That would be the reason, then. I’ve never seen fireproofing like that, only the spray-applied kind or intumescent paint...

I guess since there’s no steel decking, it just made sense to soffit all the beams instead to get the desired fire rating.

I think Chicago was on a bit of a gypsum block craze in the 20’s and 30’s. My old apartment building had interior walls made of gypsum blocks and that also wrapped the steel. In Detroit, all of the fireproofing was done with orange structural clay, but I’m sure both were common everywhere.

The floor slabs probably aren’t reinforced concrete either. I’m assuming it’s a composite block slab topped with concrete. Reinforced concrete was around in the 30’s but I doubt it was practical for a building this big. Too new and variable at the time. I’ll bet if the the plaster and grout is scraped away from the ceiling you’ll see that orange block

I've totally seen buildings from that vintage that are reinforced concrete. The Pickens Kane building on Goose Island (now Lost Arts) is pretty enormous, it has entire reinforced concrete construction with drop panels and mushroom capitals (photo from 1909).

My condo building has this kind of reinforced concrete construction and it's even a little older than that so yeah it was around for a while.

Looks like reinforced concrete slab to me. If it were something similar to a clay tile arch system with topping slab, like most of the 1920s Loop office buildings, you would see the exposed clay tile from the underside. Since many of these floors were designed to be driven on reinforced concrete was probably a better method.

The gypsum block wrap however is new to me, I am much more familiar with the clay tile fireproofing system, which is pretty pervasive in Chicago too. The columns btw are most likely steel I shapes encased in concrete, That was a standard construction in highrises for the period.

Another reason to sprinkler the building, aside from protecting the structure, contents, and occupants: Chicago code allows you 50% greater egress distances to a stair when a space is sprinklered. Considering the massive size of the building, that may have been a requirement to make the exiting work without adding new fire stairs all over the place.

You've never seen steel protected by block or clay? Or are you just saying you've never seen it protected by gypsum block?

They have been fireproofing steel with block since literally the day they started using steel here. Here's a picture of one of the columns in Louis Sullivan's 1882 Jewelers Building. Notice the cast structural steel exposed from within a circular layer of clay coated with plaster.

https://i.imgur.com/9F8kYXE.jpg

Cool info. I didnt know it went that far back in history. When did they invent/start using the spray-on. Fire proofing?

The "office building" fronting Van Buren is steel frame with tile arches, 12'8" floor-to-floor. The 1921 building on the east side is steel frame, floor details not known, 16 ft floor-to-floor. The workroom building is steel frame encased in concrete, 19 ft floor-to-floor. The details of the floors isn't explicit in any of my sources, but it sounds like they were poured concrete that enclosed the steel beams. Since the ceilings would be left unfinished, plywood or pressed wood was specified for the formwork. The structural bays were a rather unusual 29'10.5" x 44'9". The real engineering legerdemain was transferring the building's loads to caissons that avoided the railroad tracks underneath, which required unusual transfer girders and even caissons with oval bells! Magnus Gunderson of Graham, Anderson, Probst & White was the main engineer.

Assuming I'm not the only nerd who might be interested, I'll spend some pixels to post his description from the Oct. 1931 Journal of the Western Society of Engineers:

https://i.imgur.com/n1HhWXA.jpg

^ That's what I figured, though the workshop zone of the building appears to be more of a hybrid which is unexpected.

Reinforced concrete construction in older buildings corbels up in some way around the columns to diffuse the forces either with the conical "mushrooms" as ardecila mentioned or angled haunches , which isn't really the case with the post office. Plus there's a ton of steel spaced closely together, meaning the slab doesn't have all that much tensile strength.

While reinforced concrete construction was around at the time, it was still a new construction method, and there were probably not too many skilled contractors around to be commissioned on a building this large. I would imagine the US Post Office and their engineers proceeding with reliable and known methods of construction to deliver the project on time. I don't know what concrete encased around steel beams would be called as far as terminology, but it's not the common construction we see in modern times where grids or strands of rebar lend that tensile strength.

Anytime you see steel frame in an older building, odds favor that the slab will be clay tile.

This looks like the floor slabs are sitting on top of the steel structural framework. Surely they wouldn't enclose structural "stringers" that were sitting up on top of the beams. By 1930, the symbiosis of rebar and exothermic concrete would have been well-enough understood to just make these ordinary reinforced slabs same as today, right?

Post Office developer picks up another site nearby
By Danny Ecker

The developer overhauling the Old Main Post Office has added another property three blocks west of the massive office project, expanding its portfolio in an area it hopes will turn into a major corporate destination...

http://www.chicagobusiness.com/reale...in-post-office

Interesting. That part of the South Loop (bounded by 290, 90/94, Taylor St & the river) needs to take better advantage of its proximity to transit (Clinton Blue and Union Station), adjacency to UIC, and excellent connections to 90/94 and the Loop proper. Right now it has a very industrial/back office vibe. Hopefully 601W can jump start it with the OPO and whatever they have planned for this site.

I might get chastised for this statement by some, but I wonder if the Amazon bid may be the reasoning behind this purchase?

When they purchased that other lot of land next to the Holiday Inn, I wondered whether they were going to purchase the land anyway or only do it if they had solid agreements or leads. The stupid naive part of me wants to say "yeah! They have a lead!" or something, but in reality I'm guessing they were going to purchase these sites no matter what. Worse comes to worse, they can probably just increase the property values on this land, or at least some of it if OPO is successful enough. Probably a decent investment no matter what for them.

That entire block holiday inn and parking lot across the street looks ripe for tear-down once this is done and the area starts to heat up.

^ Which is great, because that Holiday Inn looks like it belongs along an off ramp on 294 in Lake County.

I cant picture this holiday inn that your talking about. Any photos? How much land are we talking about?

Nevermind I found it. That lot is huge perfect for a supertall.

I can't express into words how happy I am that they're saving this building from rotting away. I can't think of a more glorious Art Deco Lobby on earth.

https://farm5.staticflickr.com/4602/...519b576d_b.jpg

https://farm5.staticflickr.com/4708/...6208d4cc_b.jpg

For sense of scale, there are two dudes on the roof. Try to find them.
https://farm5.staticflickr.com/4745/...e1327ded_b.jpg

https://farm5.staticflickr.com/4626/...6db286e2_b.jpg
From Today

Could/would they ever convert the Merch Mart roof into parkland, resteraunt, meeting space, ect.?

That's the old post office not MM, but yeah, green roof/patio please