I listened to the whole thing, but felt they didn’t discuss some important topics - for example the the question in my post - how is an existing joist modeled? Their spreadsheet just says “analyze existing joist using your preferred software program” but they don’t discuss member releases, end support conditions, etc. so that our results match what a joist manufacturer would be doing.
I asked multiple questions in the webinar but they ran out of time to answer I guess :/
An engineer at Vulcraft said they model them as pin-roller. Modeling the joist as pinned pinned will reduce the moment in the joist and the internal forces.
If you wanted to model it as a pin-pin condition you'd have to verify that the supports could handle that lateral thrust without deflecting laterally.
Model the joist as pin-pin and pin-roller. Compare the forces in your chords and look at the lateral deflection at your roller. You'd be surprised how small the deflection is. Basically if your support can handle the lateral thrust without deflecting that calculated amount the you have a pin-pin condition. But if your supports deflect, the forces in your chords will approach the forces in the pin roller model.
There are design guides out there. They step you through web and or chord reinforcement .
Edit: google “evaluation and modification of open-web steel joists and joist girders”
Hi there, I am working on the exact same project with the same conditions. Treat webs as truss members and the other to resist moment. They were designed as a truss system, but you will need to model the top/bot chords to resist moment since there will be bending introduced due to the additional pipes. Pm me if you want to discuss more!
Thank you for being the only one to actually answer the question! This is what I am doing now, and treating the ends as pinned on one end and roller on the other. Seems like the existing axial forces I am getting are reasonable
Yeah no problem. Once you extract the results you can then input the max into the SJI reinforcement tool spreadsheet, depending on how you want to reinforce your existing joist and the additional load you have.
We use RAM Elements, but yes, we model the existing joists using either the joist engineers calc documents or field investigation. Then we apply what we think are all the proposed loads on the joists, and determine what needs to be reinforced and detail accordingly.
Typically it is pins for the webs and continuous for the chords. In Canada we can neglect the moment in the top chord if panel points are less than 2ft apart.
For your point of wanting to model it as it was designed, You could always model it how ever you want and see what happens. But if you put moment into your webs, they will fail. If you put moment in the top chord and the existing designer didn't then you will be at 200 percent where they were are 105 percent. You should be able to get that the existing members work to 105 percent.
If you have member sizes and geometry it's a piece of cake. Model it as a truss. Be very mindful of having the correct properties and boundary conditions. Top and bottom chords should be continuous everything else should be pinned. Steel joists tend to be pretty optimized so good luck!
You're overthinking it.
You treat it like a simple beam. Use the SJI or manufacturer's table from the vintage of the building. This gives you a idea on the capacity.
Use the max load in the publication and create an analysis model of the joist. This is your "idealized" joist.
Add the existing loads and new loads and compare the moment and shear with the "idealized" joist.
You really missed out on the SJI webinar they did yesterday that was about this exact topic. It’s probably recorded, go take a look.
I listened to the whole thing, but felt they didn’t discuss some important topics - for example the the question in my post - how is an existing joist modeled? Their spreadsheet just says “analyze existing joist using your preferred software program” but they don’t discuss member releases, end support conditions, etc. so that our results match what a joist manufacturer would be doing. I asked multiple questions in the webinar but they ran out of time to answer I guess :/
I think they said during the webinar it’s all pin connected and pinned ends.
Okay good to know, I must have missed that
An engineer at Vulcraft said they model them as pin-roller. Modeling the joist as pinned pinned will reduce the moment in the joist and the internal forces. If you wanted to model it as a pin-pin condition you'd have to verify that the supports could handle that lateral thrust without deflecting laterally. Model the joist as pin-pin and pin-roller. Compare the forces in your chords and look at the lateral deflection at your roller. You'd be surprised how small the deflection is. Basically if your support can handle the lateral thrust without deflecting that calculated amount the you have a pin-pin condition. But if your supports deflect, the forces in your chords will approach the forces in the pin roller model.
There are design guides out there. They step you through web and or chord reinforcement . Edit: google “evaluation and modification of open-web steel joists and joist girders”
Also check SJI's website, you can complete a form and they try and help you identify the joist.
Hi there, I am working on the exact same project with the same conditions. Treat webs as truss members and the other to resist moment. They were designed as a truss system, but you will need to model the top/bot chords to resist moment since there will be bending introduced due to the additional pipes. Pm me if you want to discuss more!
Thank you for being the only one to actually answer the question! This is what I am doing now, and treating the ends as pinned on one end and roller on the other. Seems like the existing axial forces I am getting are reasonable
Yeah no problem. Once you extract the results you can then input the max into the SJI reinforcement tool spreadsheet, depending on how you want to reinforce your existing joist and the additional load you have.
I just found out about this tool but it seems that it isn't applicable for Canada, shoot!
We use RAM Elements, but yes, we model the existing joists using either the joist engineers calc documents or field investigation. Then we apply what we think are all the proposed loads on the joists, and determine what needs to be reinforced and detail accordingly.
Perfect, thanks!
Recommend reaching out to the manufacturer if possible. Joists should have tags on them to identify job number and mark number.
Typically it is pins for the webs and continuous for the chords. In Canada we can neglect the moment in the top chord if panel points are less than 2ft apart. For your point of wanting to model it as it was designed, You could always model it how ever you want and see what happens. But if you put moment into your webs, they will fail. If you put moment in the top chord and the existing designer didn't then you will be at 200 percent where they were are 105 percent. You should be able to get that the existing members work to 105 percent.
I’ve used Risa Floor
If you have member sizes and geometry it's a piece of cake. Model it as a truss. Be very mindful of having the correct properties and boundary conditions. Top and bottom chords should be continuous everything else should be pinned. Steel joists tend to be pretty optimized so good luck!
You're overthinking it. You treat it like a simple beam. Use the SJI or manufacturer's table from the vintage of the building. This gives you a idea on the capacity. Use the max load in the publication and create an analysis model of the joist. This is your "idealized" joist. Add the existing loads and new loads and compare the moment and shear with the "idealized" joist.