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1981 - 2009 Cantilever Roof Framing
Design Resources
A Graphical Method for Design of Cantilevered and Suspended Span Beams, Mehta, AISC Engineering Journal, 2nd Quarter 1981. (as of 5/10/24 free to download).
[Mehta only references codes, no other research articles]
Roof Collapse, Magic Mart Store, Bolivar, Tennessee, Bell and Parker, Journal of Performance of Constructed Facilities, Volume 1, Issue 2, May 1987.
Discussion of a roof collapse on July 2, 1983. Building was constructed in 1973. There are a lot of "field" changes from the original design.
6" Standard pipe columns, a 32' x 40' grid, W16x36 beams typically, cantilever length unclear as of now, and 24" steel joists at 6'-4 3/4" on center. Roof slope provided of 1/8" per foot.
Commissioner Inquiry Report on the Station Square failure, August 1988
On April 23, 1988, Station Square Mall in Burnaby, British Columbia, a bay collapses shortly after opening of a Gerber girder (i.e. Cantilever) roof system which had no stiffener at the column (text and photo in Jones and Nathan). They had a roof that was used for parking on this one.
Supermarket Roof Collapse in Burnaby, British Columbia, Canada, Jones and Nathan, ASCE Journal of the Performance of Constructed Facilities, Aug 1, 1990.
There are a lot of pieces to this collapse, all having some contribution (Jones and Nathan)
The Jones and Nathan report indicates a W24x76 beam, HSS 12x12x1/2 (or 7/16") column. Note this discussion appears to contemplate or consider the inflection point as a brace, which is now regarded as inaccurate, (See FAQ 1993 - Buckling of Braced Beams with Inflection Points. (there are earlier discussions of inflection points being ineffective as braces for beams in reverse curvature, circa 1993, according to Yura and Helwig's 2010 article).
The Jones and Nathan report does concede that the brace considered at the inflection point may have been ineffective, but considers that due to the low tensile stress in the beam.
Report has some results of calculations, in summary form, you'd have to do the calculations to confirm. Column stability is not explicitly checked.
Note there is a Discussion and a Closure to the article as well.
Closure mentions there were checks of the beam web for crippling and buckling, but does not seem aware of the global stability of the column.
Roof Framing with Cantilever (Gerber) Girders and Open Web Steel Joists, CISC, July 1989
This was apparently issued in full view of the Station Square collapse.
Contains checks for stiffener or design checks to omit stiffener at the beam bearing on the column. Example is for a wide flange column turned so web is perpendicular to the beam span. Unclear if these checks are current or sufficient based on newer research. Note the calculations are in metric (of course). There is quite a bit of treatment of the column/bracing.
Cantilever Roof Framing using Rolled Beams, AISC, circa 1994. [10/20/24 Bill discusses this a bit and gives 1994 as the date in that article.]
Protypical beam sizing (in LRFD) for 30x30, 30x40, 40x30, 40x40, 40x50, 50x40, and 50x50' spans.
Live load = 12 psf, Dead load = 18 psf, Uplift = 14 psf.
Live load = 20 psf, Dead load = 20 psf, Uplift = 14 psf.
Live load = 30 psf, Dead load = 18 psf, Uplift = 14 psf.
Live load = 12 psf, Dead load = 35 psf, no uplift.
Live load = 30 psf, Dead load = 35 psf, no uplift.
Note that beam flange width may be too small for joists to be aligned and still meet minimum bearing per SJI. Joists may need to be staggered to meet minimum bearing. This is probably more true for the lighter load cases and the smaller bays where W12, W14, and W16 beam sizes appear.
Beam sizing example
Beam splice connection calculation example (plates)
Double angle connection (to end column) calculation example.
Zero figures or beam moment diagrams for those interested in that.
I do not see column designs.
The AISC guide does not show stiffener at the web of the beam framing over the column. It's doubtfulI'm unclear if this is an omission but there's a specific note on the various drawings that a stiffener is not required. As it says "Top of columns are laterally restrained by the tie joist/girder."
Design of Steel Beams in Cantilever-Suspended-Span Construction, Essa and Kennedy, ASCE Journal of the Structural Division, Nov 1995.
Design Guidelines for Continuous Beams Supporting Steel Roof Structures, Rongoe, 1996 North American Steel Construction Conference.
Several graphics, a few moment diagrams, a design example for DL=20 psf, LL=30 psf, 42' x 30' grid and 7' joist spacing in LRFD.
Decent bibliography to other articles. Note that Rongoe [9] appears to refer to Jones and Nathan, but says it was in the ASCE Journal of the Structural Division. V4 No. 3. This is the same volume and issue where the paper in J. Perf. Const. Fac. appears so I think this is an error.
Cantilever Beam Framing Systems, Hemstad, AISC Engineering Journal, 3rd Quarter 1999. (as of 5/10/24 this is a free download). This article contains a cost comparison of various framing schemes. [ References Essa and Kennedy, but says it's in AISC Eng. Jour. which I think is in error, I can't find any Engineering Journal article from Kennedy in 1995.]
Cantilever and Suspended Span Roof Framing System - Herbert L. Bill, Jr. - J. Archit. Eng. 2000 - Makes an oblique mention of "... the designer must be cautious in using this system, because it may require bearing stiffeners over the columns..." and also mentions some concerns with pattern loading, while there isn't a derivation of the system, it does mention it's impractical to cantilever the beams to the point that the double cantilever span has an equal positive and negative bending moment (wl[sup]2[/sup]/16). It seems that's pretty clearly made in the AISC publication as well.
Roof Drainage Design and Analysis: Structural Collapses, Responsibility Matrix, and Recommendations, Jordan ASCE Forensic Engineering 2005. Contains another roof failure where there's no stiffener at the bearing point with cantilever framing.
Cantilever Roof Structural Failure Caused by Excessive Construction Loads Flawed Design, and Poor Maintenance, Hal Cain, ASCE Forensic Engineering, 2009 [You can reach Hal Cain via linked in, I got a reply 5/9/2024.] This is an older building, leaking roof, corroded steel deck, that partially collapsed during roof replacement while they were removing ballast/gravel, beam in question is W14x22. Text mentions it is a W8x24 column.
Update 6/3/24 - Changed date of the inflection beam FAQ based on another article, which was published around 1993. Corrected comments on Cain (2009), column was a W8x24 per the text.
Update 10/14/24 - Added Jordan article from 2005.
Update 10/20/24 - Information from reading Bill (2000).
[Mehta only references codes, no other research articles]
Roof Collapse, Magic Mart Store, Bolivar, Tennessee, Bell and Parker, Journal of Performance of Constructed Facilities, Volume 1, Issue 2, May 1987.
Discussion of a roof collapse on July 2, 1983. Building was constructed in 1973. There are a lot of "field" changes from the original design.
6" Standard pipe columns, a 32' x 40' grid, W16x36 beams typically, cantilever length unclear as of now, and 24" steel joists at 6'-4 3/4" on center. Roof slope provided of 1/8" per foot.
Commissioner Inquiry Report on the Station Square failure, August 1988
On April 23, 1988, Station Square Mall in Burnaby, British Columbia, a bay collapses shortly after opening of a Gerber girder (i.e. Cantilever) roof system which had no stiffener at the column (text and photo in Jones and Nathan). They had a roof that was used for parking on this one.
Supermarket Roof Collapse in Burnaby, British Columbia, Canada, Jones and Nathan, ASCE Journal of the Performance of Constructed Facilities, Aug 1, 1990.
There are a lot of pieces to this collapse, all having some contribution (Jones and Nathan)
[li]Beam shown as W24x104 changed to W24x76 without a revision mark[/li]
[li]EOR called for additional bottom chord extensions, then cancelled when informed there would be a charge.[/li]
[li]Peer review of the structure and collaboration with the original EOR to revise, mostly by analysis tweaks, eventually accepting the beam "as is"[/li]
[li]"bottom" (structural slab) thickness increased from 2" to 3" (+12 psf)[/li]
[li]bottom structural slab actually cast 1/2" thicker[/li]
[li]topping portion (above the insulation) cast 1/4" thicker than on the details (another +9 psf for the two)[/li]
[li]expansion of walkway using concrete versus the insulation the structural engineer understood to be intended[/li]
*Not discussed is the lack of a stiffener at the beam where it lands on the column.*[li]EOR called for additional bottom chord extensions, then cancelled when informed there would be a charge.[/li]
[li]Peer review of the structure and collaboration with the original EOR to revise, mostly by analysis tweaks, eventually accepting the beam "as is"[/li]
[li]Incorrect consideration of live load reduction[/li]
[li]Reduction of beam moment to "face of column"[/li]
[li]Increased yield strength used for beam from 1 coupon test taken from beam web in the same heat (not statistically significant to justify)[/li]
[li]Reduction of beam moment to "face of column"[/li]
[li]Increased yield strength used for beam from 1 coupon test taken from beam web in the same heat (not statistically significant to justify)[/li]
[li]"bottom" (structural slab) thickness increased from 2" to 3" (+12 psf)[/li]
[li]bottom structural slab actually cast 1/2" thicker[/li]
[li]topping portion (above the insulation) cast 1/4" thicker than on the details (another +9 psf for the two)[/li]
[li]expansion of walkway using concrete versus the insulation the structural engineer understood to be intended[/li]
The Jones and Nathan report indicates a W24x76 beam, HSS 12x12x1/2 (or 7/16") column. Note this discussion appears to contemplate or consider the inflection point as a brace, which is now regarded as inaccurate, (See FAQ 1993 - Buckling of Braced Beams with Inflection Points. (there are earlier discussions of inflection points being ineffective as braces for beams in reverse curvature, circa 1993, according to Yura and Helwig's 2010 article).
The Jones and Nathan report does concede that the brace considered at the inflection point may have been ineffective, but considers that due to the low tensile stress in the beam.
Report has some results of calculations, in summary form, you'd have to do the calculations to confirm. Column stability is not explicitly checked.
Jones and Nathan said:
Note there is a Discussion and a Closure to the article as well.
Closure mentions there were checks of the beam web for crippling and buckling, but does not seem aware of the global stability of the column.
Closure said:
Roof Framing with Cantilever (Gerber) Girders and Open Web Steel Joists, CISC, July 1989
This was apparently issued in full view of the Station Square collapse.
Contains checks for stiffener or design checks to omit stiffener at the beam bearing on the column. Example is for a wide flange column turned so web is perpendicular to the beam span. Unclear if these checks are current or sufficient based on newer research. Note the calculations are in metric (of course). There is quite a bit of treatment of the column/bracing.
Cantilever Roof Framing using Rolled Beams, AISC, circa 1994. [10/20/24 Bill discusses this a bit and gives 1994 as the date in that article.]
Protypical beam sizing (in LRFD) for 30x30, 30x40, 40x30, 40x40, 40x50, 50x40, and 50x50' spans.
Live load = 12 psf, Dead load = 18 psf, Uplift = 14 psf.
Live load = 20 psf, Dead load = 20 psf, Uplift = 14 psf.
Live load = 30 psf, Dead load = 18 psf, Uplift = 14 psf.
Live load = 12 psf, Dead load = 35 psf, no uplift.
Live load = 30 psf, Dead load = 35 psf, no uplift.
Note that beam flange width may be too small for joists to be aligned and still meet minimum bearing per SJI. Joists may need to be staggered to meet minimum bearing. This is probably more true for the lighter load cases and the smaller bays where W12, W14, and W16 beam sizes appear.
Beam sizing example
Beam splice connection calculation example (plates)
Double angle connection (to end column) calculation example.
Zero figures or beam moment diagrams for those interested in that.
I do not see column designs.
The AISC guide does not show stiffener at the web of the beam framing over the column. It's doubtful
Design of Steel Beams in Cantilever-Suspended-Span Construction, Essa and Kennedy, ASCE Journal of the Structural Division, Nov 1995.
Design Guidelines for Continuous Beams Supporting Steel Roof Structures, Rongoe, 1996 North American Steel Construction Conference.
Several graphics, a few moment diagrams, a design example for DL=20 psf, LL=30 psf, 42' x 30' grid and 7' joist spacing in LRFD.
Decent bibliography to other articles. Note that Rongoe [9] appears to refer to Jones and Nathan, but says it was in the ASCE Journal of the Structural Division. V4 No. 3. This is the same volume and issue where the paper in J. Perf. Const. Fac. appears so I think this is an error.
Cantilever Beam Framing Systems, Hemstad, AISC Engineering Journal, 3rd Quarter 1999. (as of 5/10/24 this is a free download). This article contains a cost comparison of various framing schemes. [ References Essa and Kennedy, but says it's in AISC Eng. Jour. which I think is in error, I can't find any Engineering Journal article from Kennedy in 1995.]
Cantilever and Suspended Span Roof Framing System - Herbert L. Bill, Jr. - J. Archit. Eng. 2000 - Makes an oblique mention of "... the designer must be cautious in using this system, because it may require bearing stiffeners over the columns..." and also mentions some concerns with pattern loading, while there isn't a derivation of the system, it does mention it's impractical to cantilever the beams to the point that the double cantilever span has an equal positive and negative bending moment (wl[sup]2[/sup]/16). It seems that's pretty clearly made in the AISC publication as well.
Roof Drainage Design and Analysis: Structural Collapses, Responsibility Matrix, and Recommendations, Jordan ASCE Forensic Engineering 2005. Contains another roof failure where there's no stiffener at the bearing point with cantilever framing.
Cantilever Roof Structural Failure Caused by Excessive Construction Loads Flawed Design, and Poor Maintenance, Hal Cain, ASCE Forensic Engineering, 2009 [You can reach Hal Cain via linked in, I got a reply 5/9/2024.] This is an older building, leaking roof, corroded steel deck, that partially collapsed during roof replacement while they were removing ballast/gravel, beam in question is W14x22. Text mentions it is a W8x24 column.
Update 6/3/24 - Changed date of the inflection beam FAQ based on another article, which was published around 1993. Corrected comments on Cain (2009), column was a W8x24 per the text.
Update 10/14/24 - Added Jordan article from 2005.
Update 10/20/24 - Information from reading Bill (2000).