Steel Roof Joist Concentrated Loads

[kwinner]

Regarding mechanical equipment loading on prefabricated steel roof joists, I am wondering what your experience is with mechanical units being located in their proper locations in the field. Typically, I see units shown on roof plans with dimensions and total load. I ask this because I know the joist manufacturers apply their mechanical loads as point loads in specific locations. As we all know, many times HVAC plans like to change, and contractors aren't as concerned as we are when placing units in their final location. I anticipate chord bending issues if units are not located in the precise location they were designed for. Has anyone experienced issues with this?/How do you deal with this in the design phase?

On my last project, we wanted to build in extra capacity in the roof joists to allow for multiple scenarios in which the mech unit layout has not been completed. We achieved this by applying the mech unit loads as add-loads (which may be placed at any panel point along the length of the joist) and applying the max bend-load along the length of the joist. This gave us a lot of flexibility with mech unit placment in the field, but was very economical and it was difficult to work with the fabricator as it appeared they had not performed this analysis regularly. Is this a common practice?

Thanks,
Kevin

 

[phamENG]

Kevin - you came close. An add load between panel points is never going to be efficient. You want a mechanical zone. You lay out the area where the mechanical units may go, and you write into your joist spec that the fabricator is to design for a point load of X on any one panel point for all joists in the zone. Or perhaps a point load of Y on adjacent panel points of all joists - you get the idea. The trick is to keep it to the panel points. Bending the chords removes most of what makes a joist such an attractive structural system.

Check out this article - it has some good pointers.

 

[kwinner]

Pham,

Thanks for the reply and the article. I like the idea of a mechanical zone, and I will be using that in the future. As a follow-up, I agree that loading the chords away from panel points is not the most economical, but what would you do when the unit location is modified and is to be placed with bearing points not at panel points? What we have done is designed a sub-frame to distribute the loads to panel points, or add web reinforcing to the joists. Our particular client prefers the sub-frame over the joist reinforcement, but ultimately they prefer no additional work.

 

[KootK]

I'm a big fan of the KCS joist series for this purpose. Spec that and leave some fat in your girders and you're only left with the local joist reinforcement / subframe business.

 

[phamENG]

I've always used panel reinforcement - an angle from the point of load application to the closest panel point on the opposite chord. I haven't seen the subframe - mind posting a generic detail?

Good call on the KCS, KootK. That can be a really helpful tool when there are unknowns on a fast tracked project or a retail with future build-out options. I believe they're discussed in that article, too.

Really, these should be field repairs provided by the contractor to fix a mistake and not a planned procedure. Everything is a trade off - with OWSJ you get a lightweight but strong and efficient structural system for much less than putting up steel beams as joists, but you don't have the flexibility of putting the mechanical units wherever you want with impunity. The contractor - and the owner - need to understand that.

 

[kwinner]

One important clarification: the structures I am referring are industrial facilities which rely heavily on the ability to modify and upgrade equipment as quickly as possible to avoid shutdown time (this was understood by all parties prior to design). This also comes with working on existing structures and adding new units and reinforcement when needed. See the attached detail which was used to apply new mech load to specific joists, but can also be used to distribute load to panel points to avoid the web reinforcing.

Thanks again for the input!

 

[phamENG]

Subframe for the unit - got it. I thought you meant some new frame idea within the joist.

I'm not a fan of joists for industrial facilities unless you're talking about something like a sloped mill roof where you're not going to be putting units out on the joists anyway. If there's going to be a lot of equipment and potential variability, steel beams all the way. That's the best way to ensure minimal downtime for the production line. Going all KCS could be an option, but you're losing a lot of efficiency without saving much in cost over steel beams (or so it seems to me).