Joist Load Questions and Designations 1

[cgstrucg]

Hello,

I have a task in a project in Philadelphia, USA in which I have to create a loading diagram for vulcraft. Loading diagram is for numerous joist at roof. I have really very little information about long span joists and am trying to go through the guide vulcraft has provided and wants to ask a few basic questions which are really confusing me and any help will be really appreciated.

1. In our contract drawings we mention joists like 16k3, 18lh08, etc. I understand that number before alphabet is joist depth. What about the numbers after the alphabet? Everywhere in the guide they have mentioned that the last digit is in the load table. But we are the ones putting this in load table then how will we decide the last numbers?

2. In our contract drawings, we don' call out for any support connection type for joists right now. I believe all joists to a column having bottom chord attached to the column would be classified as joists with end moments (A typical type given in manual). In that case, we can give end moments by presenting in a table.

Does that mean all other joists automatically are simple span joists with no end moments or are there any typical connections I can look at which can help me differentiate between them?

3. In one of the table, they mention provide live load continuity moments. What does this mean? Do they mean in-plane live load end moments?

Thanks

 

[JAE]

The last number is simply an index number that SJI (and Vulcraft) use to designate a joist chord size.
A larger number represents a more significant (stronger) joist.

The Steel Joist Institute (SJI) designates these "standard" joists with load tables stating each joist size's load capacity and stiffness.

When you say "We are the ones putting this in the load table..." I'm not sure what you mean?
When you build a joist load diagram for Vulcraft, as you state, you are simply providing the joist supplier with the span, loads, and deflection criteria necessary for them to design a "special" joist.
A special joist is one that is NOT STANDARD and generally does not use the last chord index number. Rather, you are simply having them custom design a joist for you.

Your item 2. above is generally correct. If you are attaching the bottom chord to a column and then creating a moment-type connection, you need to tell the joist supplier that is your intent and also provide them with the required end moment values. You also need to remember that these joists will require, perhaps, some additional bridging as the bottom chord is partially in compression near the columns and there is no decking to laterally brace it.

Not sure what your question 3 is about. Any continuity moments would be negative moments at your columns should you make your joists continuous over supports.
Most joists, however, are treated as simple spans unless there is a top chord extension (cantilever) or a full depth extension. I've never connected them in a continuous beam system.
Joist girders are many times used as moment frame members with ends fixed to columns.


Check out Eng-Tips Forum's Policies here:
faq731-376

 

[cgstrucg]

Hello JAE,

First of all, thanks for the response.

1. I got it. I mingled multiple things together therefore got confused.
2. Got it.
3. That's what I thought. Please see the attached image. This is what Vulcraft shows on how we have to provide them end moments. They call out here to provide live load continuity moments and I am not getting what they mean by it. I think they mean in-plane live load moments, but wanted to make sure if its something else.

Thanks

 

[JAE]

Yes, per that table, those are live load moments due to the joists being made continuous over their supports.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[JAE]

The "SP" designation tells Vulcraft that this is a "special" joists and not a "standard" joist with set chord numbers. Thus you need to provide ALL the design requirement to them.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[cgstrucg]

In my case, all the joists are either getting terminating at a beam or column. When you mention joist continuous over supports, do you mean the top chord extension over supports or like an extended seat connection?

 

[JAE]

I mean any support condition where the joist is connected in such a manner where there is moment fixity on the overall joist. So the joist would behave like a continuous beam over the columns with negative moment bending.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[cgstrucg]

Thank You for your help JAE