Minimum Beam/Joist Size in roof UL Assemblies 1

[TopKnot]

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?

_________________________
TKE

 

[JAE]

I believe that in the UL book they have some language to the effect that alternative sizes can be used provided those sizes meet certain criteria.
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.

 

[Triangled]

I have had officials object to assemblies because they were not an exact match. On some of them, I have gotten my assembly through by developing a reasoned argument that my structural member would be equivalent to or better than the assembly structural element, for the relevant property, be it fire rating or sound transmission.

 

[TopKnot]

The W6x16 is the common minimum size. It seems like someone should know what the minimum size for a W10 and a W12 should be. I don't have the UL book, and I don't understand what a minimum size applies at all, if everything is fire-proofed. My guts says ignore it.

_________________________
TKE

 

[JAE]

Here is the UL text on bar joists:
[blue]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:[/blue]

And for beams...

[blue]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....[/blue]

Here is the link to the info above: Link to UL text

 

[TopKnot]

That is confusing. I don't see minimum beam size expressed as W/D. UL P723 only says W6x16 is the minimum size. It's W/D is 0.584.

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 WD TABLE June 7 2007.pdf

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.

_________________________
TKE

 

[JAE]

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.

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.

 

[TopKnot]

Thanks JAE for the input. When I read "minimum size = W6x16", I first assumed that we cannot use sizes smaller than that. In reality we can use any size of beam or joist. It is the fireproofer's job to increase thicknesses where required. Does any one disagree?

_________________________
TKE

 

[paddingtongreen]

Even though you may use different beam/joist sizes, it might be well to stick with the book if you can. Part of the job you do for your clients is to walk the design through the building code official review, and not all of them are willing to waiver the approved shapes, even if it is in accordance with UL rules.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin