Lean-to frame systems (addition to existing industrial warehouse) 2

[deactivated]

I am looking for a good design information regarding lean-to systems as shown on page 6 of pdf document in the link below.

http://www.pdhonline.com/courses/s120/s120content.pdf

According to this document the lean-to frame is designed using vertical loads only. Does that sound right? I'm a bit concerned since the largest loads according to my preliminary calcs are wind loads. Our client already has a lean-to system in place where the beams are 24' long but he wants to extend it even further up to 58' in clear span and re-use existing beams by splicing a new beam to it and reinforcing existing one if necessary after getting rid of existing exterior lean-to columns. I am looking for any information that could help me out in designing something like this and things to watch out for. One other question that I had is about analyzing lean-to frame. From the graphic in pdf document it looked like a cantilever frame but I would feel more comfortable by putting a moment connection where the lean-to beam meets existing building column since we have a huge existing column there. Thoughts?

 

[KootK]

Both of these references deal with lean-to systems to a minor degree.

AISC Design Guide
Metal Building Systems - Newman

I don't know of a specific, extensive design guide tailored specifically to lean-to additions. We have some very experienced PEMB folks here at eng-tips however (Not me). Hopefully one of them will prove me wrong. I would expect that the general principles that govern the primary PEMB structure design would probably be able to see you through the lean-to addition design.

According to this document the lean-to frame is designed using vertical loads only. Does that sound right?

Right if perhaps a bit incomplete. The lean-to system relies on the original structure for lateral support. So, while the lean-to can be designed for gravity only, the original building building will need to be evaluated for the lateral loads imposed on it by the lean-to structure. A partial list of these would include wind loads, seismic loads, thermal loads, crane loads, and general, out-of-plumb/P-delta stability demands. Also note that the lean-to may be stabilized by the original building in some directions but not others. There may be opportunities to take advantage of the fact that the lean-to may shelter the original structure from future wind loads (depends on the set up and governing codes).

From the graphic in pdf document it looked like a cantilever frame but I would feel more comfortable by putting a moment connection where the lean-to beam meets existing building column since we have a huge existing column there. Thoughts?

I think that sounds expensive. The existing column would likely require new, field installed stiffeners among other upgrades. If possible, I'd stick with rafters that are fixed at the lean to columns and pinned at the existing columns. That said, upgrading the clear span from 24' to 58' is likely to be a tall order and that may necessitate the cost and design complexity of the moment connection to the existing columns.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[haynewp]

The existing columns, anchor bolts and footings under new gravity and uplift would also be what I watch out for.

 

[ajh1]

Lean-to frames are generally pinned to the main building column, particularly if they attach at some location other than the eave. (See attached detail) Proper design of stiffeners, bearing at the bolts, flange/web weld design (if the main building column is 3-Plate or PEMB construction), etc. is critical because you don't want loads from the lean-to affecting your main building at this localized position. Generally the lean-to takes only vertical loads and loads perpendicular to the building frames. Lateral loads parallel to the frames will be transmitted into the main building. For wind loads, this is for the most part simply a repositioning of the existing loads on that building from being uniform up the wall surface to acting 1/2 at the connection point between lean-to and main building and 1/2 at the base of the lean-to column. Longitudinal loads (perpendicular to the building frames will require bracing in the lean-to for that purpose.
The lower connection between the lean-to column and the lean-to rafter also is generally pinned to facilitate easy analysis. Fixing this location, which can be done for longer lean-to spans, creates a complex analysis problem as the designer needs to know the relative stiffness of both the lean-to structure and the main structure to properly distribute the lateral loads.
Where either the main building column or its foundation is inadequate to carry the lean-to vertical loads, an additional high-side column may be framed under the lean-to rafter at the main building to remove that load from the main building. This column may also be beneficial if the wind bracing at that column line in the main building is inadequate to carry the additional lean-to longitudinal wind forces, as a new set of bracing can be added between the lean-to columns in that plane.
Increasing the span from 24' to 58' is going to jump your moment in the rafter by almost 6 times (assuming a simple span rafter). This is likely more than you could accommodate by simple reinforcement of the existing beam in any cost-effective manner.