Tension in Buildings

    Posted 01-24-2024 10:17 AM

    I think in the instance of ballasted solar arrays, using D+0.6W vs. 0.6D+0.6W makes a significant difference, since the design output is the required amount of dead load rather than member strength.  You'll specify 40% less ballast if you design for 0.6D+0.6W.

    I certainly see your point on geotech.  Only once have I seen a geotech report in LRFD, and it was clear that they had converted at the end, like we do for US Federal projects "in Metric" (i.e. design it in Imperial, then change the dimensions on deliverables).  I get frustrated with Geotechnical Engineers sometimes for putting the burden of managing separate load combinations on structural engineers, but I don't think LRFD promises as much to geotech.  When most of the engineering centers around gravity loads it must seem trivial to focus on anything besides "how much weight?"  In structures, the greatest technical challenges are in lateral, so it's more obvious to update environmental hazards to a more consistent basis, and bring the gravity loads along for the ride.  Not speaking with any authority here of course.

    One item I found interesting in the ballasted solar panels issue is that they state specifically that the 0.6 factor is not to account for variability in resisting weight.  Apparently, engineers have been mistakenly using 1.0D on the reasoning that they are 100% certain how much the ballast will weigh since they are specifying it.  The SEAOC statement clarifies that the 0.6D is intended to reduce the frequency of wind events that could rip everything off the roof, because the ASD 0.6W load is too low to design right at failure.

    P.S. stay warm up there!  I was tracking the insane temperatures in Calgary all last week.  Beautiful city, though.



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    Christian Parker P.E., M.ASCE
    Structural Project Engineer
    Chicago IL
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  • 10.  RE: Factor of Safety for Net Uplift Tension in Buildings

    Posted 24 days ago

    Here is a more clear explanation of what I said.

    If I require there is no uplift load to occur below the structure.  I would consider both equations

    Given W is the ultimate wind load  ( what use to be considered 1.6 x te 50 year wind load.)

    D +0.6 W  = 0         requires a dead (ballast) load =0.6 W 

    0.6 D + 0.6 W =      Requires a dead (ballast) load =1.0 W   

    I would take the more conservative of the two, 1.0 W   which would have given me a factor of safety of 1.6 in the past

    If considering the compressive bearing pressure under the ballast when checking the soil   I would use  D +0.6 W   = 40 % of the wind uplift   or D if W was zero.

    As I mention previously was what is missing the resistance factor for the ballast



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    David Thompson P.E., M.ASCE
    Principal
    KTA Structural Engineers Ltd.
    Calgary AB
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