21 Boston – 06 – PT Slabs

Immediately following the pouring of the Section 1 ground slab, construction began in preparation for the first PT (post-tensioned) slab one level above.

In an incredibly short amount of time the team erected the temporary support scaffolding in the northwest corner (above), and began the decking that would support the PT slab (below – far corner)

The scaffolding is spaced dimensionally for sheets of plywood. Special pieces of plywood are cut to fit around the future columns (areas where the reinforcing bars are sticking up above the deck level – below).

The grid is worked around unique elements, such as the elevator shaft shown here below, and the plywood decking will similarly be fitted up to the shafts. It’s important that when the slab is finally poured, the slab and shaft concrete should tie together.

This can be seen more clearly with the plywood deck in place (which was essentially done in a day with skill saws and nail guns).

Below you can see clearly how the plywood was crafted around the column areas and the large elevator shaft and pit.

Stepping back a bit (below), you can see two different, major operations going on at the same time. On the left, the vapor barrier is being installed over the crushed stone base in preparation for pouring the concrete of the next section of ground slab; and on the right, work has already begun on the reinforcing for the PT – post-tensioned – slab to be poured there.

Here are a couple of view of that work up on the deck.

As can be seen in these two views, there are a lot of different types of reinforcing being installed. The smaller, black bars are the normal reinforcing that strengthens concrete slabs, one set near the bottom of the slab, and another set near the top. In addition to these, however, are a set of what look like orange reinforcing bars. These are actually plastic-sheathed steel cables. Also noticeable is the fact that they don’t like flat but are draped higher near the columns and lower between columns. This has lot to do with how PT slabs work structurally.

In a PT slab, after the concrete is poured and has cured or set up a few days, the post-tensioning bars are pulled from each end, increasing the tension on them. This has two major effects on the slab. One is to cause the draped bars to want to straighten and raise up in the areas between columns. They can’t do this of course because of the weight of the concrete; but the stress they put on the concrete helps it resist the weight of parked cars and other loads on the slabs. The other effect is that the tensioning increases the compressive strength of the concrete in general, which increases its normal efficiency and allows the overall slab thickness to be reduced, saving materials. Working together, these two effects make for very strong floors.

Below is a look at the ends of the post tensioning rods (left side) where, if you look closely, you can see that the individual rods are threaded. These are the locations a hydraulic stressing ram will pull on them to increase the tension.

Notice also the little metal gadgets that look like spiders, but are often called chairs, that hold the reinforcing systems up off the deck. This is to prevent the reinforcing from touching the deck which would expose the reinforcing under the slab when the decking is removed from below and make the reinforcing vulnerable to corrosion.

One other critical detail is the wire that is being twisted around the places where the reinforcing bars cross over each other. This keeps the grid generally in the right place when the concrete is being poured so the structure will perform as it has been designed.

Then finally two more layers are added. First, the layer of electrical conduit (blue and gray) that goes on top of the bottom layer of reinforcing.

And then the top layer of reinforcing. The idea here is that the layers of reinforcing perform best when they are near (but not literally at) the upper and lower surfaces of the concrete; and other elements, like conduit, do best near the “neutral axis” (center) of the slab thickness where any effect they have on the structure will be reduced.

That’s a lot of energy going into Section 1, the northwest corner of the project; but it doesn’t mean that other areas are being neglected. Right next door, in Section 2, the ground slab is being poured so that it will be ready for the scaffolding and decking teams to start there work for the next PT slab.

That slab pour starts, of course, up at street level with concrete delivery and pumping.

Farther round the site, in sections 3, 4, and 5, the exterior wall reinforcing is being installed in preparation for the shot-crete walls. These walls need to be completed up to the level of the next floor before the PT deck is constructed so the wall and floor can be tied together.

And overhead – and often out of sight / out of mind – the tower crane lifts all sorts of things into and out of the site – a necessity when you’re working “in the hole” and can no longer drive in and out. Lifting things overhead is serious safety work; and a senior rigging director supervises every lift (steps 1 -4) and blows a loud whistle so everyone on the site is aware that loads are travelling above.

Finally, a look at Sections 1 and 2, ready for the next steps. In this photo (below) you can see that the scaffolding elements have already been stacked on the new ground slab in Section 2, ready to be set up.

And here in Section 1, the PT slab is nearly ready to be poured

This project is moving !

21 Boston – 01 – Introduction

A significant new project is currently under construction on Queen Anne Avenue.

The project is called 21 Boston and is being developed by an entity called CG 21Boston with developer barrientos RYAN, a firm that has done multiple housing and mixed use projects in Seattle. Runberg Architecture Group did the design; and the project is being built by Compass Construction.

The project provides a new 50,000 sq ft Safeway store and 325 units of housing, including 65 affordable units. The view above shows the building from the Queen Anne Avenue and Crockett Street intersection. It expresses the approach that guided the final development of the design. The overall design continues the “urban village” approach that has been used on Queen Anne Avenue for the past ten years: a street level of retail with housing above. This means that the continuity of the street will be extended and enhanced.

Within that framework the design breaks the building into separate elements to mitigate against the size becoming overwhelming. This is done is several ways. First, the street level retail and entrance to the Safeway building are handled distinctly as a one-story element clad in brick that reflects other retail across the street.

Here’s a close-up of the retail, showing the public plaza planned for the corner of the site. As part of the design process with the neighborhood, the retail stores have been designed so that they are accessible both from the street and from the Safeway store inside.

Second, the housing above the retail is broken into multiple elements to reduce its scale, and stepped back several times to reduce its massing. And finally, the overall plan organizes the housing into 3 buildings instead of one large one. These approaches are emphasized by changes in the materials of the façade.

Here is an overall plan of the podium level of the project, clearly showing the three main housing elements.

The space between the buildings has been developed as a series of landscaped courtyards that both let light and air into the units that face them, but also provide pleasant outdoor spaces for gathering and relaxation.

This approach continues at the top floors as well (shown below) where the floors set back to provide terraces and landscaping, some of which have been planned for group use. This plan also emphasizes the modest size of the buildings at this level, compared to the overall size of the project.

A couple other views show some of the variety of the project. This one below shows the 21 Boston building that gives the project its name and is the main residential entrance. It faces the northeast corner of the site and has been given its own distinct expression.

And below is the Crockett Street façade showing both a secondary residential entrance and an entrance to parking, just around the corner from the Queen Anne Avenue retail stores. The various setbacks of the different levels of the building are clearly articulated here.

All in all, the 21 Boston project will make a significant addition to the Queen Anne neighborhood: improving retail services, increasing much-needed housing and enhancing Queen Anne Avenue, the “main street” of Queen Anne’s “Urban Village”.

In subsequent posts I’ll show the project under way as well as some of the construction techniques used on projects of this size and scale.

Seattle Snow – 2019

It may seem odd to write about snow in the winter time, especially in the mountainous state of Washington; but, surprisingly, in Seattle, along temperate Puget Sound, snow shows up only every 5 years or so. Mostly, our snow comes as rain. The real stuff coats the Cascade and Olympic mountains. Then, every so often we wake up to a surprise.

This view normally shows downtown Seattle framed dramatically by Mt. Rainier; but today the Space Needle at the Seattle Center barely shows up, with the downtown skyscrapers ghosted in as a background.

We have a popular ‘overlook’ walkway around this part of Queen Anne hill; and the occasional person was out enjoying it.

The various ‘hillclimbs’ that run perpendicular to the overlook didn’t draw much traffic though – the footing was tricky.

The local shrubs took the snow in stride;

but the Japanese cherry trees were less certain.

Luckily for the Junkos, though, the bird lady was on the job with extra seeds.

As for the rest of us, we celebrated that the local Bakery and Coffee Shop were open.

Happy New Year !