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?

 

[dik]

Maybe technically, but as long as the flange is laterally supported the provided beam is stronger and a lot stiffer than that prescribed. It's likely a call by the building inspector, who may not be aware of the difference. The length of the beam is too long, and the person providing the drawing should have known and should have provided a splice. The metal fabricator can do this and ask for EOR/drafting service approval.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JAE]

I think you are correct to be concerned about LTB with the bottom chords not braced. One could add bracing of course.
Also concern about beam rollover if there aren't vertical stiffener plates at each column.

 

[STpipe]

Hi dik,

Thanks for the response. The beam does have a splice.

There is no lateral support to the bottom flange - shouldn't LTB be a consideration at that point?

 

[dik]

as JAE noted... a bit of concern.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[jayrod12]

I agree that through our truly theoretical lens, potentially a problem.

However, anecdotally I've seen countless houses with this configuration (in excess of a hundred), both steel beams and old wood beams, and nothing has performed poorly.

 

[STpipe]

JAE,

That could be a solution, although would impact how the finished ceiling looks. For the stiffener, similar to jayrod12's comment, anecdotally, I can't recall ever seeing one on a beam installed in a basement.

jayrod12,

I think wood beams are a little bit different. It's a bit easier to codify the lateral support requirements and it's material failure that governs. Similarly, I'm pretty sure I've seen this configuration as well, but it could be that those beams were designed accordingly. I ran rough numbers in forteweb, and it's showing that my W8x18 fails, however upsizing to a W8x31 passes.

I guess I'm trying to figure out if this is something that should be flagged, or if this is just standard and acceptable practice for residential construction.

 

[jayrod12]

I've probably seen almost a hundred steel beams. And likely the exact size you're indicating.

Now, I'm not saying that by the theoretical numbers it should work, it doesn't. But there has not been a rash of failures that I'm aware of. And if I've seen hundreds, there's probably hundreds of thousands of houses built this way. You'd think we'd of heard something.

Also, even for the wood beams, in new builds, I see continuous beams with dinky thin little telepost plates with maybe a screw or two in them.

I wouldn't be surprised if the allowable spans in part 9 carried over from the old days when the inflection points counted as brace points. And since that idea was disproven, no one's changed anything that wasn't broken in terms of performance in the field.

 

[STpipe]

jayrod12,

That's actually a really good point on inflection points being considered brace points in the past. Definitely something that's before my time, but I've read enough about it on these forums.

 

[JAE]

I think a lot of homes that have steel beams like that were beams that weren't loaded anywhere near their capacity so the LTB just never kicked in.

 

[STpipe]

So at the end of the day, is this something to let go, or something that needs to be addressed. At least based on people's experience here, this shouldn't be an issue because it's standard home construction practice. However from a code perspective, it seems to me that the requirements for steel beams are pretty clear - they should be for simple spans only.

 

[SWComposites]

So why are you evaluating the house? Remodel? Increased loads?

 

[phamENG]

What you need to do is related to why you're doing it.

While I haven't seen these fail from LTB, I have seen them fail from web buckling at unstiffened continuous beam over column connection.

If you're assessing it as is, I would note that it doesn't meet current code and it likely didn't meet the code when it was built (unless it did? I don't know much about Canadian codes or how old the building is). It appears to have performed adequately in service, but shouldn't be considered reliable at design level loading.

If you're adding/changing the load, I'd have it brought up to current standards.

 

[jayrod12]

I'd be going with Pham's plan. If you aren't modifying the loading at all, make a comment and call it done. If you're modifying it somehow, then you've got to make sure it works by the numbers now.

 

[STpipe]

SWComposites, phamENG

You're right that I should have added a bit more detail in my initial post. I'm involved because there were some construction deficiencies, even though it passed all inspections from the AHJ. I guess "existing home" wasn't the best description, as it hasn't been inhabited.

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.

 

[phamENG]

Oh, well then it's easy. Doesn't meet code. Have them fix it.

 

[SWComposites]

So was the original plan designed and sealed by an engineer? Or is it a “builder’s special”?

Agree with pham, ensure it meets code if you are signing off on it.

 

[Enable]

Can you post the commentary you are referring to? I'm from Ontario and the majority of houses I've seen built under prescriptive requirements have a continuous beam. We put posts at ridiculously small spacings such that pretty much every basement for a typical residence has at least two. If the Table 9.24.4.3 is applicable only to single span members it would be pretty much useless given our building conventions.

 

[dik]

Likely what was provided was as stable or more stable than that prescribed... just as an afterthought...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[STpipe]

phamENG,

Thanks for the insight. That's what I was leaning towards.

SWComposites,

Houses under a certain size fall under the prescriptive portion of the code. It allows 'designers' to draft up plans for home construction even if certain aspects would fall under a discipline of engineering. These designers (which could be an architect, developer, contractor etc.) have their own certification process that allows them to submit plans for permit.

By 'stamping' the drawings, they attest to having followed the provisions of the building code. If there is an aspect of the construction that falls outside the prescriptive requirements, then it is their responsibility to kick that back to the correct qualified individual to design that aspect of the home.

Enable,

Here is what the commentary states:

dik,

In what sense?