Architecture, Engineering & Planning EVstudio | Denver & Evergreen Architect | Colorado & Central Texas | Blog » Foundation

Repairs to Concrete Foundations Made Simple Using Carbon-Fiber Kevlar Grid Straps

Jim Houlette — Sat, 31 Dec 2011 21:20:52 +0000

From Fortress Stabilization Systems

EVstudio recently met with Pinnacle Construction where they introduced us to Carbon-Fiber and Kevlar Grids used to repair foundation walls. Carbon-fiber has recently been introduced into the construction industry due to its very high tensile strength, lightweight and workability. Sections of the grid can be adhered to the concrete surface using a high strength epoxy bonding agent.

Fortress Stabilization Systems appears to be the leader in introducing this new technology to the construction industry, applying it to many different situations from bridges to basements. The most useful application for residential would be the Carbon Staples, Kevlar Neckties, and various shapes of Carbon-Fiber/Kevlar grids used to control cracks and provide additional strength to an otherwise failing foundation wall.

EVstudio's structural department is very excited to research more about Carbon-fiber and other applications it can be used in. Carbon-fiber is a great alternative to commonly used steel repairs.

From Fortress Stabilization Systems

Originally posted 2010-03-02 00:01:01.

Foundation Wall Height Design Considerations for Residential Garages

Kacy Landsittel — Sat, 31 Dec 2011 21:20:46 +0000

There are many ways that openings like doors and windows come into consideration with respect to how we, at EVstudio, design foundation walls. More specifically, there are special criteria that go into a foundation wall design for residential garages because the threshold of door openings are at grade elevation. It is firstly critical that the wall maintain a minimum height of 6 inches above grade to prohibit moisture penetration to the typical wood framed structure it supports and of course the interior too. The foundation wall has to also maintain proper frost depth, which is a specific distance below grade, determined by local building codes and varies by location and state. Typically in our plans, the heights of a foundation wall are all relative to the finished floor elevation. However, when it comes to incorporating a swing door or an overhead door, the distance that a portion of the foundation wall we specify to be dropped is relative to the wall at that particular location.

There are some general rules we go by and different circumstances can result in a variety of solutions. For a typical above grade garage accommodating one car length in depth, we normally set the top of foundation wall height a minimum of 6-12 inches above the lowest point of the top of the garage floor slab. There are a variety of reasons the top of foundation wall is set to where it is and some architects and builders take that into consideration on a case by case basis, others just conform to standard practices. Also, we specify to slope the garage floor, and is usually 4-6 inches of fall from back to front depending on the depth of the garage and the presense of any floor drains. The result then at the rear of the garage is that the foundation wall ends up at 2-8 inches above the rear of the garage floor. For the allowance of a swing door or an overhead garage door, whether it is at the rear of the garage or the front, not only does the foundation wall need to be lowered for the purpose of the door threshold, but it also needs to be dropped further to allow for the floor slab to be thickened for structural support of the slab itself, stabilization of the adjoining foundation walls, and also the load of vehicles above. This thickened slab is formed and poured after the foundation walls have cured, which is why specifying the distance of the drop itself is important for the workers in the field who set up the form work for the concrete that is to be poured. We set the vertical depth of the thickened slab at a minimum of 8 inches below the 4 inches of actual floor slab, or 12 inches below the lowest top of slab height .

Once we have determined what height the foundation wall needs to be above the lowest top of slab height and take into account the 12 inches of thickened slab, the drop in the wall for an overhead garage door is pretty easy to calculate. For the graphic example above, I have set the foundation wall height at 8 inches above the lowest point of my top of slab height, so the net distance for dropping the foundation wall is 8 inches plus 12 inches, or 20 inches. While an above grade garage will usually have a continuous foundation wall height, we specify the necessary drops in it so doors can be incorporated. All in all, each project requires its own analysis to determine foundation wall heights and these design considerations are critical for every project.

Originally posted 2009-11-01 02:00:36.

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.

Spread Footing Foundation

Jim Houlette — Mon, 12 Dec 2011 23:46:24 +0000

This has to be the most typical foundation type with basement or crawlspace construction. The spread footing design is simple; provide sufficient footing area to distribute the weight of the structure onto the soil. Typically, a 16” wide x 8” concrete footing is used.

The width of the footing distributes the dead and live load of the structure on the soil allowing a typical 8” wide concrete wall to be placed on the footing in lieu of a wide 12” to 16” concrete wall. In addition, the poured footing helps to provide a level surface for the concrete foundation walls. The thickness of the footing will vary due to unlevel soil conditions. The designer should also keep in mind that even if a 12” wide footing satisfies soil bearing, the foundation crew may need at least 16” to allow enough area to set the wall forms on the footings and allow for dimension errors to be adjusted.

A typical spread footing foundation is also referred to as a ‘T wall’.

PROS
Cost effective
Common Construction
Low Tech

CONS
Fails with Swelling Soil

Learn more about different types of foundations

Originally posted 2009-09-13 02:09:33.

Wall on Grade Foundation

Jim Houlette — Mon, 12 Dec 2011 23:46:22 +0000

Simple and cost effective, but limited in design capabilities. A wall on grade is just as simple as it sounds. If the bearing capacity of the soil is moderate to high and the load from the structure is light then a typical spread footing can be omitted. In most foundations an 8” wide foundation wall is used so the bearing area would only be .66ft per linear foot. If the soil bearing capacity is 2,000psf the foundation could only support 1,332plf and that must include the weight of the foundation wall. Therefore, you will find in most situations that the wall on grade foundation can only support light structures.

I have used a wall on grade foundation for detached garages and small homes when saving every dollar counts. The walls can be constructed by trenching and setting forms as needed above grade or excavating for a typical crawlspace and setting forms for the depth required for frost depth.

The wall width does not need to be only 8”. You may find it cost effective to use a wall on grade that is 10” or 12” wide for a 36” tall wall in lieu of a spread footing. The volume of concrete may be higher for the wall on grade as the width of the wall increases, but you can construct the wall with a single pour in lieu of two pours; footing then foundation wall.

A wall on grade foundation can sometimes be mistaken as the same foundation type as a turn down edge slab-on-grade. The difference between a wall on grade and a slab-on-grade is the separation of the foundation from the slab. For a wall on grade the bearing area is only of the wall, and for a slab-on-grade turn down edge you can also take into account a percentage of the slab surface for bearing. You can read more about the slab bearing area from a previous discussion about post tension slab on grades and bearing area here.

PROS
Cost effective
Common Construction
Low Tech

CONS
Fails with Swelling Soil
Limited Design Capacity

Learn more about different foundation types.

Originally posted 2009-10-06 00:01:51.

Hydronic Radiant Floor Heating in Post Tension Slab-on-grade Foundations

Jim Houlette — Fri, 09 Dec 2011 23:50:09 +0000

The structural department recently designed and observed the construction of a post tension slab-on-grade foundation that incorporated a hydronic radiant floor system. Although there is nothing special about a hydronic radiant floor system in the basement floor, many builders and owners wonder if it can be done with a post tension slab.

In fact, with an engineered post tensioned slab cracks and isolated movement is reduced and allows for a radiant floor system to be used. The slab should be at least 5” or more to allow sufficient depth to accommodate the tendons and the radiant system.

This project used a 6” uniform slab with no interior ribs and utilized the exterior basement walls to provide additional stiffness. A hybrid PT foundation, but cost effective when compared to using a deep foundation system and elevated structural floor. The foundation was first designed using spread footings, but then changed to a deep pier foundation during the open hole observation by the geotechnical engineer. With that unexpected change in cost, EVstudio helped design a cost effective foundation for the client that would still maintain a shallow foundation type and provide the additional strength to resist the possible expansive material found.

Originally posted 2011-01-19 10:41:45.

Fastening to Concrete – Wedge Style Expansion Anchors

Adam Denton — Fri, 09 Dec 2011 23:50:00 +0000

A 5/8" diameter wedge style expansion anchor.

There are many situations were it may become necessary to attach a timber ledger to a concrete wall. It is desirable in many new homes to lower the elevation of the finished floor to make a home more wheel chair accessible. An example of the differences between the finished floor elevations that can be achieved with a framing ledger are shown below. Additionally, when an addition is made to a residential home a timber ledger is installed so that the finished floor of the new construction can match the finished floor elevation of the existing construction while utilizing the existing foundation wall.

Conventional Floor Framing vs. Ledgered Floor Framing

Wedge style expansion anchors work because of the friction forces developed between the studded collar (see photo at top) and the concrete. To begin a typical installation a hole must first be drilled into the concrete. All manufacturer’s instructions for hole size and depth should be followed carefully because the recommendations will vary between bolts sizes and manufacturers. Once a hole is drilled and cleaned the anchor may be driven into the hole with a hammer. The studs on the collar are slightly larger than the widest diameter of the bolt so initial contact with the concrete is only at these areas. Once the bolt is driven to its specified depth a pre-drilled timber ledger may be installed over the bolts. Finally, the nuts (with supplied washer) may be installed. As the nut is tightened, the studded collar with catch on the edges of the hole in the concrete and will be forced onto the wedge. The collar will expand as it is driven onto the wedge creating a greater frictional force against the concrete. With the nut installed to the proper torque the anchor may be safely loaded to the manufacturer’s design specifications.

Originally posted 2010-07-30 23:04:52.

Another ICF Foundation Home Under Construction

Kacy Landsittel — Fri, 09 Dec 2011 23:49:51 +0000

We have designed several homes that feature ICF construction for foundations. This home is currently being built high above Idaho Springs, Colorado. In this case, ICF (Insulated Concrete Form) is only being used for the walk-out basement. They offer great thermal performance and streamline construction with the forms serving double duty of insulation and a form for pouring the concrete.

Below are some related articles that have additional information about ICF construction.

Steel Screw Pile Foundation

Jim Houlette — Mon, 21 Nov 2011 08:02:27 +0000

A deep foundation system that is gaining popularity in areas with difficult soil, contaminated soil and due to its speed of construction is steel screw piles.

At first glance screw piles look like the typical helical pier. However, there are significant differences that make a dramatic change in capacity. First, the screw piles are installed as a single continuous piece in lieu of sections bolted together. Second, the machinery used to install screw piles is larger and capable of producing a higher torque.

The components of a screw pile are simple; a steel shaft, steel helix at bottom of shaft, and a connection plate at the top of the shaft. Screw piles can vary in size to accommodate different design loads. The steel shaft size is a function of axial load, torsion during installation and thickness due to corrosion. The helix is a plate that is shaped just like the threads of a wood screw and engages the soil to distribute the axial load and it locks into place to provide tensile loads as well.

One of the first questions people have is, “How long will it last until failure from corrosion?” There are several factors that go into corrosion of steel; oxygen, moisture, dissolved salts and other corrosive chemicals. Since there is very little oxygen in the soil, corrosion is limited and can be controlled by using thicker material to allow for expected corrosion. The life of steel screw piles can be 50 years to an indefinite period depending on the type of structure and life span needed.

I designed foundations in Stapleton where the site was located over the old terminal and contaminated soil was discovered. If typical straight shaft drilled piers were used the spoils from drilling the hole would have had to been bagged and disposed of properly. To avoid bringing contaminated soil up to the surface steel screw piles were used.

I list a lot of PROS below and for good reason. I have been very impressed with screw piles and the wide range of uses of them. EVstudio was recently visited by Alpine Site Services for a lunch and learn. Mike Oliveira discussed how screw piles are installed and the many advantages to their system over other typical foundation types. What really impressed me was the speed of placement of screw piles. Alpine can place 30 to 50 screw piles in one day!

Another great advantage to screw piles is that they can be placed at an angle to provide bracing and tension resistance that is aligned with the force of the tension. An example is a typical counterfort that braces a foundation wall. You can see the angled screw pile above.

PROS
Fast Installation
Can be Proof Tested
Negligible Uplift Pressure from Swelling Soil
High Capacity up to 500kips
High Tensile Resistance
Made from a reusable material (LEED point)

CONS
Specialized System
Large Equipment
Concerns with Corrosion

Learn more about different foundation types.

Originally posted 2009-10-10 00:21:43.

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.