Minimum Beam/Joist Size in roof UL Assemblies
Minimum Beam/Joist Size in roof UL Assemblies
(OP)
UL roof assemblies list minimum sizes for steel supports that work in that assembly. For instance, UL P723 says W6x16, 10K1 or 12K5 steel joist minimum size. That is pretty common.
I cannot find any basis for that "minimum size". Weight per LF? Depth? Flange thickness? Web thickness? Is a W10x12 too small? What about a 12K1? What say you wise ones?
Maybe more importantly, has any building official ever interpreted a size on your plans as too small for the roof UL listing?
I cannot find any basis for that "minimum size". Weight per LF? Depth? Flange thickness? Web thickness? Is a W10x12 too small? What about a 12K1? What say you wise ones?
Maybe more importantly, has any building official ever interpreted a size on your plans as too small for the roof UL listing?
_________________________
TKE






RE: Minimum Beam/Joist Size in roof UL Assemblies
I can't recall what that criteria was - I seem to remember something about minimum weight per surface area or some such comparison with the actual listed/tested size.
If you have the UL book, look in the early pages of the book where they provide an extensive commentary on the use of the UL listings.
RE: Minimum Beam/Joist Size in roof UL Assemblies
RE: Minimum Beam/Joist Size in roof UL Assemblies
_________________________
TKE
RE: Minimum Beam/Joist Size in roof UL Assemblies
7. Steel Joists
The specified minimum-size joist in floor- or roof-ceiling designs is the joist that meets the requirements for both the minimum depth and the minimum weight per foot. Joists that exceed the specified minimum size may be used, provided the accessories are compatible. The dimension from the bottom chord of joists to the ceiling, whether given or calculated, is a minimum.
Spacing between joists may be increased from that specified to a maximum of 4 ft on centers if the floor slab meets structural requirements and the spacing of the hanger wires supporting the ceiling is not increased. Where it is necessary to provide support for the ceiling hanger wires between the joists, this may be accomplished by using 1-1/2 in., 16 gauge or larger cold-rolled steel channels. Each channel with its web oriented vertically should be placed on top of and perpendicular to the joist's bottom chord and tied thereto with a double strand of 18 SWG galvanized steel wire.
The area of bridging bars or angles specified in the individual designs is a minimum. Larger bridging may be necessary in order to meet the structural and/or code requirements.....K-Series joists may be substituted for other joists specified in floor- or roof-ceiling designs as follows:
And for beams...
1. Beam Size
For fire-resistance purposes, the minimum beam size is expressed in terms of a W/D ratio, where W is the weight of the beam per lineal foot and D is the perimeter of protection material at the interface between the steel section and the protection material. Accordingly, beams of the same configuration and having a greater W/D ratio than the beam size specified in the fire-resistive design are considered larger than the specified minimum-size beam and may be used in that design.
W/D values are published by the American Institute of Steel Construction. In 2001, the method used to calculate the perimeter was refined to include the fillets of hot-rolled sections rather than assuming right-angle intersections. An example of this change results in the W/D value for a W8x28 section changing from 0.80 to 0.819.
Application of equations in the Fire-resistance Directory that include proportional relationship of the (W/D) value are not affected by the change in the calculation process for (W/D), provided the (W/D) values used are determined by a single method....
Here is the link to the info above: Link to UL text
RE: Minimum Beam/Joist Size in roof UL Assemblies
If the W/D must be higher than that, then the minimum sizes are W8x24, W10x26, W12x30, W14x34, W16x31, and everything larger. That cannot be right.
TABLE for W/D:
http://www.adfire.com/images/MD%20WD%20TABLE%20Jun...
That link says:
How May W/D, M/D and A/P Rations be Used?
When the size of a structural member differs from the minimum size specified in a fire test design, compare the W/D, M/D or A/P ratios. If the W/D, M/D or A/P is greater than or equal to that of the test design, the fire resistance rating and method of protection (minimum thickness, etc.) are applicable to that member.
The UL “Fire Resistance Directory” and ULC “List of Equipment and Materials” contain equations for adjusting thickness of directly applied fire resistive materials for beams and columns of alternate size. The ratios from the attached tables may be substituted directly into those equations. Some fire resistance designs include proprietary thickness equations that utilize W/D, M/D and/or A/P ratios.
Doesn't that mean that the fireproofer must increase the thickness of the spray applied fireproofing for members less than the minimum? It doesn't mean that you cannot use them. Do you agree? We can specify an W8x10, and they just have to increase the spray applied fireproofing from 7/8" to something larger they calculate.
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TKE
RE: Minimum Beam/Joist Size in roof UL Assemblies
I'm not sure I agree with you - a larger beam size feels, intuitively, stronger, but under fire exposure, deeper beams can buckle easier under local flange or web distortions than shallower beams.
What you suggest about the thicker fireproofing on beams less than the minimum sounds correct to me.
RE: Minimum Beam/Joist Size in roof UL Assemblies
_________________________
TKE
RE: Minimum Beam/Joist Size in roof UL Assemblies
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin