70's Era Drawings Interpretation

[TRAK.Structural]

I recently got called out to look at some corrosion on some steel columns in an industrial building constructed in the 70's. General structural system of the building is steel framed with w-shape cols and roof joists/joist girders with w-shape spandrel beams. Exterior walls are CMU and appear to be non load bearing. Its only 1-story and FFE to roof is about 24 feet. Column spacing in the area of interest is 20 feet. Review of the plans has a few curious things I'm trying to understand as it may dictate some of my recommendations for repair of the corroded steel. See below screenshots. The foundation plan only shows enlarged spread footings under every other column at the perimeter. Wall sections at the perimeter away from columns do show a continuous footing. To go along with that, the roof framing plan shows the spandrel beams going over the top of the columns that have no enlarged foundation noted, and framing into the columns that do have foundations noted (this framing configuration was visually verified). Additionally there are anchors embedded in the CMU wall that connect to the steel columns at a regular vertical spacing (on all columns). And finally, X-bracing (threaded rods) is sprinkled throughout the perimeter and includes columns with and without enlarged foundations noted.

1. Are the columns with no enlarged foundations just for wind bracing of the exterior walls (and some relatively insignificant vertical loads from the spandrel beam)? Presumably these are founded on the continuous wall footing at the perimeter as the visual assessment showed that the base of all columns is below the slab on grade elevation.
2. If the answer to question 1 above is yes than the CMU would have to be horizontally spanning (or at least 2-way spanning). Was that a common way to do it back then?
3. If the answer to question 1 above is yes than it would seem the loads from the X-bracing must be small in order to justify not having discrete enlarged footings at some braced bay columns?

 

[canwesteng]

That's an interesting one. Only thing I can thing of is restricting differential movement at the joint in the block. What's the connection between the block and the roof/floor diaphragm?

 

[bones206]

The wind columns most likely connect to the 16B26 spandrel beams at the roof level. That way the CMU spans vertically to the spandrel, distributes the top wall reaction laterally to the columns.

There is probably something (maybe bridging?) acting as a strut in the roof plane to deliver the wind column reactions to the vertical lateral system. Are those W16x40s moment frames?

The masonry ties at the column help keep the block wall in plane with itself, since there’s a vertical control joint there.

 

[TRAK.Structural]

canwest - Nothing concrete for roof diaphragm connection at the top of the walls, at least on the dwgs.

bones - Yes there is a connection where the beams run over the top of the columns, but I'm struggling to see a load path for the masonry to span vertically.

See below some more dwg snips and some photos

 

[canwesteng]

If it was just a wind post, I'd expect that connection to the girder to look different, generally some kind of tab with slots. Maybe the post is just supporting that beam and the masonry restrains buckling?

 

[bones206]

I don’t think they thought through all the details and load paths very well.

 

[simplebm]

If the CMU is not connected to the roof deck or spandrel beams it will only span horizontally to the wind columns and main columns.

 

[bones206]

Are those column ties reliable as structural supports for a wall? I’ve never seen published capacities by any of the masonry tie manufacturers. I’ve always considered them to be prescriptive accessories.

 

[TRAK.Structural]

canwest - Yes the columns are certainly taking load, but I think its minimal since the spandrel beam only has a few feet of trib width for roof loading. I doubt the CMU anchors can be counted on for column buckling restraint since they are only engaging one flange.

bones - I would think the ties are sufficient given that they haven't failed yet (other than getting rusted out). The aspect ratio of the wall panels is fairly square (20 col spacing and 22-ish feet tall) so if there is some connection at the roof level maybe some out of plane load is going to both the cols and the roof diaphragm.