Reinforcing existing open web steel joists 3

[jdbpe]

I have a project with existing 28 inch deep open web steel joists spaced 8 feet apart. New construction is putting snow drift onto the joists creating an overstress situation. How do I reinforce these? I cannot add more joists because of existing piping everywhere between joists. Its not the same case as welding a WT to the bottom flange of a W10 or so because of axial loads in each member of the truss? Do I have to analyze the joist as a truss and determine overstress in each individual member and reinforce accordingly?

 

[miecz]

Are you going to replace the existing deck?

 

[jdbpe]

Jmiec:
The deck will remain because it is supporting snow drift at the other end of the joists that is more then new snow drift on this end. I guess joists were designed for that snow drift but not for any more load

 

[JAE]

Analyze the joists as a single simple span "beam" with the new required loads.

This gives you a shear/moment set of envelopes.

Now determine the shear/moment capacity of your 28 in. joist from its published capacity table for the given span. This gives you a shear/moment capacity envelope. Keep in mind the SJI specification that requires a minimum shear capacity in the middle portion of the span equal to 25% of the total shear capacity at the end of the span.

Overlay these two envelopes and compare. Wherever the shear required exceeds the shear capacity, you must add additional web diagonals to make up the difference. We usually use small 1" or 2" angles on the outside of the web diagonal, welded to the chords and patterned to match the existing diagonals.

For moment improvement, use A36 rods (unthreaded) placed in the re-entrant elbow of each angle chord member and welded with stitch welds.

Also check the deflection as the diagonals and chord upgrades won't do a whole lot to minimize deflection as the depth is not changed.

 

[UcfSE]

Order this book from Vulcraft:

"Designing with Vulcraft Steel Joists, Joist Girders, and Steel Deck"

http://www.vulcraft.com/

It is well worth having and describes the procedure JAE mentions and includes examples. In addition to retrofitting, there is a great deal more information about designing with steel joists.

 

[VTPE]

Be sure to be careful is extensive welding is to be used. Welding weakens the joist during the welding process. be sure to shore the joist or to unload the joist of any major loads prior to welding.

 

[DJYork]

I've used the same method as JAE discribes.

 

[Lutfi]

I used the same method that JAE talked about. As a matter of fact, you can download this simple software from Vulcraft, called "Vulcraft Assistant” that will assist you in performing your analysis. Here is the link for this software:

http://www.vulcraft.com/

Regards,
Lutfi

 

[Wrightguy]

I have seen the A36 rods used as mentioned above along with plates being added to the bottom of the joist.

 

[jjeng2]

How you do the analysis is up to you but I reinforce joists by welding rods to the insides on the chords(if L's) and welding rods or addl. L's to the web members. Its easy to do in field if joist members are all L's but sometimes ive run across strange cold formed shapes and that can be difficult to reinforce.

 

[mrengineer]

A.I.S.C. Engineering Journal has article by James Fisher "Strengthening Open-Web Steel Joists".

 

[jopalu]

JAE,

In your reply, you use the phrase "re-entrant elbow" of each angle chord member.

Would you or anyone else please explain or describe this "re-entrant elbow"?

FYI: I may need to reinforce newly installed steel joists, to support a relocated 0.75K RTU. First, I will contact the joist detailer to see if they will provide an engineered field fix as is often provided for wood roof trusses for such a case as this.

Thank you ...

 

[STR04]

All,

How are you allowed to use A36 bars should your chords be made of GR 50? There is a strain compatibility issue when mixing materials unless you're assuming the built-up section is entirely made of A36, but that may be too conservative.

 

[JAE]

Re-entrant elbow - its the location on the inside of angle - at the juncture between the two legs -