StoneTree^® Concrete Fence

What is the IBC?

The IBC is the International Building Code, which is developed and maintained by the ICC (International Code Council). This code is updated on a three year cycle (2000, 2003, 2006…). The IBC is intended to establish minimum performance requirements for building systems, such as Stonetree^® Walls System, to safeguard the public health, safety and general welfare. The building code relies heavily on referenced published standards. In the precast concrete fence industry some of the typically referenced standards are; ASTM (American society for Testing and Materials), ACI (American Concrete Institute) and ASCE-7 (Minimum Design Loads for Buildings and Structures by American Society of Civil Engineers).

The IBC has been adopted at the state or local level in 50 states plus Washington, DC (Figure 1). The building code becomes law when it is formally enacted or adopted by the appropriate authority. Local (State/County/City) permitting offices that have adopted the IBC are listed on the ICC web page www.iccsafe.org/gr/Pages/adoptions.aspx

Particular States/Counties/Cities have included additional requirements to the IBC codes, such as; Florida or California. The IBC does not nullify any additional provisions by local codes. The most restrictive code will always govern.

Local building officials enforce the requirements of the code through permits. The permit application typically will include construction documents. Usually, the general information shown on the construction documents indicates “the location, nature and extent of the work proposed and show in detail that it will conform to the provisions of this code and relevant laws, ordinances, rules and regulations, as determined by the building official.”¹ The construction documents may also ask for the manufacturer’s installation instructions. The structural documentation required for the Stonetree^® system may include the following:

The local permitting office may require the construction documents for the Stonetree^® system to be prepared and stamped by a registered professional engineer. Even when a permit is not required by the permitting office, the building code still requires that products, such as the Stonetree^® Precast Stone Wall System, meet the standards of the IBC code and/or the jurisdiction. This is to ensure the safety of the public and to reduce liability.

¹ International Code Council. International Building Code 2003. Section 106.1.1. pp5

When is Permitting & Design Required?

The International Building Code (IBC) is a set of rules that specify the minimum acceptable level of safety for constructed buildings and non building structures. The IBC has identified certain work that does not require permitting. If Stonetree^® Wall does not require permits it does not mean that the work can be “done in any manner in violation of the provisions of this code or any other laws or ordinance of the jurisdiction.”² In synopsis, even if a product does not require permitting it still needs to follow the code. According to the traditional (remember local authorities can augment the IBC with higher standards) IBC, permits are not required for “fences not over 6 feet (1829 mm) high”³ (≤6 feet).

As mentioned previously, local requirements maybe more stringent than the IBC and should always be checked. As an example:

• Local requirements could change the permitting requirements for the height of the fence. As an example; the city of Tulsa, Oklahoma revised building code, exempts from permitting the following; “Fences not over eight (8) feet high,unless of masonry or precast construction over four (4) feet high.”⁴ Therefore, the StoneTree^® precast wall above 4 feet in height would require permitting in Tulsa, Oklahoma.
• Local requirements or permitting agency may view the Stonetree^® Wall system as an exterior non-load bearing wall instead of a fence. As a wall (the Stonetree^® Wall system) will require a permit. A wall restricts or prevents movement across a boundary and blocks vision.

The IBC assigns Use and Occupancy Classifications to buildings and structures. This classification relates the risk of fire and life hazard to a structure’s occupancy. The Stonetree^® precast stone wall is in the Utility and Miscellaneous Group U with agricultural buildings, greenhouses etc. These structures have a low occupancy category which yields a low importance factor used in the design of the structure. The importance factor can range from .87 (low hazard to human life) to 1.15 (essential facilities i.e. hospitals, fire, police).

The IBC allows that if a structure is not capable of being designed by approved engineering mathematical analysis, it instead can be evaluated by the use of destructive testing. The testing method is required to be developed by a registered design professional that simulates the actual loading conditions and “the building official shall approve the testing method”⁵. The materials (concrete mix), and workmanship of the test wall needs to represent what will be used on the project. In the test, the structure will be subjected to an increasing load until failure occurs or until the applied load is equal to 2.5 times the desired design load. So, the allowable load on the wall is then the lesser of:

• Failure load / 2.5
• Max load applied / 2.5

The testing safety factor of 2.5 is to account for any variations in material and workmanship of the structure..

² International Code Council. International Building Code 2003. Section 105. pp 3.
³ International Building Code 2003. Section 105. pp3.
⁴ Title 51, Tulsa Revised Ordinances. Chapter 1. Section 105.2.1.2
⁵ International Building Code 2003. Section 1714. pp 358.

Liability

The Stonetree^® Wall system can fall under many forms of liability. The key objective is to understand your liability and how to minimize it. This section is just a brief overview of liability. You should consult with your local legal console for a more comprehensive review.

Liability can fall under either commercial (contract) or tort. Tort liability is concerning property or reputation, and finding responsibility for the compensation of damages incurred. Product liability falls under tort law. Product liability is the area of the law in which manufacturer’s are held responsible for the injuries their products cause.

Claims typically associated with product liability are negligence, breach of warranty, strict liability, and various consumer protection claims. Product liability laws are determined at the state level and vary widely from state to state.

Negleigence

A basic negligence claim requires the following; duty owed, breach of that duty, an injury and that the breach caused the injury. There are different forms of negligence concepts which have arisen to deal with specific situations, such as, Negligence per Se.

Negligence per Se is the violation of a statute or regulation. An example of Negligence Per Se is if a Stonetree^® manufacturer produces a wall that was not according to the building code. Remember, even if a permit is not required the standard is still the building code⁶. It can easily be established if a wall is made to code through calculations by an engineer. Assume that the wall collapses and someone is injured by the wall’s collapse. The violation of the building code (statute or regulation) establishes Negligence Per Se and the manufacturer can be found negligent.

Now, assume that the design was stamped by a registered professional Engineer (P.E.) and the manufacturer made the wall according to the design. The liability transfers from the manufacturer to the P.E. who did not design the wall according to the stated building code.

Warranties

Warranties are statements by a manufacturer or seller concerning a product during a commercial transaction. Warranties are usually classified as either express or an implied warranty.

Strict Liability

Strict liability claims focus on the product and does not prove fault. Under strict liability the manufacturer is liable if the product is defective, even if the manufacturer was not negligent in making the product defective. The manufacturer is forced to pay for all injuries caused by the products.

There are many ways to protect your company from liability claims. One example is to document your company’s Quality Control Manual – include testing, inspection, traceability (i.e. batch numbers), how to handle customer complaints, product recall steps, typical design standards and a documented filing system for designs when P.E. stamp required.

⁶ International Code Council. International Building Code 2003. Section 105. pp 3.

IBC Wind

The IBC (International Building Code) is intended to establish minimum performance requirements for structures, such as the Stonetree^® Wall. The building code relies heavily on referenced published standards, such as ASCE 7, for the calculation of wind loads. The Stonetree^® system would classified as a “solid sign or a freestanding wall”, in the wind load calculation per ASCE 7. The design code requires two (2) pieces of information from the project site to calculate the wind load:

• City, State
• Local terrain.

The city and state are used to determine the basic wind speed (V in mph). The basic wind speed is identified by a map in ASCE 7 (Figure 2). As you can see from the map, fences built in coastal regions of the east will be subjected to higher wind loads. The local authority may adjust the loads to account for recorded higher local wind speeds.

The local terrain of the project site affects the Exposure Category. The Exposure Categories or the ground surface roughness conditions, according to ASCE 7, are as follows:

A – (No longer in use)

B – Urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of single – family dwellings or larger
C – Open terrain with scattered obstructions having heights generally less than 30 ft, i.e. flat open country, grasslands, and all water surfaces in hurricane prone
regions
D – Flat, unobstructed areas and water surfaces outside hurricane prone regions, i.e. smooth mud flats, salt flats and unbroken ice.

In summary, Stonetree^® systems designed in unobstructed areas (D) will have a slightly higher wind load than an urban area (B) where buildings can interfere with the wind pressure.

Design Process

The structural design process leads to an end product that is proven to be strong and stiff enough for the intended purpose throughout its intended lifetime. The Stonetree^® Wall needs to be designed and constructed to be able to support the nominal loads without exceeding the allowable stresses for the materials used in construction. To achieve this end product and/or to obtain permitting:

1. The calculation process used needs follow a reliable and reproducible path.
2. The structural materials used in the design need to have known, reliable and reproducible short and long term properties.

The structural design process understands the flow of forces through a structure. The analysis provides a complete load path capable of transferring loads from their point of origin to the load-resisting system. The lateral wind force is distributed to various vertical elements of the Stonetree^® Wall in proportion to their rigidities. As an example, the top half of the 3 ½ inch thick panel (tributary area) will contribute the wind load to the top beam and the bottom half of the panel will contribute the wind load to the bottom beam. In essence, this is how the applied wind load is transferred from one structural component to another.

The load capacity or the load the structure can carry has to be larger than the load applied. In the design process a factor of safety is used to account for uncertainties in the loads, uncertainties in structural analysis, manufacturing process, material properties and failure criteria, etc.

Load Capacity > Safety Factor x Load Applied

The inputs required for the design of the Stonetree^® Wall:

• Wall Height
• The calculated uniform wind load (qdesign)
• Special loading conditions
• Concrete 28 day compressive strength (f’c)
• Reinforcement – (see Section Reinforcing Types)

How the Stonetree^® Wall resists the loads can be investigated by assuming a uniformly distributed wind load acting on the face of the panel:

1. The horizontal uniform wind load hits the center (3 ½ inch thick) panel which induces bending in the vertical panel (Figure 3). The panel needs to be designed to resist the bending or flexure. The tributary area on the panel transfers the load to the top and bottom beams, and to the columns.

2. The wind on the beam and the load from the panel, places the horizontal top and bottom beams into bending (Figure 4). The design for the beams is required to ensure that the beams can withstand the accumulation of both of the loads. The end forces from the beams, load the columns.

3. The load transferred from the top and bottom beam to the columns is a concentrated shear force (Figure 5). The force is located at the joint between the column and the beam. The design calculations should verified that the end of the beam will not fail in shear at this location.

4. Horizontal shear forces can also form in the column fork connection where the cross-sectional area decreases
(Figure 5). Structural computations need to confirm that the fork column connection can resist the induced loads.

5. The loads will be transferred from the column to the structural post member, typically a W4, to the foundation. The W4 and the foundation are outside the scope of this report.

6. Finally when the panel is still green and being lifted from the form (Figure 6), it is assumed to have approximately half of its 28 day compressive strength (approximately .5 x f’c). The self-weight of the panel will load the bottom beam in tension. The bottom beam needs to be checked to confirm the panel can carry the loads generated during the lifting process.

Why Do We Reinforce The StoneTree^® Wall?

Concrete is a mixture of cement and aggregate. When mixed with a small amount of water, the cement hydrates to form a microscopic crystal structure encapsulating and locking the aggregate into its rigid structure. Typical concrete mixes have high resistance to compressive stresses; however, any appreciable tension (i.e. due to bending) will break the rigid structure resulting in cracking and separation of the concrete. Stonetree^® walls are subjected to bending and/or tension stresses during lifting, transportation, setting and after installation due to wind loads. The end product can simply be damaged due to lifting loads prior to ever reaching the site if it is not reinforced against bending.

Steel has high strength in tension. When steel is placed in concrete, the composite material (reinforced concrete) resists compression but also bending, and other direct tensile actions. Therefore, a reinforced concrete section is where the concrete resists the compression and steel resists the tension. The reinforced concrete structures can behave as a single structural entity. The bending loads developed in the Stonetree^® wall (shown in the Design Process Section) can be withstood by reinforced concrete. For the concrete to engage the reinforcing a small crack must occur. Once the concrete tensile crack has reached the reinforcing, the load is transferred to the reinforcing.

The Stonetree^® system can contract and expand due to changes in temperature from night to day. The coefficient of thermal expansion of concrete is similar to that of steel. The comparable expansion properties eliminate internal stresses that could cause cracking.

In addition to tension, concrete can require reinforcing due to shear forces, for example through the column connection of the Stonetree^® Wall system (see Design Process Section). A shear force acts parallel to the material surface (Figure 7). Historically, shear reinforcing, in concrete has been handled with stirrups (Figure 8). Stirrups require a time consuming job of bending the rebar and tying it into position in the reinforcing system. Recently, steel fibers have been recognized as shear reinforcing, according to ACI 544.4R Section 3.3.

Reinforcing Types

Concrete has been traditionally reinforced with rebar or welded wire reinforcement(WWR) for bending. Today concrete is also reinforced with fibers.

Reinforcing Bars (Rebar):

Rebar can be utilized to meet the reinforcementrequirements of the Stonetree^® Wall System. Rebar reinforcing is designed to be placed in specified locations in concrete to counteract calculated tension and shear stresses. As an example, (see Design Process section) wind on the Stonetree^® wall face, willplace the top and bottom beam in tension (Figure 9). Rebar will be required to be placed a specific dimension away from the beam face to counter act the tension load. The rebar needs to be held in place during the curing of the concrete.

Structurally reinforced concrete has a minimum area of steel reinforcement requirement in elements under flexural load. As an example, in the top beam of the Stonetree^® Wall (b=12 in, d=3.5 in, fy= 60,000 psi) the minimum steel areas requirement would be 0.14 in² of rebar or a number 4 bar.

Structurally reinforced concrete has a minimum area of steel reinforcement requirement in elements under flexural load. As an example, in the top beam of the Stonetree^® Wall (b=12 in, d=3.5 in, fy= 60,000 psi) the minimum steel areas requirement would be 0.14 in² of rebar or a number 4 bar.

Rebar reinforcing can include the time consuming task of tying the rebar into location and bending the steel for the hooks and stirrups. Additionally, care needs to be taken during consolidation/vibration to ensure that the reinforcing is encapsulated by the concrete. If the reinforcing is congested proper consolidation can be difficult.

 

 

Welded Wire Reinforcement:

Typically, Welded Wire Reinforcement (WWR) can be used in combination with rebar to reinforce the Stonetree^®precast wall system. WWR is used when placing reinforcing sheets at controlled spacing. WWR is designed to be placed in specified locations in the concrete to resist calculated loads. To illustrate, the WWR could be used to reinforce the 3 ½” thick panels against wind loads. The WWR would be place in the center of the panel. The WWR can also be bent into required shapes i.e. the WWR could be bent into the shear stirrups in the columns (if required).

Fiber Reinforcement:

Concrete reinforced with steel fibers is less time consuming to place than traditional reinforcing, while still increasing the flexural toughness many times. Fiber reinforcing is uniformly dispersed into the concrete mix for an overall reinforced wall system.

The properties of fiber reinforced concrete (FRC) are related to the fiber aspect ratio (length/diameter), fiber material, fiber tensile strength, anchorage, dosage, and shape. All these items are taken into account by through testing of the FRC and obtaining a Re value for different fiber dosages. The Re value is the Residual Strength of the FRC after cracking. To determine the Re value the testing arrangement, relies solely on the concrete and fibers to determine the strength of the composite material (Figure 12). The beam sized used in the Re testing is the same size beam utilized in the standard Modulus of Rupture test (making the test information relatable). Utilizing the Re value and the concrete’s modulus of rupture the components of the Stonetree^® system can be design to resist specified loads.

Note: Re Values are not the same as ARS Values (Average Residual Strength). The test for an ARS relies on a steel plate during the loading phase of the test (Figure 13). This test method is for “comparative analysis among beams containing different fiber types”⁸. The ARS test’s beam size, is a different size than the test beams utilized to determine concretes modulus of rupture.

Steel Fiber Reinforcement

Depending on the wind loading conditions and the Re Value, steel fiber reinforcement may be used alone or in combination with rebar to reinforce the Stonetree^® Wall. The Re value is utilized to determine the flexural strength of the FRC per the design methodologies developed in ACI 544-4r⁹ for steel fibers (see Design Process, Item 1).

The steel fiber can also be employed to resist the shear forces that can develop in the Stonetree^® column in high wind loads.

Different types of steel fibers are defined in ASTM A820¹⁰ (Drawn wire fibers, Cut sheet fibers, Melt-extracted fibers, Mill-cut fibers, and Modified cold drawn wire fibers). This specification covers the minimum requirements for steel fibers; defines measurement dimensions, tolerances, workmanship, minimum physical properties (tensile and bending requirements). This specification ensures the steel fibers have a reliable and reproducible design values (see Design Process, Item 2).

Synthetic Fiber Reinforcement

“Design methods for particular applications using low volume synthetic fibers have not yet been developed.”¹¹ Utilizing the steel fiber design methodologies and the products’ Re Value synthetic fibers can be evaluated. Depending on the wind loading conditions and the products’ Re Value, the synthetic fiber reinforcement may be used alone or in combination with rebar to reinforce the Stonetree^®Wall. Synthetic fibers can not be employed to resist the shear forces that may develop in the Stonetree^® column in high wind loads. In these high wind load cases stirrups may be required to carry the shear forces.

Currently, synthetic fibers do not have an ASTM Standard Specification defining their minimum requirements.

⁸ ASTM C1399. Standard Test Method for Obtaining Average Residual-Strength of Fiber-Reinforced Concrete. ASTM International, 100 Barr Harbor Drive, P.O. Box C700 West Conshohooken, PA 19428-2959, USA
⁹ ACI 544-4r. Design Considerations for Steel Fiber Reinforced Concrete. Reported by ACI Committee 544. American Concrete Institute, P.O. Box 9094 Farmington Hills, MI 48333
¹⁰ ASTM A 820/A. Standard Specification for Steel fibers for Fiber-Reinforced Concrete. ASTM International, 100 Barr Harbor Drive, P.O. Box C700 West Conshohooken, PA 19428-2959, USA.
¹¹ ACI 544-1r. State-of-the-Art Report on Fiber Reinforced Concrete. Reported by ACI Committee 544. American Concrete Institute, P.O. Box 9094 Farmington Hills, MI 48333

Fiber Mixing

Fibers are engineered to disperse uniformly throughout the entire concrete mix to create a very effective multi-dimensional reinforcement system for the Stonetree^® precast stone wall system. To ensure that the fibers are spread though out the concrete, care is required during the mixing process. Fibers are either added in the concrete truck or at the batching plant.

Fibers added concrete truck:

Set the mixers to normal charging speed (12 to 18 RMP) and add the fibers. To ensure that the fibers do not bunch, ribbon feed the fibers into the mix. In other words, a bunch of fibers going into the mix can lead to a bunch of fibers coming out in the concrete mix. The charging mixing speed is required to carry the fibers away as they enter the mixer. After all the fibers have been added to the mixer, set the mixer to the highest mixing speed. Continue to mix until the combination is homogeneous; approximately 70 revolutions, or for 4 to 5 minutes.

Fibers added at batch plant:

In a batch plant arrangement, the fibers can be added a multitude of ways as long as the fibers are not the first component in the mixer.

1. The fibers can be added to the aggregates on the conveyor belt and mix in a normal manner. The fibers should not pile up or form clumps on the way to the mixer.
2. If it is not possible to add the fibers with the aggregates, then add the fibers to the mixer after the aggregates are introduced to the mixer.
3. Finally, the fibers may be added to the aggregate weigh hopper in the plant after the aggregates have been weighed. Usually, this arrangement works best with a conveyor belt. Then fibers and the aggregates flow into the mixer together.

The fiber clumping means that the individual fiber strands do not disperse uniformly throughout the concrete mix, and therefore they may fall short of imparting the desired reinforcement to the resultant hardened concrete. Most fiber clusters occurs as the fibers are added to the concrete mixture and can be eliminated by controlling the rate of fiber addition or by the use of collated fibers (glued fibers). Collated fibers are separated by the concrete mixing action which also disperses the fibers throughout the mix. Once the fibers enter the mixture clump-free, the fibers generally stay clump-free.

Typical Causes of Bugholes

The surface appearance moves into a prominent position when the concrete is used as an architectural building material, such as the Stonetree^® Wallsystem. One of the primary influences affecting the surface quality of concrete is bugholes. Bugholes are surface air voids in the concrete. The voids are the result from the migration of entrapped air (and to a lesser extent water) in the surface of the formed concrete during placement and consolidation. Bugholes normally occur in vertical concrete surfaces.

During consolidation, the air bubbles seek the nearest route to reach pressure equilibrium. In the vertical form liner, the closest distance for the air bubbles’ migration is to move horizontally out to the form, then up to the concrete surface. If these bubbles are not directed vertically to the concrete surface, the bugholes will be present, if not abundant.

These surface voids are primarily an aesthetic problem for exposed concrete and should not affect the structural strength of the Stonetree^® system. However, problems can arise if the concrete surface is to be painted. The permissible size or number of bugholes isn’t defined for the smooth-form finishes according to ACI 301-99.

What are common causes of bugholes?

• The most common cause of bugholes is improper vibration. The vibration sets the air and water bubbles into motion. The proper amount of vibration sends both entrapped air and excess water to the free surface of the concrete or the top of the wall. There is a fine line between too little and over vibration. Too little vibration will lead to surface air voids on the concrete wall face. Over vibration will cause over-consolidation of the concrete resulting in aggregate segregation and bleeding.
• Bughole formation can also be effected by the wrong form-releasing agent with the Stonetree^® form material. It is imperative that form releasing agents are used according to manufacturer’s recommendations and with specified form material.
• Mix design can also be considered a significant contributor to bughole formation. A sticky or stiff mixture that does not respond to consolidation can be directly linked to increased surface void formation.
• Self-Consolidating Concrete (SCC) is becoming increasing popular with the Stonetree^® licensed manufacturers to improve surface quality. SCC will typically cause any scratches on the form, concrete paste buildup, etc., to become extremely visible. SCC should not be used with barrier type release agents since the appearance of the formed Stonetree^® finish is important. If the barrier agents are applied thinly, the concrete does not release well from the form, and the surface of the concrete “peels.” When applied heavily, the barrier agent traps large numbers of air pockets. SCC should have test specimens cast to ensure the SCC works well with the release agent.

Additional Information:

Consult “Guide for Surface Finish of Formed Concrete,” a publication prepared by the ASCC Education and Training Committee.

See ACI 309 or PCA’s Design and Control of Concrete Mixtures for a full description of consolidation using vibration.

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