Precast Concrete

Precast concrete is a sustainable building system that provides slender, efficient structural elements.  Precast plants provide many advantages over concrete cast in the field.  Better control over conditions, materials, and workmanship gives a higher strength, higher quality product in a safer environment.  Repetitive forms can be reused.  The limitations of transportation, however, means widths over 14' can be problematic and expensive to move.


Testing is similar to that of site-cast concrete.

Precast concrete is often pre-stressed - prestressing allows a much more efficient distribution of compressive and tensile forces, thus it allows for smaller, cheaper concrete members.  Tensioned steel squeezes the concrete, putting the entire concrete section under compression, reducing cracking.  The strands can be shaped to more closely match the path of tensile forces through the beam, which tends to be at the top near the ends, but at the bottom near the center of the span.  T








Prestressing comes in two flavors:


PRE-TENSIONED:

Manufactured with steel cables or bars, called “tendons,” are pulled taut prior to the casting of the concrete, which bonds to the tendons and the force is transferred along the surface area of the rebar.  This site goes slightly further in depth.

POST-TENSIONED:
The tendons are not allowed to bond during curing by sheathing them.  After the concrete has cured, the tendons are jacked taut and anchored to the ends of the concrete, pulling the concrete section into compression.  Most commonly used in large building projects such as high-rises and bridges. Post-tensioned concrete slabs-on-grade also can be found where there are unusual soil conditions.

Precast pieces were used to reinforce the stage floor prior to Kirstie Alley's appearance on Dancing With The Stars.

The NY Times is down with Steel Connections

The New York Times has a great series of photos showing steelworkers, and inadvertently, connection details of the Freedom Tower (1,776 feet, baby!)  Click through the gallery here. 

Iron worker on the Freedom Tower, credit NYTimes.com.

Shear studs are used to connect beams to the concrete floor structure, effectively bonding the two.  The floor structure then becomes the top flange of the beam, increasing its capacity to resist bending.  This composite beam action is very efficient because it doubles up on usage of the concrete slab's strength.  This allows heavier  loads and longer spans.  Walking on shear studs 75 stories in the air also causes you to become a certified bad-ass, even if you are hooked up to proper safety equipment.

That gusset plate is pretty cool.  What's it gonna do?  You can see the continuous fillet weld connecting it to the top flange of the beam - that bad boy was probably shop welded, as that would be an intense weld to perform in its current place.  As many welds as possible should be done in the shop rather than the field - it's cheaper, easier, and the results are better.  Pre-formed holes indicate it's gonna get bolts.  I assume a diagonal beam will be bolted at an angle perpendicular to the right face of the plate, and a big connector clip will cover all the bolt holes and connect it to the gusset plate.

Flip through the gallery and look for other cool stuff.  If this don't jazz you up, more than you already are for football season, I don't know what will.

In conclusion, STEEL! WOOOO!

NYC Hurricane Irene Special! - Intro to Wind Forces

Bad weekend to start your construction project in the Tri-State area.  Or end your project.  Or be a contractor on a project:  just look at all of their responsibilities to comply with the NYC's Deparment of Buildings:  


To secure construction sites, builders, contractors and developers should take all precautionary measures including but not limited to the following: 

• Tie down and secure material and loose debris at construction sites.
• Cover electrical equipment from exposure to the weather.
• Store loose tools, oil cans and extra fuses in a tool box.
• Secure netting, scaffolding and sidewalk sheds.
• Suspend crane operations and secure crane equipment when wind speeds reach 30 mph or greater.
• Suspend hoist operations and secure exterior hoists when wind speeds reach 35 mph or greater, unless manufacturer specifications state otherwise.
• Brace and secure construction fences.
• Secure all windows.

Owners don't have it any easier:

• Bring inside loose, lightweight objects such as lawn furniture, potted plants, garbage cans, garden tools and toys.
• Anchor objects that would be unsafe outside, such as gas grills or propane tanks.
• Close up and secure patio umbrellas.
• Secure retractable awnings.
• Clear rooftop drains, gutters and leaders.
• Secure all windows.
• Remove aerial antennas and satellite television dishes.

Architects' responsibilities include:

• Do not go in to work.  Drink if necessary to prevent this.
• Continue to peruse job postings on Craig's List.

Now, on to wind:

EFFECTS OF WIND ON BUILDINGS

• direct pressure
• drag
• suction
• rocking, buffeting
• vibration
• clean-off effect:  wind gusts have more effect of building parts than the whole building

CRITICAL WIND EFFECTS

Codes provide critical wind velocity and design wind pressures.  Wind speeds are taken from the code, and are based on historical data measured at a standard height of 33' above the ground.  Factors considered by code are building size, building shape, openness and sheltering effect of surrounding landscape, and various special conditions.

Wind forces determined are:  

-  inward pressure and outward pressure (suction) on exterior walls

-  pressure on roof

-  overall force on the building

-  sliding and overturning forces - resisted by dead weight of building)

-  harmonic effects - resisted by stiffening/bracing/tightening of elements)

-  effects of openings - forces are increased at openings, these can be calculated via a wind tunnel test
torsional effects - twisting of elements about the centroid, or center of stiffness

Seismic Forces and You

 I hope you're taking the structural exam soon, because today was class experiment day for seismic loads.  As such, the internets a-sploded with great seismic resources:

A summary of modern seismic-mitigating devices is here.
http://gizmodo.com/5833664/how-buildings-stay-up-when-the-earth-shakes

Here's a feature story on the world's most state-of-the-art earthquake-resistant structure.
http://www.wired.com/wiredscience/2009/11/worlds-largest-earthquake-safe-building/

You can see the Mercalli Scale in action here, where real people are asked to rate the amount of shaking and damage they experienced.  Totally subjective - I've never been in an earthquake before, so I would throw off their system, because I give it "two thumbs up."

This article relates directly to FEMA's report that should be read, in which existing structures are evaluated for seismic resistance.  This quote hits close to home:

The [structures] that are of a particular concern are unreinforced masonry. The brownstones, six-story, turn of the century. Those are the buildings that don't have much ability to withstand lateral forces, and they tend to crumble.

Concrete Curing & Testing

Concrete gains about 70% of its strength during first week of curing; it takes 28 Days to fully cure, like Sandra Bullock's heart.

TESTING BEFORE CURING

Slump test.

Slump test: measures consistency of concrete; amount of slump desired depends on how concrete will be used, but typically in the range of 2" to 6".

Even a real Kelly ball looks dirty
on an R. Kelly album cover.

Kelly Ball Test: also "ball penetration" test; hemispheric mass of steel with a calibrated stem is dropped onto a slab of freshly laid concrete. Amount of penetration is measured and compared to 1/2 the values of slump test.

K-slump test.

K-Slump test: uses 3/4" tube that contains a floating scale - the distance the scale floats out is read and measures consistency of concrete.

TESTING AFTER CURING

I will crush you.

Cylinder Test: measures compressive strength; tested in laboratory; During

the initial pour, samples of the concrete are poured into 12 inch by 6 inch

cylinders to be tested. After seven days of curing, the first sample is

given a compression test, and again after 28 days (full strength).

Core cylinder extracted from cured concrete.

Core Cylinder Test: used when a portion of the structure is in place and

cured but needs to be tested; a cylinder is drilled out of the concrete and

tested in lab to determine compressive strength.

Impact hammer: non-destructive field test of concrete strength after it has

hardened; rebound of plunger snapped against surface is measured.

TESTING FOR MOISTURE

Moisture is a critical factor in determining the ultimate strength of concrete.

Calcium chloride test (moisture dome test): common test for moisture in

concrete. Contractors use calcium chloride to test the dryness of concrete

before putting down flooring; tester weighs the container again on the same

scale. The difference in weight represents the water vapor emitted.

Hygrometer test (relative humidity test): moisture emission by measuring the

relative humidity RH of atmosphere confined adjacent to the concrete floor;

test standards recommend that moisture sensitive flooring not be installed

unless RH is 75% or less.

Polyethylene test: presence of visible water indicated concrete is

insufficiently dry for application of finishes. Similar to mat test.

Electrical Impedance test: moisture content in slab is read out directly.

TESTING FOR ALKALINITY

pH level of concrete should be tested -  concrete normally has pH of 12.0-13.3 (7 is neutral).

Alkalinity can screw you in 2 ways; high alkalinity on surface of slab

can damage a tile installation by causing the adhesive to re-emulsify, or

revert to its liquid state. Alkalinity is also responsible for Alkali-Silica

Reaction (ASR), in which cement begins to dissolve sand and rock within concrete.

ASR is reduced by using low lime content, aggregates not susceptible to ASR,

proper curing and not finishing with hard trowel surface.

Titration test: lab test of alkalinity in concrete.