Purdueenginerd

On 12/20/2019 at 3:45 PM, wxman said:

Can somebody please explain why soil sampling is important? I’m purely ignorant on the subject matter and would like to know. 
 

I assume that the soil sampled a block or two over (Likely sandy loam/clay) would be the same for said property. I doubt the soil changes that drastically from block to block.

I forgot another good example I had where the geotech report was handy. 

I have a renovation project of a two story building in Dallas. When I was able to recover the structural drawings from the city, the one kind of alarming thing we observed is that it had drilled piers 70 feet down! Which is generally excessive for a 2 story building. The first floor was also called a structural deck, in that its not directly bearing on the soil and is supported by structure. (there was a nice horrifying crawl space I had to get into to make my measurements). As part of the renovation, the owner was creating a large outdoor patio and ordered a geotechnical report to support this structure. The geotechnical engineer found that there was a landfill at the site from 1820-1880 and from 1880 to about 1950 there was cattle slaughter house. There ALSO used to be a small river running through the site which had been filled in in the 19th century. What did this mean? It meant that for about 50'--- the soil was ... actually trash. So when the building was constructed, they drilled piers right through it to native soil to get the engineering values they needed. We could have done that as well, but any soil removed from the site would need to be treated (environmental considerations) and so the owner didn't want 70' piers (lots of soil removed from site). We ended up driving micro-piles which are a different foundation system all together. (they remove less soil). Fundamentally, the geotechnical report altered the engineering of both the original building and our renovation that would not have been predicted if we assumed that soil conditions were identical to adjacent Dallas blocks. 

On 12/20/2019 at 3:45 PM, wxman said:

Can somebody please explain why soil sampling is important? I’m purely ignorant on the subject matter and would like to know. 
 

I assume that the soil sampled a block or two over (Likely sandy loam/clay) would be the same for said property. I doubt the soil changes that drastically from block to block.

Anytime a building is constructed, the structural engineer needs to design the foundation. The foundation design is based off of the soil conditions at the site. The geotechnical engineer consultant will bore down in several locations at the project site and provide important information for the design team. For the structural engineer, depending on the project, the geotechnical engineer will provide test data with actual values for the structural engineer to use. For example: for a spread footing design, the geotechnical engineer will provide an "Allowable" bearing pressure at a given depth. Why does this matter?  Without it, a structural engineer is forced to design the foundation based off of prescriptive minimums denoted in the code. As you might imagine, those values are not good. For the owner, getting soil sampling can reduce construction costs and make the building more durable. 

The geotechnical engineer also provides recommendations to the civil engineer! For example, if the building has a slab-on-grade, but the top soils have a high-plasticity and are susceptible to movement and swelling, the geotechnical engineer might recommend removing 4-8 feet of the top soil and replacing it with select fill!

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I doubt the soil changes that drastically from block to block.

You'd actually be quite surprised. I have a project right now where I'm designing four 1 story buildings all within about 200' of eachother. The geotechnical report took 8 samples. There's a limestone gravel layer with a nice allowable bearing pressure of 30,000 PSF (thats a fanastic value). The depth of that layer is 5' in one spot and 30' in another. So one building has drilled piers 35' and another building, its only 10'. In an area like Houston, with a lot of alluvial deposits soil conditions below grade can and do change drastically. Buffalo bayou has not had a static location for the majority of its existence and proto-streams and different conditions likely flowed through downtown Houston eons before even humans were on the north american continent. 

Now you may ask, what are normal values and was is PSF?

PSF stands for Pounds per Square foot, and I'm simplifying things here a bit but there are two main directions loads or energy get transmitted into the foundation, Laterally (ie, wind an seismic), and Vertically (Gravity, Dead, Live, and overturning from wind and seismic). When a building a designed, the structural engineer will size the foundation element based on the values provided by the Geotechnical engineer. Lets say he/she gave me a value of 4000 PSF at 8 feet down (which is more or less normal for Houston). Lets say I'm just designing a spread footing. A spread footing, for lack of a better terminology is a block of concrete, typically directly beneath a column or vertical member. Most of the time they are square, but technically can be any geometry. (ive had several petro-chemical projects with octagonal spread footings). Anyway, lets picture a 5'x5' (looking down, in plan) spread footing. Thats 25 square feet. I would expect that to carry 100,000 lbs load (vertical only). (25 *4000 psf = 100,000 lbs). Not bad right? Now lets picture if the owner elected not to get soil sampling done and I had a column with 100,000 lbs of force. How big would my spread footing need to be?  100,000 lbs/1000psf = 100 square feet. Or 10'x10'. It would also have to be thicker for other reasons I'm not going to touch on right now. Now, imagine a building with 50 columns, those cost savings will add up quick!

2 hours ago, zaphod said:

I hate to be that guy, and to be clear I'm not opposed to public owned affordable housing development, but...

 

Isn't it really expensive to build high rise apartment units compared to other forms of housing? Especially buildings like this, it appears to be far more high end than a standard midrise block or concrete slab tower. What kind of factors or variables lead to the decision to put public housing resources into mixed income high rises in one of the ritziest areas of town?

 

If I was in charge I think I would strategically buy up land that's currently occupied by noxious, trash land uses that hurt surrounding communities, like junkyards and concrete plants around Acres Homes and the Northside. Then I would plow all that under and build tidy looking houses and small apartments there, with trees and stuff. The land must be very cheap, and so would the construction methods for that kind of housing. And instead of it all being apartments, I would mix in some housing to sell to specially qualified persons. Create a real neighborhood.

 
Keep in mind that some affordable housing developments are merely that because they set aside a certain number of units for lower income residents, instead of concentrating poverty into one large tower.

I pretty much categorically oppose your proposal that the poor should be relegated to trash land, like junkyards and concrete plants. Land, even in midtown, is relatively cheap and is a good spot for middle, lower, and upper classes to co-mingle and go to work together. As for the costs associate with building a high rise. Yes its typically more expensive than a low rise building, BUT, it uses less land to create the same number of units, which also has costs associated with it. 

5 minutes ago, wxman said:

Can somebody please explain what's going  on here? I've never seen or heard of this before. What makes this different from all the other pours where they pump the concrete in and it's done?

Concrete curing procedures are based on ACI 308.1 (http://dl.mycivil.ir/dozanani/ACI/ACI 308.1-98 Standard Specification for Curing Concrete_MyCivil.ir.pdf) (link is for the 1998 version, I think the latest version is actually 13). Can't tell what they've put down from the photos but at least from 200' away they appear to be complying with section 2.

A contractor, should follow curing procedures on all of their concrete placements. Outdoor Flat work pours are especially susceptible to damage if procedure isnt followed. 
 

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SECTION 2—MOISTURE RETENTION 2.1—

General 2.1.1 Description—This section covers methods and procedures for curing concrete using plastic sheets, plastic sheets bonded to water-absorbent fabric, or reinforced paper.
2.2—Products

2.2.1 Plastic sheets shall be polyethylene with a minimum thickness of 0.1 mm (0.004 in.) or 4 mil plastic sheets bonded to water-absorbent fabric shall have a minimum thickness of 0.1 mm (0.004 in.) or 4 mil polyethylene film bonded to a layer of cotton, burlap, manufactured fabric, or other absorptive material. Sheets shall meet the requirements of ASTM C 171.

2.2.2 Reinforced paper shall consist of two layers of kraft paper bonded with a layer of bituminous adhesive, reinforced with non-asbestos fibers. Reinforced paper shall comply with ASTM C 171. 2.3—

Execution 2.3.1 Place material on the concrete surface as soon as possible without marring the surface. Cover all exposed concrete surfaces and beyond the edge of the concrete surface. Securely tape sheets together or lap them. Maintain the integrity of the material and the ability to contain the water on the concrete surface throughout the curing period. 2.3.2 Verify that the concrete is continuously wet under the sheets; otherwise, add water through soaker hoses under the sheets.

2.3.3 Use black or dark colored plastic sheets when the daily high ambient temperature is below 15 C (60 F). Use white or similarly reflective plastic sheets when the daily high ambient temperature is above 30 C (8 5F). Use any color or transparency of plastic sheet at temperatures between 15 and 30 C (60 and 85F)

 

The reason for performing these actions is because when concrete cures, the chemical reaction of water and cementitious materials puts off heat, ie, it is an exothermic reaction. A temperature differential between the interior and the exterior faces of the concrete can built up tensile stresses and cause large cracking in the face of the concrete (this is bad!) In addition, for flatwork and/or outside pours, air flow, evaporation, and sunlight can change the strength characteristics of exposed sections of the concrete(this is bad!). Curing procedures are put in place to make sure these bad things dont happen or are mitigated. IE, ensuring that exothermic concrete can dissipate heat more quickly, or alternatively, make the concrete uniformly warm. 

For Mass pours.. or concrete placements that are huge. Additional steps have to be taken to mitigate these affects. ACI207.1R-14 (http://dl.mycivil.ir/dozanani/ACI/ACI 207.1R-05 Guide to Mass Concrete_MyCivil.ir.pdf) (i've provided an older version). Provides guidelines and specifications on how to handle that. The history section of the commentary is quite educational if you'd like to learn more. There are additional specifications for Hot weather and Cold Weather concreting. 

20 hours ago, Luminare said:

 

Wouldn't it be fun if you could actually "bake" a building. In this example with all the tarps the building could come out like a souffle. Just pull the tarps and out grows a full building.

A few years ago I had a repair project at petrochemical facility where we had a mass-pour with a high-early strength concrete. A 2' thick concrete roof deck for a below grade structure.  After the concrete set (to give you context on how fast it was setting, I could walk on one side of the roof while the other side I would sink into), the contractor basically flooded the top of the concrete with 3 inches of water. The next morning, I recorded the water temperature at 120ºF. It was stunning to me how much energy it put out curing. The contractor indicated they had a fire break out several years back because of extreme exotherming of the setting concrete. 

15 hours ago, hindesky said:

Hogg building getting a paint job. This is some big rebar, looks like 1" think to me. I would think a mat pour will be coming up in the short future.

 

 

 

Fun Fact! 

#3 bar, Diameter 3/8" 

#4 bar, Diameter 4/8"

#5 Bar, Diameter 5/8"

#6 Bar, Diameter 6/8"

#7 Bar, Diameter 7/8"

#8 Bar, Diameter 8/8"

#9 Bar, Diameter 9/8" (also 1 in^2 in cross area)  - Pre 1940's, #9 bars would traditionally be square 1"x1" bars. 

#10 Bar, Diameter 1.27" (They stopped using the previous numerical system)

#11 bar, Diameter 1.41"

#14 bar, Diameter, 1.693"

#18 bar, Diameter 2.257"

Here's a picture of #18 bar I took the other day