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Raft Slab-on-Grade Foundation

Jim Houlette — Sat, 31 Dec 2011 21:20:36 +0000

A Raft Slab-on-Grade foundation can be as simple as a 4” thick slab with no reinforcement and a shallow perimeter rib to an 8” thick slab heavily reinforced with deep ribs spaced roughly 10’o.c. The wide range of concrete thicknesses and reinforcement is a function of many things such as soil type, loads, exterior finishes, allowable deflections, climate, and many more factors. The primarily factor that influences the slab thickness, reinforcement and rib sizes of a slab-on-grade foundation is the soil type.

If the soil is a sandy material with non-swelling fines to allow good compaction then the slab-on-grade foundation will be evenly supported, and has a low risk of soil movement. This raft foundation will just need the minimum slab, ribs and reinforcement.

If the soil has swelling material that is susceptible to changes in moisture content then the raft must be designed to resist this movement. The use of reinforced ribs is an economical way to strengthen the slab. Ribs are below the slab and are spaced evenly to create a grid or waffle appearance. This is done in lieu of thickening the entire slab because you gain more strength with depth and will typically use less concrete and steel reinforcement to achieve the same design capacities.

Raft foundations have been used for residential structures, commercial structures and heavy construction structures. The reason for the wide range of uses is simply because the raft slab-on-grade foundation is simple to construct and one of the most cost effective foundations. Typically, little earthwork is needed, sites are leveled, ribs are trenched, reinforcement placed and then concrete is placed. Also, since the raft foundation is just reinforced concrete, access to plumbing can be achieved by removing sections of the slab and then patching as needed. If the slab is post tensioned then accessing the plumbing is much more difficult.

But the raft foundation has its problems as well. On sites with high risk of soil movement the slab can become expensive if deep ribs and heavy reinforcement is needed. That is when a post tensioned slab may become cost effective. In addition, the control of visible cracks in the slab can be difficult. Concrete shrinks as it cures and if control joints are not placed appropriately or sufficient reinforcement is not used the owners may be upset with visible cracks. Cracks caused by shrinkage do not indicate failure, but can be an eye sore and a difficult conversation with the owner.

PROS
Cost effective
Common Construction
Low Tech
Sections may be removed to Access Plumbing

CONS
Limited Design Capacity
Does not respond well to Soil Movement

Learn more about different foundation types.

Originally posted 2009-11-03 10:42:22.

Wind or Fallen Tree Damage Report by a Structural Engineer

Jim Houlette — Sat, 31 Dec 2011 21:20:34 +0000

From 9News

Unfortunately, we are experiencing some high winds as the year ends. Although structures built in the last 20 years should have been designed to withstand a 90mph wind gust or higher in some areas along the front range it doesn't protect the structures from falling trees or other forms of wind damage. There are of course many structures built prior to the building codes that may not be able to withstand the high winds.

If your structure is damaged from these high winds or anything else (fire, snow, water, soil movement) you may need a structural engineer to inspect the structure to determine if it's safe and what repairs are needed.

EVstudio can help you get through this. Our structural engineers are based in Denver and Evergreen and live throughout the Denver area.

In an emergency please call or email Jim Houlette, PE at 720-975-5368 or jim@evstudio.com for help. Anytime.

It's Okay to be a Little Shifty

Dean Dalvit — Mon, 12 Dec 2011 23:46:20 +0000

This post is for all of those framers who diligently adhered to a structural framing plan to the letter and wound up with more backout framing than they knew what to do with once the plumber and mechanical contractor had their way with the joists.

According to Weyerhauser, the makers of the TJI joist products...
TJI® joists that do not occur under a joint in the sheathing may be shifted to avoid cutting them for plumbing or other mechanical. Joists other than the single joist to be shifted must remain at their specified locations. Joists meeting the clear spans of the Trus Joist L/480 live load span table may be shifted up to 6” (but not exceeding a maximum spacing of 24” o.c.).

Framers, have your guys lay out the plumbing and heating penetrations through the floor deck so you can adjust the layout as necessary and save yourselves considerable headaches during backout. While it is a bit unorthodox, we like to show the plumbing fixtures on our structural plans for this very reason. We don't always know how the mechanical contractor is going to run his equipment, so it's better to ask twice and cut nothing at all!

Originally posted 2008-08-11 15:05:28.

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Differences in Metric and Imperial Reinforcing Bar (Rebar) Grades and Sizes

Dean Dalvit — Fri, 09 Dec 2011 23:50:04 +0000

All Contractors should be intimately familiar with Steel Reinforcing Bar (Rebar) grades and sizes. With materials coming from all over the globe these days, it is ever important to know the comparison between the Imperial and Metric systems for bar grading and sizing. Starting in 1997, CRSI member steel mills have adopted new "soft metric" standards in their grading and sizing.

The charts below should be used to cross reference imperial grades and sizes with metric grades and sizes. Watch out because if your specs are in metric and your supplier is still using the imperial system, you may wind up with some really huge bar diameters on site. As you can see, the old #4 (1/2") bars are now being called out as #13, so be very careful here to avoid these kinds of potentially costly mistakes.

Soft Metric Bar Sizes vs. Inch-Pound Bar Sizes:

Metric Bar Sizes	Inch-Pound Equivalents
#10 #13 #16	#3 #4 #5
#19 #22 #25	#6 #7 #8
#29 #32 #36	#9 #10 #11
#43 #57	#14 #18

Inch-Pound Grade	psi* Minimum Yield Strength	Metric Grade	MPa** Minimum Yield Strength
40	40,000	280	280
50	50,000	350	350
60	60,000	420	420
75	75,000	520	520

* pounds per square inch
** megapascals

For more information, please visit www.crsi.org/rebar

Originally posted 2009-08-17 17:36:51.

What in the World is a Ufer Ground?

Adam Denton — Fri, 09 Dec 2011 23:50:01 +0000

In EVstudio’s structural engineering practice this question comes up almost every week. In contemporary construction, a ufer is a concrete encased ground electrode, which is most often a 20 foot long piece of #4 rebar cast into a foundation wall that will protrude from the top of wall so that the structure’s electrical system may attach to it. Technically this falls within the realm of electrical engineering, but enters the realm of structural engineering because the 2008 National Electrical Code requires an installation observation. Because the structural engineer typically will already need to be present on site at some point to observe the rebar construction in the foundation walls many building departments are happy to pass the additional liability on to the structural engineer and require them to observe and document the ufer installation as well.

The ufer was invented during World War II by Herbert Ufer, who was tasked with finding a cheaper alternative to the current method of grounding explosive ordinance storage structures against lighting strikes. The previous grounding method required driving hundreds of feet of electrical ground into the poorly conductive soils found in Arizona. Ufer discovered that due to the naturally high ph levels in concrete (uncured “wet” concrete is caustic and will burn human skin on contact*) the readily available supply of ions will conduct electricity better than almost all soils. The interaction of steel and concrete provide a better ground than the interaction of copper wire and concrete so it was natural connection to utilize the steel reinforcing bars already required of most foundations.

* The author has first-hand knowledge of the validity of this claim.

Originally posted 2011-04-14 00:07:08.

How To Read Metric Rebar Marks on Steel Reinforcing

Dean Dalvit — Mon, 21 Nov 2011 08:02:22 +0000

In order to identify the basic information about the rebar that is delivered to a jobsite, it is important to know how to read the mill markings that are stamped on every bar. This is especially critical to ensuring that your materials match up with the specifications in the plans.

These mill markings illustrate the "soft metric" system adopted by CRSI member mills in 1997 (see our post about metric and imperial markings):

Images and bar marking description courtesy of CRSI

The top letter or symbol identifies the producing mill and deformation pattern.

The next marking is the bar size.

The third marking symbol designates the manufacturing material — usually either "S" for carbon-steel (ASTM A615) or "W" for low-alloy steel (ASTM A706).

Finally, there will be a grade marking (4 or 5, for 420 or 520) or by the addition of one line (420) or two lines (520) that must be at least five deformations long.

For more information, please visit www.crsi.org/rebar

Originally posted 2009-08-20 07:00:19.

Seismic Testing of a Mid-Rise Wood Framed Structure

Adam Denton — Fri, 18 Nov 2011 20:22:21 +0000

NEESWood Capstone Test Structure

After the destruction caused by the 1995 Kobe earthquake the Japanese government took it upon themselves to construct the world’s largest seismic testing table, often referred to as a “shake table”. This testing facility, called Earth-Defense, was built to its enormous size out of necessity. Space to build new structures is a rare commodity in Japan and it was essential to be able to test large mid-rise (4 to 7 story) structures that maximize usable space while minimizing the building footprint. The Earth-Defense shake table is 50’ x 65’ and can support an astonishing 2.5 million pounds while “shaking” test buildings in three directions at once.

The NEESWood Capstone project shown in this video was conducted last summer to test the viability of wood framed mid-rise structures. Mid-rise structures are more traditionally constructed out of steel and concrete, but with the trend towards “green” construction it is becoming desirable to use renewable construction products like wood in larger commercial, residential and mixed use projects. The NEESWood Capstone project tested a seven story mixed use building consisting of a steel framed commercial (retail) main level and six stories of wood framed residential units. This test building was constructed in its entirety within the walls of Earth-Defense. The test building was constructed with many proprietary wood and steel products along with new NEESWood construction methodologies, all of which being beyond the scope of this article.

The portion of the results this test that was of most interest to me as an engineer was the “real life” forces experienced in the structure. As engineers in school we are handed exhaustive volumes of information that we are expected to accept as “the unquestionable truth”. There is an understanding that all of this information has been determined by hundreds of collective years of expert design experience, material testing and are always in a continuous state of refinement. As an engineering student I accepted that this was the adopted standard practice of my trade, but I always wondered how all the design theory I was being fed related to the real world. Hundreds of gauges were installed throughout the Capstone structure to test the forces actually experienced during an earthquake. It was quite satisfying as a design professional to see that the calculated anticipated forces at play in the structure were very close to those actually recorded.

For more detailed information on NEESWood Capstone project please follow this link.

Originally posted 2010-07-15 00:01:06.

Structural Modeling in Revit

Anthony Ries — Thu, 10 Nov 2011 07:39:06 +0000

With each new project we use more and more of the tools in our BIM toolbox. The latest tool that we have developed to the level of prime-time is our structural steel framing within Revit. We are currently working on a very unique custom home in the foothills of the Rocky Mountains which has some unique requirements. This home will be making extensive use of architectural concrete and structural steel.

Normally we can create a 3D walk through of a project as an additional architectural service. With this current project we have created the entire structural steel system in Revit and with a few mouse clicks are able to create a structural steel walk through. Below is a short video showing the preliminary structural steel layout only. No bearing walls or concrete is shown.

As a design and documentation tool we are able to pan, zoom and rotate in 3 dimensions to thoroughly vet our structural design and also more quickly identify problem areas where structure may be interfering with other trades or design features. Additionally it provides a solid model that indicate lengths of beams, joists and columns. This particular model will be shared with the steel fabricator in order to improve their accuracy, speed and reduce questions. We also use it internally to investigate how various pieces come together in a direct way.

With all of that said, 3D modeling is no substitute for expertise, education and practical experience. We use the software as a tool to further explore and document our designs, not as a crutch to develop the design.

Helical Pier Foundations

Jim Houlette — Sun, 30 Oct 2011 06:38:02 +0000

Helical piers are typically used when repairing failed shallow foundations. The helical piers are a deep foundation system that can be placed alongside existing foundations with very little interference.

However, helical piers are also used for new construction as an alternative to typical straight shaft drilled piers. EVstudio recently designed a new foundation using helical piers due to high water table and swelling soil. With a small surface area on the steel shaft the helical pier is very stable is highly expansive soil. We have also recently used helical piers for an addition to a home. There was no soils report, but the house was on drilled piers. A drill rig for new drilled piers could not gain access to the backyard, so helical piers installed using smaller machinery was recommended.

The real advantage helical piers have is their use for repairing moving foundations. The entire existing structure can be underpinned using helical piers that are bolted to the bottom of the foundation. Once the structure is underpinned the moving soil below the footing can be removed, establishing a void space separating the structure from any future movement.

PROS
Fast Installation
Can be Proof Tested
Negligible Uplift Pressure from Swelling Soil
High Tensile Resistance
Moderate to Small Equipment Needed

CONS
Specialized System
Concerns with Corrosion

Learn more about different types of foundations

Originally posted 2009-12-11 00:54:22.

Modern Mountain Home in Evergreen Colorado Nearing Completion

Dean Dalvit — Wed, 26 Oct 2011 06:33:25 +0000

We're very excited to be seeing the completion of a modern mountain home we designed in Evergreen Colorado near completion. The Certificate of Occupancy was recently released and the owners are moving in. EVstudio provided all of the Architecture, Structural and Civil Engineering on this project and the seamless integration of each discipline within the others resulted in a true transcendence of form and function.

Modern Mountain Colorado Home

The home nestles naturally into the site and steps with the site. The excavator remarked how simple the excavation was and how perfectly the cut and fill balanced so no costs were incurred in hauling material. Expressing the structural frame was also a strong element in this design. We needed moment frames to handle the lateral loads, but didn't want the huge thermal bridge in the wall that they ultimately create. So instead, we expressed them on the surface of the wall with steel C channel and used the structural element as an architectural trim detail. In fact, all of the headers were handled in this way and the thermal envelope of the walls is significantly better. Note the square head bolts used as an additional touch...

Exposed Steel C Channel Moment Frame

Not only are the forms on this home distinctive offering a fresh interpretation of modern in an organic mountain setting, but the materials are also very rich in both texture and their very nature. Reclaimed weathered barnwood siding, rusting steel and natural stone are prominent elements, respecting the relationship between a home and its climate.

Mountain materials of stone, reclaimed barnwood and steel

The proportions of the steel siding were based on Fibonacci's golden section. We studied not only the proportions of the home, but also of the aspect ratios of the elements themselves to arrive at the sizes, then the contractor had them manufactured to our exacting specifications.

Metal Siding With Proportions of Golden Section

Attention to detail was a hallmark of this home both inside and out. We'll post the interior shots after the owners get settled in, but the exteriors received as much attention as the inside. For our canted steel I-Beam posts, we inserted a reclaimed wood timber with square head bolts.

Seamless Structural and Architectural Detailing

With everything having come together in such a harmonious way, The owners are thrilled with the outcome, as is the neighborhood, not to mention the EVstudio team. We were fortunate to have clients who were so great to work with and open minded to new ideas for their home!