OBC Part 9 Steel Beam Spans

[STpipe]

I'm reviewing the structure of an existing home. There is a continuous W200x27 beam that's supported on posts at every 10-14' by jack posts. Wood joists bear on the top flange of the beam, and the overall length of the beam is ~64-65 ft. The building was designed under Part 9 of the Ontario Building Code. This is my first time diving into the prescriptive requirements of the building code, but this raises a few questions:

1. Under the assumptions stated in the commentary, the prescriptive requirements are for simple spans with a floor providing continuous lateral support to the top flange of the beam (thus LTB cannot happen). So to me, once they make the beam continuous, shouldn't that push the design to Part 4, and the original designer should have sent this off to an engineer to design?

2. I'm assuming that the jack posts provide no lateral stability to the bottom flange, therefore for the purposes of LTB the unbraced length would be the total length of the beam (64'-65')? That would also be the reason why the prescriptive requirements are limited to simple spans since LTB doesn't come into play and it's simple to establish span guidelines?

 

[lexpatrie]

This doesn't apply to the Canadian code, because I don't know it well,

While the beams are sized for simple supports in the prescriptive part of the code, as noted the continuous / cantilever condition changes that. Force distribution and stress reversal (compression on the bottom flange) may/should occur. There are equations for "tension flange continuously braced" for Cb in the AISC Steel Design after College slide show (page 24). It might be justifiable there, via computation, but that doesn't sound like the job, here. Just mentioning an avenue. I'm not clear the OP is actually on the hook for signing off on the design.

Steel Design After College

You also have to consider the extra force drawn into the interior column, and the stability of the beam web against the concentrated load/K factor for the interior column. Same slideshow discusses that as well.

Regards,
Brian

 

[skeletron]

Personally, I don't trust or use the steel beam span tables in Part 9. It's so g'damn easy to run a steel beam with either the CISC beam selection tables or a program like Woodworks Sizer or ForteWeb. A quick cruise through the W200x27 beam design using 1.5kPa / 1.9kPa (DL/LL):

@ 7.3m --> DCR = 87%, L/233 for live load, L/126 for total load
@ 6.9m --> DCR = 97%, L/221 for live load, L/120 for total load
@ 6.6m --> DCR = 106%, L/211 for live load, L/115 for total load
@ 6.3m --> DCR = 112%, L/208 for live load, L/114 for total load
@ 6.1m --> DCR = 120%, L/201 for live load, L/110 for total load
@ 5.9m --> DCR = 125%, L/198 for live load, L/109 for total load
@ 5.8m --> DCR = 134%, L/187 for live load, L/103 for total load

For that final span (5.8m above) to work in bending, the loads would need to be adjusted to 0.72kPa (15psf) / 1.7kPa (35psf) (DL/LL). To get the spans to work for L/360 live load deflection, the live load would need to be adjusted to just under 20psf.

Even a spot check of a W250x39 does not pan out:
@ 9.0m --> DCR = 109%, L/191 for live load, L/103 for total load
@ 8.4m --> DCR = 125%, L/177 for live load, L/96 for total load

One key point, is that the Note from the Commentary also indicate this statement (not shown in the above screenshot):
"The calculation used to establish the specified maximum beam spans also applies a revised live load reduction factor to account for the
lower probability of a full live load being applied over the supported area in Part 9 buildings."

With regards to the OP's question:
#1: If the continuous span of the beam is >40ft (12.2m), I would consider this outside of the prescriptive requirements of Part 9. My interpretation is supported by Clause 9.4.2.1.(c) and Table 9.23.4.3, which does not list span lengths over 11.4m.
#2: Yes. I would consider the jack posts essentially ineffective in supporting the bottom flange.

 

[Winterpeg]

When looking at part 9 in Canadian codes for the first time, it helps to understand that all of the prescriptive requirements of part 9 are simply canned part 4 solutions presented in tabulated form so that home (or small building) builders don't have to hire an engineer for every build if they stick with standard systems. The assumption of simple spans may just mean that the beams are sized based on simple span moments, which would be conservative.

 

[STpipe]

Winterpeg,

The assumption is the compression flange is fully laterally supported, which means the flexural capacity of the beam is governed by the plastic capacity of the beam (assuming Class 1 or 2), outlined by the requirements of Section 13.5 of S16-14. Once you add an intermediate support, then the bottom flange goes into compression in the negative moment regions near the interior support, continuous support on the bottom flange is not provided, therefore that pushes the design of the beam to Section 13.6.

Take a W200x27 - simply supported you're looking at a flexural capacity of ~87 kN m because the top flange is continuously supported.

Take that same W200x27 and make it 12.2 m long with a jack post supporting the beam at mid-span (beam span of 6.1m). Your moment capacity reduces to ~32 kN m.

Table 9.23.4.3 allows you to use a W200x27 at that span up to a supported joist length of 4.8m. For both a simple span and a two span beam (and considering the live load reduction) you get a factored moment of ~98 kN m, more than 3 times the capacity of the beam. And that's assuming that you're providing sufficient lateral support at the post to take your unbraced length as 6.1m. If you're not, then the moment capacity drops to ~15 kN m.

 

[Winterpeg]

Taking a quick look at this particular case, and you're right, it doesn't even seem to work by their own requirements (based on max allowable span). Even if simply supported such that the table applies, it seems to fail using their own loading, and doesn't meet their stated L/360 deflection limit.

As you state, continuity is no help as the negative moment still exceeds Mr so the Lu for negative moment regions discussion is moot. I wonder how many tens of thousands of these are installed...

 

[WinelandV]

We were brought on because the municipality wanted an inspection and letter from a structural engineer to indicate whether the structural aspect of the home meets code or are adequate. Everything hinges on the interpretation of that prescriptive clause for determining allowable steel beam spans.

What prompted the AHJ to get an outside engineer (you) involved? I would imagine the AHJ's inspector can look at the construction and determine if it meets the prescriptive requirements.

Please note that is a "v" (as in Violin) not a "y".

 

[STpipe]

Winterpeg,

I think it's all contingent on what they use for their live load reduction factor. I calculated mine based on Part 4, and the flexural demand is ~12% higher than the capacity - not the end of the world. Deflection I'm at around ~17% higher. Again, not the end of the world.

I think it all hinges on the statement in the commentary: "The calculation used to establish the specified maximum beam spans also applies a revised live load reduction factor to account for the lower probability of a full live load being applied over the supported area in Part 9 buildings".

I interpret that as using a different LLR than what's presented in Part 4, but they don't actually indicate how they calculate that value. Nevertheless, even if I could calculate the correct LLR, the reduction in strength from LTB should still make that table not applicable for continuous spans.

WinelandV,

We got involved because there were questions about whether the AHJ's inspector did a satisfactory job in their inspection. It's a small town and I think this is the only inspector they had. An independent opinion so to speak to ensure all parties (homeowner & AHJ) were satisfied with the structural aspect of the home.