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.

Anger Management #2: Pass/Fail Letters

The ARE process makes me want to start fires. Nothing huge, just a small toilet paper fire in the bathroom of the testing center. But that would only hurt the maintenance staff of the office building involved, and that’s not who an ARE candidate should direct rage toward. These rants are an outlet to prevent any harm coming to you, Prometric-tenant building maintenance workers. You’re welcome.

I know you must grade a lot of exams. I mean there were around thirty from my school’s alums alone last year! Whew! So I guess we’re lucky to get any feedback at all. Positive feedback is a waste of space, so that is not given, ergo being given no information equals a shiny gold star – you got it, dude! But failure prompts a little more info, so we can correct what we did. Emphasis on “a little” more. Yes, you FAILED MISERABLY on the stair vignette. Everything else was fine except SOMETHING ON THE STAIR. We’re not going to tell you what, as you certainly remember everything you did on that particular drawing – you know what you did, dumbass. You now get to slowly lose your mind as OMIGOD DID I DRAW THE STAIRS UPSIDE DOWN? DID I END THEM 6 FEET ABOVE THE FLOOR I CAN’T REMEMBER! GAAAH! Maybe you emptied out that gutter right over a door way, maybe you didn’t… there’s no proof you didn’t, and thus, no proof that you are not a retard. So suck on that for six months, and try harder next time. Good job on everything else though! Except that thing. You know.

Masonry Mortar

Don't mistake wine for mortar; however, both depend on quality of ingredients

Mortar can be similar to wine in that you should be aware of the labels because each type is different. Similar to wine, Mortar is created by several different ingredients making some mortar stronger than others. There's suitable mortar for colder climates and there's a variety of colors to choose from. I am not encouraging the use of wine on job sites, btw. This is just to help you remember the differences in mortar types.

Mortar is made up of portland cement, hydrated lime, and sand (aggregate) & water.

Lime is added to impart smoothness and workability. Its produced by burning limestone or shells in a kiln to drive off carbon dioxide and leave quicklime (calcium oxide).

Hydrated Lime (slaked lime): released large quantities of heat.

Mortar mix can be obtained in shades ranging from pure white to pure black; makes up 20% of exposed surface area of brick wall, therefore color is important.

Mortar composition standard for unit masonry refer to: ASTM C270

TYPE N: used for most purposes

TYPE M & S: higher strength structural walls (or for severe weather). S is used below grade, as well.

TYPE O: most economical and used for non-bearing walls

For more information on selecting the right mortar for your job refer to Masonry Contractors Association of America

Movie Building Inspector #1

It's time for episode 1 of Movie Building Inspector, where today we are responding to reports of violations at a certain school for wizards and witches... I hear it's one of those liberal ones over in Europe.  Let's just take a look in the main hall and GAAAH!

Hogwart's Dining Room

Holy crap, Hogwart's is a fire hazard if I have ever seen one!

Sure, heavy timber construction is better than light frame, but lets back those 5,000 open flames away from the roof structure, unless you have an anti-litigation spell for all those angry parents of flattened children - class action suiticus dismissum! The more resistant the structure is to heat, the bigger the building can be! Also, should Hogwarts have a proper sprinkler system (which they do, although the fairies-with-tiny-buckets-of-water system is only approved for use in England), the structure could be enlarged and travel distances to exits lengthened. See the chart below for Construction Types.

Click to enlarge

Who isn't a fan of magic stairs that swing from corridor to corridor? Of course, this design would get an immediate stamp of rejection from the DOB...

Stairs to Gryffindor Hall

If you want to bring these up to code, here's a few design rules you should know:

• Handrails at the bottom of stairs shall extend 12" plus extend the slope for a horizontal distance the depth of one more tread. 
• Vertical distance between landings may not exceed 12'-0" with headroom clearance no less than 6'-8".
• Monumental Stairs are stairs 88" in width or wider and require intermediate handrail.
• Avoid single steps; where there are more than three steps, handrails must be provided.
• Handrails must be located 34"-38" above nose of stair or ramp.
• Width of an egress stair is based on occupancy: in most cases, 44" is the minimum but you can go to 36" in a residential occupancy.
• American Disabilities Act requires the below dimensions for handrail design:

You should always refer to your local building code for regulations regarding stair design, as they may have added a few requirements of their own.

Rainwater Collection

My last year in school, I was a member of a studio that designed and helped construct a 1200 SF home for a Central Texas ranch manager and his wife.  We used as many green materials and technologies as we could, as the ranch's goal was not cattle-raising, but environmental preservation.  The centerpiece was a rainwater collection system that gave the house a self-sufficient water supply that did not further strain the drying aquifers of what was then a baby drought.  Flash forward 7 years and the drought has not relented, however, the house continues to maximize the little rainfall it receives and provide plenty for the house's owners.  The manager elaborates on the system in his journal here and gives a good break down of the basic functions.  As the AREs look more and more like the LEED exam, particularly the SPD exam, it's all good stuff to know.

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!

Pronunciation Manual

Sometimes it's important to get back to basics.  There seems to be some disagreement in the proper pronunciation of the word 'architect'.  Courtesy of PronunciationManual, here is the official and correct pronunciation of the word:



Visit their YouTube channel for more clarifications on difficult words.  Much better than the competing channel PronunciationBook.

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.

Anger Management #1: The ARE Forum

The ARE process makes me want to start fires. Nothing huge, just a small toilet paper fire in the bathroom of the testing center. But that would only hurt the maintenance staff of the office building involved, and that’s not who an ARE candidate should direct rage toward. These rants are an outlet to prevent any harm coming to you, Prometric-tenant building maintenance workers. You’re welcome.

The ARE Forum is great.  It serves a very important purpose in filling the void left by the major publications in that it takes information given by verified sources and manages to confuse the shit out of everybody.

"I heard handrails have to be 2” from the wall." 

"Oh yeah? Well I heard they have to be 1” and overlap exactly."

"WRONG. They are supposed to travel the perimeter of the room and spell out the name of your favorite architect on the South wall, as long as there are no windows on it.  Also, here’s my vignette for you all to grade. I don’t really contribute on anyone’s posts ever, but if you all want to take your time and grade this site plan I shat out in a half hour, it’d be awesome bro, because I take the test in three hours."  So now its up to your fellow ARE candidates, who are pissed off because they’ve been skipping happy hours to study for a week straight, to gently break the news that the blue line is water and you’re not designing a building for Venice in a totally constructive and non-snarky way. And Mr. Sajjad, I get what you’re doing making things really harder than they need to be, leg weights and such, but don’t think your name isn’t cursed to high heavens twenty to thirty times a day. St. Peter may ask what was up with all that at the Pearly Gates.

But I appreciate that without the excessive hand-wringing and sob-story fest that is the forum, we would have nowhere else to go except $400 seminars and $40 AIA “study sessions” where they go over the answers on the NCARB sample tests, even though they print the answers at the end of the test for free. Fee trap! Of course, we could always just go to our local library and read the reading list NCARB helpfully recommends at the end of each sample exam to get our knowledge on – I would do that but the reference section at the library is the homeless peoples’ new capital, and they’re always holding meetings there. Stabby meetings.

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.