Ontario Building Code, steel beam tables vs engineered calculation discrepency

[lubos1984]

Hi Guys,

Just wondered if any of you can shed some light on the steel beam tables provided in Part 9 of the Ontario Building Code.
I'll take a simple example:
According to the table a W8x18 steel beam with a 19'6" tributary width should be able to span 19'.
Beam is assumed to be laterally supported with typical residential loads applied (10DL and 40 LL) PSF

When i run the calcs i get some major failure in deflection (over 50%). I can't figure out what reduction factors are being used and what is the rationale. This is especially an issue if I start stamping drawings where architect uses these span tables yet I think they are not compliant with Part 4 of the Code (using engineering calculations).

If anyone can provide any insight here would be much appreciated.

 

[cliff234]

I am in the US and am not familiar with those tables. That said, deflection is a serviceability issue – not a strength limit state. Is it possible the tables consider only the strength limit state? If so, at a minimum, there should be a warning that the values in the table do not consider deflection.

In any case, I would never rely on a table like this for beam design. It is a bad idea, just as it’s a bad idea to use Table 3-6 in the AISC Steel Manual for mandating required beam shear connection strengths (unfortunately, a widespread bad practice)! Just do the calculations!

 

[dik]

...all more the reason that deflection should be included.

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

-Dik

 

[lubos1984]

The code should consider deflection as well since as you guys mention there are serviceable issues especially in houses where you have drywall so I’m sure it’s taken into account. it is mentioned in Part 4 of the code. I wonder if there is a reduction factor for deflection oflaterally supported beams that I am not aware of that is being applied here ?
Link to code provision is below if anyone interested

https://www.buildingcode.online/1823.html

 

[skeletron]

Check the commentary (A-Table 9.23.4.3) for the derivation of the span. Copied here for convenience. They use 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've calc'ed out the table (it might be a previous thread too...) and come to the same conclusion as you. Most are within tolerance for capacity, but the deflection significantly exceeds allowable. It's a pretty hand-wavey method for Part 9 to use a revised load reduction factor without stating what it is.

If an architect is designing the beam with the tables, let them take the liability. If they are involving an engineer, design the beam with your calculations unless you want the liability of the table.

 

[STpipe]

If you think that's bad, wait until you start considering continuous beams (a very common occurrence in residential/Part 9 construction) compared to the Part 4 calculations.

 

[lubos1984]

wow, just like that.

Skeletron big thanks for providing the commentary on this. I assume this likely applies to all the tables in Part 9 ?
I have an architect that sized everything with Part 9 but now needs me to examine one steel beam because of a point load. am i to tell him to resize all of them now that i'm involved ? unless i stamp for "THIS BEAM ONLY". This will cause all sorts of confusion now. what a mess.

 

[canwesteng]

I would stamp as "engineering for members not complying with part 9 framing requirements only" or something to that effect on the stamp. Though once you stamp it, it falls on you to make sure the framing does comply with part 9.

 

[jayrod12]

Assuming this supports only one floor, using the code prescribed live load reduction in part 4, you can reduce the live loads 17%. If it's a two storey, with the trib areas indicated, you can reduce it 32%. So there's likely some room there as well.

 

[TLHS]

If I were to put a note, I would put "Beam xxx sized based on Part 4 methods to account for xxx loading. All other beams by architect/designer/whoever." If you want to explicitly cover them, I'd replace the second sentence with "All other beams sized as per clause 9.x.x"

 

[EngDM]

Beam xxx

ahem, pg 13 please.

 

[lubos1984]

Thank you all for your suggestions. This discrepancy between Part 4 and Part 9 raises all sorts of questions for me. For example if a beam is sized for part 9 but i have a doorway opening above it that creates two small point loads is that beam supposed to be sized as Part 4 now that its no longer a continuous uniform load ? All of a sudden its much heavier or can i still claim the special powers for live load reduction ?

 

[TLHS]

If it makes you feel better (worse?), this is also happening for other elements. We had a thread a couple months ago where we dug into some tables and there are lots of little factors in part 9 that don't align even in the parts that you would expect to align. The Engineering Guide for Wood Frame Construction also has a specification that makes some little modifications to part 9, but it's incorporated by reference so that's an even more fun gray area. They don't seem to have a revised area load reduction factor though, they just reference the NBCC one.