OWSJ Top Chord Capacity

[Junior3ng]

Hello All,

I am a junior engineer working in a consulting firm, currently evaluating OWSJ's for a client. I have previous experience regarding OWSJ evaluation, however, I do not believe it made sense so my boss went with the safer option of reinforcing it.

Now I am only evaluating the OWSJ, there are no additions of equipment, so reinforcing is not really an option as there are no changes being made.

Similar to my previous project, I am getting the top chord capacity, done manually through CSA S16-19 Chapter 16, much less than the loads provided by the original structural drawings. I am assuming that in the x-x direction, length is the panel points distance, y-y direction is the deck fastening locations, and z-z the spacing between panel points and spacers. The only other option is to reduce loads or span, however, to me, these seem fixed and no where to reduce, as loads were provided by the little structural drawings we received.

I feel as though there is not much room to make errors, I have had senior engineers review and confirm the steps, and at the moment I am a little confused. I would not like to approach the client and say the top chord is at a utilization ratio of 150% even though its been standing for 20 years now.

If anyone had any advice, that would be greatly appreciated! Thank you all

***Disclaimer-no structural drawings or shop drawings were available so site measurements is how we got our sizes.

 

[phamENG]

Not sure how things are done in Canada with these, but in the States the Steel Joist Institute has their own design standard. I've never had a steel joist check out based on AISC, but if analyzed using the SJI specs it makes more sense.

 

[Junior3ng]

Apologies, I do not know how to reply to a message, maybe you can help me with that also,

Can you provide this SJI design standard? I was only able to find a few equations in the specification publications, not sure whether that is what you are referring to,

Thanks for your reply

 

[phamENG]

This is a 1990s era forum, not Reddit. There is no reply. You can quote somebody, but that's about it.

Sorry, I meant Specification. There's more than 'a few equations'...check out section 4, Design and Manufacture.

 

[Once20036]

What did your senior engineers who reviewed and confirmed your steps have to say about it? And why aren't they the ones approaching the client with the results?
When you say its 150% utilized, are those the results of hand calculations or the results of a computer model?

 

[Junior3ng]

What did your senior engineers who reviewed and confirmed your steps have to say about it?

The previous project he said that the calculations are correct, however, I am not sure whether he has experience regarding OWSJ hand calculations, so maybe the calculations are correct in regards to the CSA code, but not applicable to OWSJ top chord analysis.

And why aren't they the ones approaching the client with the results?

This is a major issue with my current firm, my boss is having me do projects essentially by myself, and it is really overwhelming. I am only with 1 Y.O.E and I do not know how to manage projects, clients, calculation, etc (almost everything other than typical calculations) (Sorry for the rant). We are still in the analysis and evaluation stage, so the client has not been approached yet,

When you say its 150% utilized, are those the results of hand calculations or the results of a computer model?

This is based of rough hand calculation of loads and member resistance.

Thanks for your reply

 

[phamENG]

my boss is having me do projects essentially by myself, and it is really overwhelming.

Sounds all too familiar. I can sympathize. If you can't get them to come around, I strongly suggest seeking out a firm with a strong mentoring culture.

 

[Brad805]

When on site did you find any tags on the joists? Do you know what year they are from? Many members have old books that might be of help.

 

[jayrod12]

What shape are the chords? I ask because I have reviewed old shop drawings and have seen extremely high yield strengths for some of the proprietary shapes. So there may be more capacity than you're calculating.

 

[KootK]

If anyone had any advice, that would be greatly appreciated!

In my experience, this usually comes down to your bracing assumptions. The joist designers will oft exploit every advantage they can in this regard (nowadays it's built into their software). What are you using for your K-values relative to the design panel point spacing? If those values are not less than 1.0, there's a very good chance that you are making design assumptions that are much more conservative than the original joist design embodied.

 

[EngDM]

I've talked to people from both Canam and Vulcraft, they take the z-z spacing as zero (or 1mm). The way the guy from Canam explained to me, is that for an angle top chord to fail about z-z it has to flatten out, which would require lifting the entire deck. I agree that this is weird and the code specifically gives guidance on this, but I have had to do that to get my Fez to not govern, and get my loads to work.

If you can't get them to come around, I strongly suggest seeking out a firm with a strong mentoring culture.

As I new engineer myself, I cannot stress enough what pham has said here. I am fortunate enough to be given projects to run with but with the ability to ask whatever questions I need without being made to feel like a burden, and it takes a lot of weight on my shoulder.

At the end of the day, you aren't the ones stamping the drawings. It's part of the EOR's responsibility to stamp for your work, so he should do some due diligence (I'm not saying he is required to hold your hand).

Can you provide your site measurements and what Cf you are getting? I can see what I come up with independantly from you.

 

[Junior3ng]

When on site did you find any tags on the joists? Do you know what year they are from? Many members have old books that might be of help.

There were but they were not readable-rust and paint

 

[Junior3ng]

What shape are the chords? I ask because I have reviewed old shop drawings and have seen extremely high yield strengths for some of the proprietary shapes. So there may be more capacity than you're calculating.

They are standard double angles, we are currently assuming 300MPa for the angle steel strength.

 

[Junior3ng]

In my experience, this usually comes down to your bracing assumptions. The joist designers will oft exploit every advantage they can in this regard (nowadays it's built into their software). What are you using for your K-values relative to the design panel point spacing? If those values are not less than 1.0, there's a very good chance that you are making design assumptions that are much more conservative than the original joist design embodied.

We are currently using 0.9 and 1.0, based on the canadian code. I am using the spacing between panel points for the x-x direction, and deck fastener locations for the y-y direction.

 

[Junior3ng]

I've talked to people from both Canam and Vulcraft, they take the z-z spacing as zero (or 1mm). The way the guy from Canam explained to me, is that for an angle top chord to fail about z-z it has to flatten out, which would require lifting the entire deck. I agree that this is weird and the code specifically gives guidance on this, but I have had to do that to get my Fez to not govern, and get my loads to work.

Thats interesting, the Canadian code opts to provide the "length as the panel point spacing or the spacers, or both, with K=0.9". I have tried to make this 0, however, the chords still fail,

As I new engineer myself, I cannot stress enough what pham has said here. I am fortunate enough to be given projects to run with but with the ability to ask whatever questions I need without being made to feel like a burden, and it takes a lot of weight on my shoulder.

At the end of the day, you aren't the ones stamping the drawings. It's part of the EOR's responsibility to stamp for your work, so he should do some due diligence (I'm not saying he is required to hold your hand).

That has been a major problem I am experiencing. Engineers are available to answer questions, however, it feels that they are getting frustrated. Also, there are no senior engineers involved in my projects, so if there are things I miss due to my inexperience either dont get caught or get caught very late, which is quite embarrassing for myself and the firm when we are facing the client. I understand that I may not be stamping or taking responsibility, but I would still like to produce safe structures, and do good work for my own satisfaction and growth, confidence, etc.

Can you provide your site measurements and what Cf you are getting? I can see what I come up with independantly from you.

Sure, thanks for your help, apologies for the metric units.

Double angles-45mmx45mmx4.2mm, spaced at 28mm
Panel spacing at 610mm (2 feet) and deck fasteners every 304mm (12")
Kx=0.9, Lx=610
Ky=1, Ly=304
fy=300MPa

I am currently getting a Cf of around 140kN-(rough calcs based on simply supported beam and dividing by height of joist (factored 5.5kN/m for 11.5 m span) I have checked typical joist tables and see that this type can work (and the evidence that the joist is still standing), however, please advise otherwise)

Let me know if you need any other information. Thanks for all your help!

 

[Once20036]

This is based of rough hand calculation of loads and member resistance.

I`ll stay away from the mentorship type stuff as that's being addressed above.
If you`re using hand calculations, that rules out software stuff. A couple things to consider

1) These old joists ignored flexure between top chord panel points, assuming the panels points were 2' apart or less.
2) We recently had a conversation with Canam, and believe that they used the SJI standards in Canada.
3) 350 MPA is more likely the case (50 ksi) per SJI specs.
4) SJI has a great design tool thats a free download from their website. It'd be a great way to double check your hand calcs.

What's your limiting compression capacity (what failure state? x-x, y-y, z-z)?
How does your compression capacity compare to your bottom chord tension capacity?

 

[EngDM]

Double angles-45mmx45mmx4.2mm, spaced at 28mm
Panel spacing at 610mm (2 feet) and deck fasteners every 304mm (12")
Kx=0.9, Lx=610
Ky=1, Ly=304
fy=300MPa

With the values you've provided I'm getting a Cr of 128kN (Feyz governs). If I set z-z bracing to 1mm I jump up to 175kN and Fex governs.

 

[dik]

I design connections for metal fabricatore. If it's an OWSJ as part of many, I generally treat the joist as a beam and check the area of steel required as 0.9 x depth for moment d to determine the axial loads in the T & B chord, and determine diagonal forces using the reactions. If something different, I generally do a quick 2D computer truss. If it's a matter of the drawings causing a point load applied to the chord (often the case for added mech equip) I add this note to my review:

-EOR/ARCH TO CONFIRM WHERE MEMBERS ARE NOT SUPPORTED AT PANEL POINTS OF OWSJ, PROVIDE A HSS 1X1X0.125 STRUT WELDED FROM POINT OF SUPPORT TO CLOSEST TOP OR BOT PANEL POINT OF SAME OWSJ. EOR/ARCH TO CONFIRM LOCATIONS, MEMBERS AND WELDS.


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

-Dik

 

[jayrod12]

Back of napkin calculation on my end is using the green tables in the steel design code. For a 10mm gap and the angle size indicated (which I've actually assumed as L44x44x4.8 because L45x45x4.2 don't exist in the canadian book) they give a Cr min between 188-198 kN. Generally speaking a larger gap provides additional capacity as it improves the system out of plane stiffness.

If you're getting an actual Cf around 140 kN, then it should be working. Or at least be closer than 150% utilization.

 

[EngDM]

If you are that far over you should be checking the web members as well. If it's an old joist it could be hollow pipe which typically is assumed is a much lower yield strength.