The specifying professional can now use joists and joist girders with either the ASD (Allowable Strength Design) or LRFD (Load and Resistance Factor Design) methods included in the Canam joist catalog, 42nd Edition SJI Specifications.
How it affects your drawings
Structural contract drawings should show which method is being used, so joists and joist girders can be designed accordingly. Canam’s framing plans and bills of material include a “check box” to indicate whether the project is ASD or LRFD. Whether you choose ASD or LRFD is dictated by the design method used for other structural steel components for the structure.
LRFD tips
When you use LRFD, provide factored loads on the contract drawings. This is a good idea because for some loads, the proportion of dead and live load may not be available or may be a matter of judgment. Doing this is also a safer, more conservative approach in case there is any confusion regarding the load factoring.
For LRFD joist girders, the letter “F” should be used, not be used—not “K” -- at the end of the designation, to show that the kip loading has already been factored.
Example: An ASD designation would be 36G7N12K , but an LRFD designation would be 36G7N18F.
NOTE: Where live loads are provided solely for purposes of a live load deflection check, these loads should not be factored, such as for “load/span” joists.
Is there any economic advantage to specifying ASD or LRFD?
Yes and no… There is virtually no difference between an ASD or an LRFD joist of the same designation.
But by choosing ASD or LRFD in some cases, you may be able to select a different designation, or in the case of special joists and joist girders, the result may be a heavier or lighter joist, depending primarily on the ratio of live to dead load.
- Live and dead loads: ASD and LRFD will produce similar results and joist selections when the ratio of nominal live to dead load is 3 to 1 (an average LRFD load factor of 1.5). For extremely light dead loads, and a ratio of live to dead load greater than 3 to 1, the use of ASD could create a lighter joist and joist girder framing system for gravity loads than LRFD. Conversely, if the live and dead loads are approximately equal, and any case where the live to dead load ratio is less then 3 to 1, LRFD could create a lighter joist and girder framing system under gravity loads. (See case studies).
Weight savings for ASD/LRFD
For “load/span” designs and the typical ratios of live and dead load on joist projects, LRFD can generate some weight savings. However, the effect of wind loads, both up and down, in combination with the gravity loads, may create a more severe effect with LRFD than with ASD.
Example: A gross uplift of 25 psf for case 5. The result of the net wind uplift load combination for ASD is 78 plf, while the result of the net wind uplift load combination for LRFD is 132 plf. The LRFD net uplift in this case has a load factor of almost 1.7 when compared to the ASD net uplift.
With ASD, the downward wind pressure on a roof may not be “additive” to the design live load. This is because a factor of 0.75 can be used for the load case with live plus wind, and the result may be less than the live load alone. But with LRFD load combinations, the downward wind pressure is always “additive” to the live load.
Canam can run comparative ASD and LRFD quotations for any particular project or load case to aid the specifying professional in making the most economical selection.
Case studies comparing ASD/LRFD & resulting designations
Case 1 – 24K, 40 foot span, 10 psf DL, 20 psf LL, 5’-0 c/c
- ASD selection: 24K4
- LRFD selection: 24K4
- “Load/span” weight comparison: equal
Case 2 – 24K, 40 foot span, 10 psf DL, 30 psf LL, 5’-0 c/c
- ASD selection: 24K5
- LRFD selection: 24K5
- “Load/span” weight comparison: equal
Case 3 – 24K, 40 foot span, 10 psf DL, 40 psf LL, 5’-0 c/c
- ASD selection: 24K7
- LRFD selection: 24K8
- “Load/span” weight comparison: equal
Case 4 – 26K, 50 foot span, 20 psf DL, 20 psf LL, 6’-0 c/c
- ASD selection: 26K10
- LRFD selection: 26K10
- “Load/span” weight comparison: LRFD 5 % lighter
Case 5 – 26K, 50 foot span, 20 psf DL, 30 psf LL, 6’-0 c/c
- ASD selection: 26K12
- LRFD selection: 26K12
- “Load/span” weight comparison: LRFD 9 % lighter
Case 6 – 26K, 50 foot span, 20 psf DL, 40 psf LL, 6’-0 c/c
- ASD selection: (beyond K12)
- LRFD selection: (beyond K12)
- “Load/span” weight comparison: equal
Case 7 – 32LH, 50 foot span, 25 psf DL, 20 psf LL, 6’-0 c/c
- ASD selection: 32LH06
- LRFD selection: 32LH06
- “Load/span” weight comparison: LRFD 10 % lighter
Case 8 – 32LH, 50 foot span, 25 psf DL, 30 psf LL, 6’-0 c/c
- ASD selection: 32LH06
- LRFD selection: 32LH06
- “Load/span” weight comparison: LRFD 6 % lighter
Case 9 – 32LH, 50 foot span, 25 psf DL, 40 psf LL, 6’-0 c/c
- ASD selection: 32LH08
- LRFD selection: 32LH07
- “Load/span” weight comparison: LRFD 4 % lighter
Case 10 – 24LH, 35 foot span, 50 psf DL, 50 psf LL, 4’-0 c/c
- ASD selection: 24LH05
- LRFD selection: 24LH04
- “Load/span” weight comparison: LRFD 3 % lighter
Case 11 – 24LH, 35 foot span, 50 psf DL, 75 psf LL, 4’-0 c/c
- ASD selection: 24LH06
- LRFD selection: 24LH06
- “Load/span” weight comparison: LRFD 4 % lighter
Case 12 – 24LH, 35 foot span, 50 psf DL, 125 psf LL, 4’-0 c/c
- ASD selection: 24LH09
- LRFD selection: 24LH09
- “Load/span” weight comparison: equal
|