Visited the construction site of a 28-story residential tower near Chinatown in Boston.
Two of the superintendents showed us around the building. They told us about issues they were having with unexpected deflections in their poured concrete slab floors, which caused errors of up to 3 inches between floors in places. This is a problem in places where you want to hang curtain walls from the slabs. They showed us the connection detail of the main steel floor beams to the reinforced concrete column core. These were actually just shear connections - the beams were simply supported between the core and peripheral steel columns running the height of the building.
One thing that struck me was the strong emphasis they put on having a super-accurate Building Information Model. Making sure that they model everything down to the last detail means that they can have more predictability when installing everything in the field. This saves on labor costs because cuts through steel can be done in the shop and not using union labor, there are fewer collisions and corrections to be made, etc.
The floor beams came from the steel manufacturer with a camber (slightly arched with the middle higher than the ends). This is so that when concrete is poured over the beams, they settle flat. Apparently there may have been some miscalculation or something else went wrong, because the beams appeared to have too much camber, meaning that some parts of the floor weren’t level, up to a large fraction of an inch. They have to actually grind down the floor and pour in self-levelling compound to make it level because the tolerance for the wood floors they plan to install is very low ( <1/16”).
In one spot, they had a cantilever of about 5 feet out from the peripheral columns, which lean in towards the top of the building. When they installed little kickers to support the cantilever, the beam they attached the kickers too was in danger of “rolling.” Anticipating this, they installed a little diagonal beam to resist the rolling.
Parts of the envelope are hanging curtain wall, and parts are precast concrete panels, or “stones,” with punch windows in them.
We got to see them installing the roof insulation and drainage system which was just a few layers of tapered foam insulation topped with a few layers of water/vapor? barrier, which was pitched to direct water into drainage pipes.
The method for casting the core was pretty interesting. The concrete company that Suffolk subcontracted with for the core has a really well-developed process where they have a climbing formwork /scaffold that uses hydraulic pistons to climb the concrete core as it’s curing. Their team and machinery were so well coordinated that they could cast a floor per day. Because they were casting during winter, they had to be careful to keep the concrete warm while it cured. So they had a 30-foot blanket skirt hanging from the climbing formwork to keep the few floors below them warm. Too cold, and the concrete might freeze and lose a lot of structural integrity. Too hot and it might cause other problems. Because they were jacking the climbing formwork up using concrete poured only the day before!, they made 8 test cylinders for each pour, and tested them to failure 8,16,24… hours after the pour to prove to the structural engineers that they could safely climb to the next floor. Using this method they were able to pour the entire concrete column in around 28 days. They used #11 rebar for the reinforcing steel in the core, which is some of the biggest rebar you can get at 11 x 1/8” = 1 3/8” diameter.