Anchor Bolt Design Philosophy
In
this page, I will tell you about the design philosophy of anchor bolt
design and also to define the different terms used for anchor bolt
design.
Anchor bolt is the most important part to transfer the load from superstructure to sub-structure (say Foundation system). So, though anchor bolt design is very simple, but still you have to give more care to design this small element as load transfer mechanism. Otherwise, super structure and sub-structure, designed separately, will not behave as a single structure, in other words, a failure of total structural system.
Whenever you start the design of anchor bolt, you will see the words like "Cast in place anchor", "Post installed expansion anchor", "Preloaded anchor", "Ductile design", "Anchor Bolt Projection" "Edge distance for shear", and "edge distance for tension". Please look below for the definition of some of the different terms:
Cast in place anchor:
When a headed bolt, headed stud, or hooked bolt is installed before placing concrete, is called "Cast in place anchor". See project standard for dimensions and areas. The recommended minimum embedment depth for cast-in anchors varies from eight (8) to twelve (12) bolt diameters (see project design code). The design engineer should note that minimum embedment requirements have an impact on concrete thickness, especially in designs such as area slabs and building slabs.
Cast-in anchors have many benefits, including the following:
Post installed expansion anchor:
When an anchor rod is installed after placing concrete, making a hole in concrete with drill-bit, is called "Post installed expansion anchor". This type of anchors rely on bond to transfer load to the concrete. Post-installed anchors have many benefits, including the following:
Preloaded anchor:
An anchor is subjected to a large tensile force by intentionally elongating the entire length of the bolt. A nut is generally advanced along the bolt threads until the desired tensile force is achieved. In general, there are no standard criteria for preloading anchor bolts. The level of the required preload generally depends on the specific application of the anchor bolt. In general, preloaded bolts will require a sleeve or bond breaker to permit elongation to occur along the entire length of the bolt. Without a sleeve or bond breaker, the concrete would bond to the shank of the bolt during construction. The bond may not be completely broken during the preloading process. Over time, the bond may become completely broken and may cause a significantly reduced preload in the bolt.
Anchor bolts used for equipment supports should be preloaded to the equipment manufacturer's recommendations when specified. This is especially true for bolts anchoring rotating or vibrating equipment.
Anchor bolts of ASTM F1554 Grade 36, A307, or A36 material (i.e., regular carbon steel bolts) should have only a nominal preload applied. It is recommended that they be tightened to a snug tight condition. Snug tight is defined as tightness attained by a few impacts of an impact wrench or the full effort of a man using an ordinary spud wrench. When bolts are anchoring equipment or are subject to possible loosening during operation, a locking device should be provided. Acceptable locking devices include double nuts or jam nuts, interrupted threads, and tack welds (for weldable materials only).
The three basic methods used for applying a preload to a high-strength anchor bolt—using hydraulic tensioners, torquing to a specified level, and using turn-of-the-nut method—are described in detail below.
Elongation Checks For Preloaded Bolts: Where precise preloads are required, the elongation of the anchor bolts may be checked as a verification that the proper preload has been applied. Elongation checks are usually performed only when tensioners are used as the preloading device. Dial gauges can be used to measure the projection of the bolt from a reference surface before and after preloading. The required elongation for a given preload can be calculated as follows:
For plain rod: de = (P*L) / (Ab*E)
For full threaded rod: de = (P*L) / (At*E)
For partial thread rod: de = (P/E)*[(Lt/At) + (Ls/Ab)]
where
de = Elongation of anchor bolt, mm.
P = Desired preload, kN
L = Effective length of bolt, mm. (usually taken from centers of nut to anchor nut)
Ab = Nominal cross sectional area of bolt (area of shank), mm2
E = Young's Modulus of Elasticity, MPa
Lt = Length of thread below nut, mm
Ls = Length of shank, mm.
At = Area of threaded section, mm2
The preload is generally considered acceptable if the actual elongation is within + 5 percent of the calculated value for the given preload.
Sleeves:
Two basic types of sleeves are partial depth and full depth anchor bolt sleeves. Partial depth sleeves typically have a corrugated profile and are made from high density polyethylene (say, Plastic Wilson or equal). Full depth sleeves are typically made from a steel pipe section with a steel bearing plate seal welded to the embedded end. Sleeve diameters are generally two to four times the diameter of the anchor bolt. Sleeves serve two purposes:
Anchor Bolt Projection
The length of an anchor bolt that projects from the concrete surface where the length is measured from the concrete surface to the free end of the anchor bolt. Any thickness of grout placed on the concrete surface must be included in the projection length.
Baseplate Leveling Systems
Some of the most common methods used for leveling base plates, are, the use of shim stacks and leveling nuts. It must be emphasized that the shims and leveling nuts should be removed before preloading the bolts. Leveling nuts may be used only on anchor bolts where preloading is not required. Use of leveling nuts on anchor bolts that are preloaded would result in bolt tension only in the region between the leveling nut and the top nut.
Ductile Design
The ability of an element to deform beyond the point of elastic yield prior to total failure is called ductility. Ductile design referring to an anchor with design strength equal to the design strength of the steel element. All potential concrete failure modes must have design strengths greater than the steel element (supplemental reinforcing may be used to increase the design strength of concrete failure modes).
Design discussions for Cast in place anchor:
As with all designs, anchor bolt designs must meet the project design criteria and project commitments to codes and standards. Here, this discussions follow the provisions of ACI 318-05, Appendix D, for the design of cast-in anchors.
Much consideration is given to ductility in the design of anchor bolts. In general, steel is a ductile material and plain concrete is not. For anchorage to concrete, ductility usually means that in the event of overload, the ductile steel anchor will yield before the concrete can fail in a brittle manner. In this discussion, ductile designs are also referred to as developed anchors. Ductile designs are therefore preferable for most applications.
During anchor bolt design, you may find that proper ductile design is not possible for some reasons and following are some cases:
Shear Load: There are several alternatives for transferring shear from an attachment to the concrete.
Now to complete the design of cast in place anchors, you can create your own calculation sheet based on ACI 318-05, Appendix D as explained step by step.
I hope this page will be very helpful to you.
Copyright 2009. All rights reserved. Please do not print or copy of this page or any part of this page without written permission from Subhro Roy.
Disclaimer: This page is prepared based on experience on Civil Engineering Design. All definitions and most of the explanations are taken from different text books and international design codes, which are referenced in the contents. Any similarity of the content or part of with any company document is simply a coincidence. Subhro Roy is not responsible for that.
Anchor bolt is the most important part to transfer the load from superstructure to sub-structure (say Foundation system). So, though anchor bolt design is very simple, but still you have to give more care to design this small element as load transfer mechanism. Otherwise, super structure and sub-structure, designed separately, will not behave as a single structure, in other words, a failure of total structural system.
Whenever you start the design of anchor bolt, you will see the words like "Cast in place anchor", "Post installed expansion anchor", "Preloaded anchor", "Ductile design", "Anchor Bolt Projection" "Edge distance for shear", and "edge distance for tension". Please look below for the definition of some of the different terms:
Cast in place anchor:
When a headed bolt, headed stud, or hooked bolt is installed before placing concrete, is called "Cast in place anchor". See project standard for dimensions and areas. The recommended minimum embedment depth for cast-in anchors varies from eight (8) to twelve (12) bolt diameters (see project design code). The design engineer should note that minimum embedment requirements have an impact on concrete thickness, especially in designs such as area slabs and building slabs.
Cast-in anchors have many benefits, including the following:
- They are capable of supporting very large loads.
- Drilling or cutting of reinforcement is not required for installation.
- Strength is not sensitive to installation procedures and techniques.
- Supplementary reinforcement may be easily included in the design.
- Labor and materials are required to create templates for placement in the form-work.
- Re-work is costly when anchors are incorrectly placed (location and elevation).
- Fabrication lead time is required to support concrete placement.
Post installed expansion anchor:
When an anchor rod is installed after placing concrete, making a hole in concrete with drill-bit, is called "Post installed expansion anchor". This type of anchors rely on bond to transfer load to the concrete. Post-installed anchors have many benefits, including the following:
- Savings in labor and materials on templates required for cast-in anchors
- Placement locations that may be adjusted as required to accommodate attachments
- Flexibility in the construction schedule
- Performance that is highly dependent on installation procedures and techniques
- Potential for cut or damaged reinforcing bars if drilling is required for installation
- Limited load capacities
- Difficulty including supplementary reinforcing in the design
- Prohibited preloading
Preloaded anchor:
An anchor is subjected to a large tensile force by intentionally elongating the entire length of the bolt. A nut is generally advanced along the bolt threads until the desired tensile force is achieved. In general, there are no standard criteria for preloading anchor bolts. The level of the required preload generally depends on the specific application of the anchor bolt. In general, preloaded bolts will require a sleeve or bond breaker to permit elongation to occur along the entire length of the bolt. Without a sleeve or bond breaker, the concrete would bond to the shank of the bolt during construction. The bond may not be completely broken during the preloading process. Over time, the bond may become completely broken and may cause a significantly reduced preload in the bolt.
Anchor bolts used for equipment supports should be preloaded to the equipment manufacturer's recommendations when specified. This is especially true for bolts anchoring rotating or vibrating equipment.
Anchor bolts of ASTM F1554 Grade 36, A307, or A36 material (i.e., regular carbon steel bolts) should have only a nominal preload applied. It is recommended that they be tightened to a snug tight condition. Snug tight is defined as tightness attained by a few impacts of an impact wrench or the full effort of a man using an ordinary spud wrench. When bolts are anchoring equipment or are subject to possible loosening during operation, a locking device should be provided. Acceptable locking devices include double nuts or jam nuts, interrupted threads, and tack welds (for weldable materials only).
The three basic methods used for applying a preload to a high-strength anchor bolt—using hydraulic tensioners, torquing to a specified level, and using turn-of-the-nut method—are described in detail below.
- Hydraulic Tensioners: Hydraulic tensioners should be used when a precise preload on large diameter anchor bolts is required. The tensioner applies a direct load to the bolt by threading onto the projected end of the bolt and then jacking against the adjacent concrete surface. Once the jacking is complete, the nut is hand tightened down to a snug position to lock the tension in the bolt. The anchor bolt must project a minimum of one diameter past the end of the nut to allow for use of the tensioner. The residual preload should be specified, as there will be a loss of pretensioning (depending on the length and diameter of the bolt) when the tensioner is released.
- Torquing to a Specified Level: Applying a specified torque to an anchor bolt is another method of obtaining a preload. This method results in a preload that varies significantly as a function of field conditions (cleanliness of bolt, existence of lubrication, etc.) and is no longer recognized by AISC. Therefore, use of a predetermined torque for preloading purposes is not recommended (with the exception of equipment manufacturer requirements).
- Turn-of-the-Nut Method: This method is described in the AISC Specification for Structural Joints for ASTM A325 and A490 bolts. The specification lists the required nut rotation from the snug tight condition for bolt lengths up to 12 diameters.
Elongation Checks For Preloaded Bolts: Where precise preloads are required, the elongation of the anchor bolts may be checked as a verification that the proper preload has been applied. Elongation checks are usually performed only when tensioners are used as the preloading device. Dial gauges can be used to measure the projection of the bolt from a reference surface before and after preloading. The required elongation for a given preload can be calculated as follows:
For plain rod: de = (P*L) / (Ab*E)
For full threaded rod: de = (P*L) / (At*E)
For partial thread rod: de = (P/E)*[(Lt/At) + (Ls/Ab)]
where
de = Elongation of anchor bolt, mm.
P = Desired preload, kN
L = Effective length of bolt, mm. (usually taken from centers of nut to anchor nut)
Ab = Nominal cross sectional area of bolt (area of shank), mm2
E = Young's Modulus of Elasticity, MPa
Lt = Length of thread below nut, mm
Ls = Length of shank, mm.
At = Area of threaded section, mm2
The preload is generally considered acceptable if the actual elongation is within + 5 percent of the calculated value for the given preload.
Sleeves:
Two basic types of sleeves are partial depth and full depth anchor bolt sleeves. Partial depth sleeves typically have a corrugated profile and are made from high density polyethylene (say, Plastic Wilson or equal). Full depth sleeves are typically made from a steel pipe section with a steel bearing plate seal welded to the embedded end. Sleeve diameters are generally two to four times the diameter of the anchor bolt. Sleeves serve two purposes:
- First, partial depth and full depth sleeves afford the opportunity to move the top of the bolt slightly when trying to align the attachment. However, the presence of a sleeve does not imply that an anchor bolt may be freely bent or otherwise deformed in order to account for placement that was out of construction tolerance.
- Second, a full depth sleeve may be used in conjunction with preloaded bolts. The sleeve permits elongation along the entire length of the bolt, and the bearing plate transfers the tension force from the anchor bolt to the concrete. It must be emphasized that the bearing plate must be sized to ensure that the anchor does not pull through or cause the plate to deform excessively. Also, the nut on the bottom of the anchor plate must be held securely in place to prevent loosening during construction activities.
Anchor Bolt Projection
The length of an anchor bolt that projects from the concrete surface where the length is measured from the concrete surface to the free end of the anchor bolt. Any thickness of grout placed on the concrete surface must be included in the projection length.
Baseplate Leveling Systems
Some of the most common methods used for leveling base plates, are, the use of shim stacks and leveling nuts. It must be emphasized that the shims and leveling nuts should be removed before preloading the bolts. Leveling nuts may be used only on anchor bolts where preloading is not required. Use of leveling nuts on anchor bolts that are preloaded would result in bolt tension only in the region between the leveling nut and the top nut.
Ductile Design
The ability of an element to deform beyond the point of elastic yield prior to total failure is called ductility. Ductile design referring to an anchor with design strength equal to the design strength of the steel element. All potential concrete failure modes must have design strengths greater than the steel element (supplemental reinforcing may be used to increase the design strength of concrete failure modes).
Design discussions for Cast in place anchor:
As with all designs, anchor bolt designs must meet the project design criteria and project commitments to codes and standards. Here, this discussions follow the provisions of ACI 318-05, Appendix D, for the design of cast-in anchors.
Much consideration is given to ductility in the design of anchor bolts. In general, steel is a ductile material and plain concrete is not. For anchorage to concrete, ductility usually means that in the event of overload, the ductile steel anchor will yield before the concrete can fail in a brittle manner. In this discussion, ductile designs are also referred to as developed anchors. Ductile designs are therefore preferable for most applications.
During anchor bolt design, you may find that proper ductile design is not possible for some reasons and following are some cases:
- Piers/pedestals or other concrete elements where the edge distances and bolt spacings preclude development of the steel anchor strength -The addition of supplemental reinforcing can often provide restraint and confinement capable of producing a ductile design.
- Large diameter bolts specified by machine manufacturers - Machine anchor sizes are often much larger than the sizes that would be required for strength considerations only. In such cases, supplemental reinforcement should be provided in order to come as close as practicable to a ductile design. Note that manufacturers sometimes specify ductile anchors as part of their design criteria.
- Supports for architectural, mechanical, and electrical components - This case consists primarily of post-installed adhesive or grouted-in anchors for medium to light duty service.
- Supports for structures or equipment where anchors are not required to be designed for an applied load - Such cases would include posts subject to gravity load only, equipment skids subject to gravity load only, etc. In these cases, anchors should be provided with minimum recommended embedment depths given in Project design criteria.
Shear Load: There are several alternatives for transferring shear from an attachment to the concrete.
- Anchor Shear: An anchor may be loaded in shear, and, in turn, transfer the shear to the concrete. Welded studs are most commonly used to transfer shear in this manner. Welded studs have the advantage of being securely welded to the attachment. An anchor bolt inserted through a hole in an attachment requires special consideration to assure shear transfer from the attachment to the anchor. Usually, the bolt holes in the attachment will be oversized to accommodate anchor installation tolerances. Oversized holes make it unlikely to achieve bearing at all anchors in a given attachment. There are two methods to deal with oversized holes: First, the number of anchors considered to resist the shear may be limited to half of the total no of anchor used; second, washers without oversized holes may be provided and then welded to the attachment to transfer the shear to the anchor.
- Friction: For cases where a sustained compressive force exists between the attachment and the concrete surface, friction will be developed. Project criteria and codes must be reviewed to determine if it is permitted to rely on frictional resistance.The friction load should be based on the dead load and any portion of the live load that causes the shear. Care must be taken not to overestimate the dead load. If the applied shear load exceeds the shear that can be transferred by friction, additional means must be provided to transfer the applied shear that is in excess of the friction.
- Shear-Friction: Shear-friction may be developed as a result of the anchors effectively clamping the attachment to the concrete surface. In this case the anchor must be designed for the tension resulting from shear friction as well as any tension applied directly.
- Shear Lugs: Shear lug is the preferred methods of shear transfer when friction is not an option or is not sufficient to resist the applied shear. This method requires pre-formed pockets in the concrete. The shear lug is generally a steel element, welded to the attachment, that transfers the shear directly to the concrete through bearing. Shear lugs are most commonly single cantilever plates but may be wide flange shapes or box sections for very large loads. Use of cantilever shear lugs greater than the thickness of the attachment plate is not recommended, and shear lugs should be designed using a minimum of 50 mm. embedment into the concrete. Fillet welds are preferred over penetration welds by fabricators for attaching shear lugs to baseplates. For the case of extremely large shear loads, shear lugs made from wide flange sections or box sections may be necessary. The behavior of large shear lugs with deep embedments will be dependent on the stiffness of the shear lug and the stiffness of the attachment.
Now to complete the design of cast in place anchors, you can create your own calculation sheet based on ACI 318-05, Appendix D as explained step by step.
I hope this page will be very helpful to you.
Copyright 2009. All rights reserved. Please do not print or copy of this page or any part of this page without written permission from Subhro Roy.
Disclaimer: This page is prepared based on experience on Civil Engineering Design. All definitions and most of the explanations are taken from different text books and international design codes, which are referenced in the contents. Any similarity of the content or part of with any company document is simply a coincidence. Subhro Roy is not responsible for that.