Editing Openai/692cb06b-b484-800b-9bb9-7b819ce20d91 (section)

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Before the skyscraper era truly begins, most office buildings in Boston and NYC are built like this:
* Exterior walls: Thick granite, sandstone, or brick load-bearing walls (sometimes 3–6 feet thick at the base to carry the weight).
* Interior structure: Heavy timber framing: large wood posts, girders, and joists (“slow-burning construction”). This is similar to mill-construction techniques of the mid-19th century.
* Floors: Timber joists or laminated wood beams. Fireproofing was minimal; fires were a major problem.

Examples of this style include many pre-1880 Boston commercial blocks and older Wall Street-area buildings.

==== By the late 1870s–1880s: ====
* Cast-iron columns begin replacing timber posts in some buildings.
* Wrought-iron beams (I-beams) begin supporting floors.
* Fireproof tile vaults appear over iron beams.

But the walls are still load-bearing masonry, meaning:

: 

This is a transitional stage. New York had many cast-iron–front buildings, but the iron was the façade—not the structural skeleton.

==== The breakthrough is the steel skeleton frame (also “steel cage” construction), pioneered in Chicago in the mid-1880s. ====

New York and Boston adopt it somewhat later, roughly:
* New York: c. 1889–1895 (e.g., Tower Building, 1889—first steel skeleton in NYC)
* Boston: c. 1890s (e.g., Ames Building 1889–1893—transitional, stone-bearing; more true steel-frame buildings appear mid-1890s onward)

Key change:
Now the steel frame supports the building, not the stone.

This means:
* Granite exterior becomes a stone veneer tied to the frame.
* Walls do not need to be massively thick.
* Buildings can climb above 12–15 stories.

==== By the turn of the century: ====
* Full steel skeletons with fireproofing (terra cotta tile around beams).
* Granite, limestone, or brick exterior cladding on the steel cage.
* Large windows, because walls no longer structural.
* Better wind bracing through steel columns and beams.

This is the era of early skyscrapers on Broadway, Wall Street, and in Boston’s financial district.