Sep. 23, 2024
In construction and engineering projects, reliable and robust fastening solutions are crucial for ensuring structural integrity and safety. Among the various options available, anchor bolts stand out as versatile, high-strength fasteners that provide exceptional stability and support.
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The performance and understanding of anchor bolts and bolted joints are often underestimated. In reality, anchor bolts act as springs, utilising elasticity to provide stability and hold machine frames firmly to foundations.
In this comprehensive guide, we will explore the function, sizing, tension, materials, and installation of anchor bolts. By understanding these key aspects, you can ensure proper selection, installation, and long-term performance of anchor bolts, maximising their effectiveness in securing structures.
Understanding Anchor Bolts &#; Their Function
Anchor bolts play a critical role in the construction industry by securely fastening machine frames to foundations. Their primary objective is to establish a strong and stable connection, ensuring the machine remains firmly in place. By placing anchor bolts under sufficient tension, they can withstand the combined horizontal forces that act upon the machine. These bolts are responsible for maintaining the necessary clamping force that holds the machine against these forces.
Anchor bolts play a vital role in effectively transferring loads from the machine frame to the foundation. This transfer of loads ensures the structural integrity and longevity of the machine throughout its operational lifespan. By bearing the weight and distributing the forces evenly, anchor bolts prevent excessive movement or displacement of the machine, minimising the risk of damage or instability.
With their crucial function in providing stability and load transfer, anchor bolts are a fundamental component in the construction of structures. Properly installed and tensioned anchor bolts are essential for ensuring the safety, performance, and longevity of machines and structures in various industries.
Anchor Bolt Sizing
Proper sizing of anchor bolts is crucial to ensure the structural integrity and stability of the machine or structure being fastened. Several factors come into play when determining the number, diameter, and length of anchor bolts.
One significant consideration is the shaking forces that the anchor bolts will need to resist. These forces can vary based on the specific application and environmental conditions. Additionally, the strength of the materials involved, such as the anchor bolts themselves and the foundation, must be taken into account. Anchor bolts should be selected to withstand the anticipated loads and forces they will experience during operation.
Concrete cracking is another factor to consider when sizing anchor bolts. Excessive or improper loading of anchor bolts can cause concrete to crack, compromising the stability of the structure. By properly sizing the anchor bolts, the risk of concrete cracking can be minimised, ensuring long-term stability and durability.
Mounting chock and grout factors should also be considered during anchor bolt sizing. These factors can affect the distribution of loads and the overall performance of the anchor bolts. Proper sizing ensures that the anchor bolts can adequately transmit the loads from the machine or structure into the foundation, preventing premature failure or displacement.
Undersized anchor bolt installations should be avoided at all costs. Insufficiently sized anchor bolts can lead to structural instability, compromising the safety and performance of the machine or structure. It is crucial to carefully analyse all the factors involved in anchor bolt sizing to ensure a proper and robust connection that can withstand the intended loads and forces.
By taking into consideration shaking forces, material strength, concrete cracking, and mounting chock and grout factors, engineers and construction professionals can determine the appropriate number, diameter, and length of anchor bolts, guaranteeing a secure and reliable fastening solution that meets the specific requirements of the application.
Anchor Bolt Tension
Anchor bolts derive their holding force through elastic stretching, comparable to springs. It is important to understand that the torque applied to the nut is not the primary factor in achieving the desired tension. Rather, the torque serves as a mechanism to stretch the bolt, resulting in the desired elongation and subsequent tensioning of the anchor bolt.
The stretch of the bolt itself is what determines its effectiveness in maintaining a secure connection. When the bolt is stretched, it develops a tension force that counteracts the external forces acting on the structure. This tension force is crucial for anchoring the machine or structure firmly in place and ensuring its stability.
It is worth noting that the torque applied to the nut should be carefully controlled during the installation process. However, it is not the sole indicator of proper tensioning. Instead, it serves as a means to achieve the desired level of stretch in the bolt. Monitoring the elongation of the bolt is essential to ensure that the anchor bolt reaches the required tension and can effectively resist the forces it will encounter.
Understanding that anchor bolt tension is primarily determined by the elongation of the bolt allows for precise and effective tensioning, ultimately ensuring the anchor bolts can provide the necessary holding force to secure the machine or structure against external loads and maintain its stability over time.
Anchor Bolt Materials
When it comes to anchor bolt materials, certain considerations are vital for ensuring a strong and reliable connection. For new installations and repairs, it is recommended to use high-strength bolts that meet ASTM spec A-193 standards. These bolts should have a tensile yield of 105,000 psi, ensuring they can withstand significant loads and forces.
Similarly, nuts used in anchor bolt assemblies should meet ASTM spec A-194 standards. Choosing nuts that meet these specifications ensures their compatibility and performance with the selected bolts. Moreover, the use of Super Nuts can offer additional advantages such as enhanced durability and resistance to loosening.
To address any lack of perpendicularity between the bolt and frame, the inclusion of hardened steel spherical washers is recommended. These washers help compensate for any misalignment and provide a more secure and stable connection between the bolt and the frame.
In terms of thread construction, rolled bolt threads are generally preferred over cut threads. Rolled threads provide a smoother and stronger thread profile, resulting in improved load-bearing capacity and reduced susceptibility to thread stripping or failure.
By selecting appropriate materials and components for anchor bolts the structural integrity of the machine or structure can be maintained, providing confidence in its performance and resistance to external forces.
Anchor Bolt Installation
Proper installation of anchor bolts is crucial for their effectiveness and the overall stability of the machine or structure. There are several key considerations to keep in mind during the installation process.
Using longer bolts during installation offers various benefits. Firstly, it allows for maximum stretch of the bolt, which is essential for achieving the desired tension. Additionally, longer bolts help move terminations and tensile stresses away from the surface of the block. This has the advantage of enlarging the cone of compression, effectively putting a larger portion of the block into compression. Moreover, this technique helps mitigate the potential effects of chock and grout creep as well as thermal expansion.
To ensure the integrity of the anchor bolt installation, it is recommended to wrap or sleeve the bolts along their entire length. This practice helps prevent shear cracks from occurring between the bolt and the concrete, ensuring a secure and durable connection. It also provides the longest possible free stretch length for the anchor bolt, allowing it to effectively bear the intended loads and forces.
In addition, designing an open relief area around the top of the bolt is beneficial. This design feature enables small bending adjustments to accommodate the frame. By allowing for these adjustments, the anchor bolt can better conform to the specific requirements and geometry of the frame, enhancing the overall stability and performance of the connection.
Stress in Threaded Nuts
In threaded nuts, the highest concentration of stress occurs within the first three threads. This means that the threads closest to the nut&#;s face bear the majority of the load. It is crucial to recognize this stress distribution when designing and assembling anchor bolt connections.
While some may believe that using double nuts on anchor bolts can increase the joint&#;s strength, this is not the case. Doubling the nuts does not significantly enhance the strength of the connection. Therefore, it is advisable to avoid using double nuts and instead focus on proper tensioning and installation techniques to ensure the integrity and stability of the joint.
Preferred Anchor Bolt Terminations
To achieve optimal performance and load transfer, careful consideration should be given to anchor bolt terminations. Several preferred terminations can effectively support and transmit loads from the machine or structure to the anchor bolts. Options such as welded plates, embedded plates, or anchor rods offer reliable and robust connections.
Welded plates involve welding a plate directly to the machine frame, providing a solid and secure termination point.
Embedded plates are embedded within the concrete foundation, allowing for a direct load transfer between the anchor bolts and the foundation.
Anchor rods, on the other hand, offer a versatile option by extending through the structure and providing a reliable connection point.
By selecting and implementing these preferred anchor bolt terminations, engineers can ensure the durability, stability, and performance of the anchor bolt connections.
Conclusion
Anchor bolts are essential components in maintaining the stability and safety of structures. By gaining knowledge about their function, sizing, tensioning, materials, and installation techniques, informed decisions can be made to optimise their effectiveness.
When it comes to anchor bolt replacement and installation, Alphatec Engineering stands out as experts in the field. Their expertise and experience ensure that anchor bolts are carefully selected, installed, and maintained, providing long-lasting and reliable support. By prioritising the proper selection and installation of anchor bolts, the safety and structural integrity of machines and structures can be significantly enhanced. Trust Alphatec Engineering for all your anchor bolt needs.
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Hilti Anchors vs. Anchor Bolts
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MVPs
(Structural)
(OP)
6 Aug 09 16:34I know that Hilti makes a wide selection of different anchor systems. It seems that the traditional anchor bolts have been the design choice for all types of construction.
When it comes to residential, would you consider using Hilti anchors for anchoring the column baseplates to the supporting footing pads or are the traditional anchor bolts better for that? How about anchoring the sill plate to the foundation wall?
Replies continue below
(Structural)
6 Aug 09 17:10Both can be acceptable if designed properly. I will generally allow for drilled and epoxied anchors for columns and sill plates. The size and embed depth will vary depending on the loads and conditions.
(Civil/Environmental)
6 Aug 09 17:16As H57 stated both are acceptable if designed properly. As for drilled and epoxy, I spech out the Hilti epoxy but just give the grade of anchor so customer has option to compare pricing between different anchor suppliers.
2
archeng59(Structural)
6 Aug 09 18:11I wish Hilti and other manufacturers had never invented the epoxy and adhesive anchors.
(Structural)
6 Aug 09 18:19Check the manufacgturer's fine print closely, as some afterset anchors are not suitable for vibratory loads, including seismic loads.
Mike McCann
MMC Engineering
(Structural)
6 Aug 09 18:26archeng59,While I do agree these anchors are heavily over used onsite, they are very good option for cyclic/fatigue load conditions like wind, if you have to use a post-installed option. I like cast-in anchors the most.I do not like to use mechanical anchors in wind applications due to the displacement in the concrete required to get ultimate strength conditions, and since you often have alternating loading conditions (ie tension to compression) mechanical anchors if loading past slip condition will fail very quickly.
When in doubt, just take the next small step.
(Structural)
6 Aug 09 20:50With good workmanship cast-in anchors are preferred however we do get calls from builders who don't get them in the right position and then we have to go with the post-installed anchor option.
(Structural)
6 Aug 09 22:39archeng59,Please enlarge on your earlier comment. Why do you wish there were no epoxy or adhesive anchors?
BA
(Structural)
(OP)
7 Aug 09 08:23Ok, I see now. What are some of the reasons for choosing the use a grouted base plate over a plate resting directly on a footing pad?
(Structural)
7 Aug 09 08:58The grouted baseplate allows the G.C. to get it level to accept the column. Rarely is the top of a footing level enough to accept a column properly.
(Structural)
7 Aug 09 09:53Grout also allows for minor adjustment of elevation.
BA
(Mechanical)
7 Aug 09 10:47Biggest issues I see in residential anchor bolts are short bolts, and missing anchors in interior load bearing walls and high truss/girder uplift connection. Wish they were properly placed, but some times there missing.
(Structural)
7 Aug 09 12:46In the region I'm practising, post-installed anchors are considered as somewhat expensive system than cast-in-situ anchors. But, in my openion, there are number of advantages you can gain when using post-installed anchors.
Advantages
* No need to do tedious setting outs. Just do the casting and then fasten. Cast-in-situ anchor positions might change during concrete operations
* Rapid construction. E.g. No need to keep starter bars from columns for lintels etc.
Disadvantages
* Close edge distance, distance between anchors etc. will reduce load carrying capacities
* Mechanical anchors exert pressure on base material when tightening
* Force-displacement curve is better in cast-in-situ anchors
There will be a bigger list.....
KC
(Structural)
7 Aug 09 12:51BAretired, because contractors will purposely omit installing anchor bolts because they want to use the post-installed anchors instead. Those products certainly can be used, but the workmanship issues are horrendous and the anchors are rarely installed properly. Also, contractors regularly want to omit dowels for CMU walls and use the epoxy/adhesive to install the dowels instead of using cast-in-place dowels.
(Structural)
7 Aug 09 14:00archeng59, I don't recall running into that problem in fifty years of practice. If anchor bolts are shown on the drawing, the contractor has always installed them. But when a bolt is required in existing concrete or the bolts are installed incorrectly, the adhesive anchors are very handy.
BA
(Structural)
7 Aug 09 14:01Considering the quality of residential construction as compared to commercial construction, the lack of training of the individuals doing the work, and the tendency of residential contractors to not understand structural necessity, I would stay away from expansion or epoxy anchors for such applications.
Make them put in the anchor bolts properly. Get really stubborn once or twice and have them tear out a footing and re-place the concrete if the bolts don't align...they'll learn how to do it right eventually.
Use leveling nuts and good grout for the baseplates. Keep in mind...residential contractors don't know diddly about steel erection...be prepared to hold their hands a lot.
(Structural)
7 Aug 09 14:44I understand where Archeng59 is coming from. In the industrial world we deal with a similar issue related to epoxy anchors. But, that's usually related to conflicts between the design schedule and the construction schedule.
We may not have the final equipment drawings yet. We may not even know which vendor is going to provide the equipment. Yet we are asked to provide final foundation drawings under the assumption that they can go ahead and pour the foundations as long as they can go back and use the post-installed anchors to correct for any issue.
It's a bit more stressful to have to rely on that sort of thing. (hence the sympathy with ArchEng59). But, it also allows us to absolutely impress clients with our ability to meet a demanding schedule.
Josh
(Structural)
7 Aug 09 16:49BA, all I can do is speak about my own experiences. Ron, I have required that the contractor remove and replace misplaced anchors or to install the cast-in-place anchors shown on the drawings. The contractor goes crying to the owner that I'm delaying the project with unreasonable requirements because Hilti's products can be used. When the owner calls and demands that I approve the epoxy/adhesive anchors so it won't delay his date of occupancy, what can I do except explain why I don't approve them? Not much. I then require that a special inspector observe the installation and provide a report. For some reason, this scenario has occurred several times on more than one project during the last couple of years. And not just on my projects, as I'm finding out. Aggravating.
(Structural)
7 Aug 09 16:55archeng59...I agree with you! Most of the time you don't have the luxury of being an a$$, but it sure is nice when you can get that point across. Yep...it's aggravating. Contractor always sounds better to the owner than we do.
(Structural)
8 Aug 09 07:06Another vote cast with archeng59. If not for post-installed bolts, contractors would have to actually take some care in setting out their cast in bolts. There are a lot of reasons post-installed bolts are not as good as cast in. Drilled in bolts depend heavily on installation workmanship and inspection, while cast in bolts just have to be in the right place. Most epoxy bolts are subject to creep (Big Dig tunnel ceiling).
(Structural)
8 Aug 09 12:08I agree with all of you that say that contractors are using post installed anchors as a crutch to avoid planning ahead. And they're not as good as cast in place.
But I've had projects where if it wasn't for drilled and epoxied reinforcing dowels, I don't know what I would of done. Say I'm adding a concrete structure to another structure and an expansion joint is not feasible. And contractors, not being completely stupid (lazy is another story), realize if I'm anchoring a concrete wall using epoxy, what's the big deal about a couple of ledgers?
So we have to take the good with the bad.
(Electrical)
8 Aug 09 17:04Gentelmen,
I am coming from the Electrical side of building construction. We do Healthcare, Civic and Educational projects. Our anchorage and seismic bracing details are reviewed by the State and Local Agencies and we utilize a lot of Hilti equipment. They have many pre-approved systems that are used for many types of equipment anchorage. It is almost a given that the Hilti "Kwik-bolt III" will appear on all of our work. These are an afterset anchor but it does not use epoxy. I am sure you are familiar with this or something like it. I was curious as to the discussion regarding post installed bolts did not include these? Are these too expensive for residential construction? Or is there another reason?
My switchgear, generator, transformer, conduit trapeze, and all manner of equipment anchorage specifications and details have called for these for many years. I've got thousands of these things installed. The Office of Statewide Health Planning and Development and The Division of the State Architect's Structural Engineers as well as our in-house Structural Engineers favor these. Perhaps not being structural myself, I've misunderstood the message in this thread. I was just curious and thought I would ask an oponion of the experts.
(Structural)
8 Aug 09 17:52All of you have raised valid points regarding the workmanship of post-installed anchors.
Do you have any performance specs indicating "x" percentage of all post-installed anchors must me qualified. I note that ACI has a publication for qualification of post-installed anchors.
How many of you have or have thought of including pull tests to be done on a certain percentage of post-installed anchors? Maybe that will help contractors to bring their "A" game during installing the anchors. If the thing pulls out well before capacity is reached, its on their pocket to get it right.
(Electrical)
8 Aug 09 18:10slickdeals,
We have seen spec's for 10% of post-installed anchors being tested. We had one particular out-of-this-world OSHPD inspector have 100% of these anchors tested. Contractor was not amused. Especially as the testing was specified to be "as required by the AHJ".
Regards,
EEJaime
(Structural)
8 Aug 09 18:11I require a certification of installation to the hilti/ramset guidelines (This has the added advantage ensuring the anchor is installed by a tradesmen not a labour), Plus proof loading for minimum 5% selected by myself (I would say at random, But I do like to select ones that are in heavy tension). However, if you only have say 3-10 chemical anchors onsite, enforcing this spec is impossible.I also have a question regarding cast-in anchors, what is the standard tolerance provided in connection? I generally aim for +/-40mm, what does everyone else aim for?EEJaime,What you are referring to is a mechanical post installed anchor. These are good for situation where loading is required almost immediately after installation, However as noted by msquared48 these anchors can have problems with cyclic loadings, and to minimise the effect of this they generally have a torque that must be applied to ensure no movement during loading. They are also more expensive generally, and have a lag time on ordering. I generally find these have less problems then chemical anchors due to the fact that a tradesmen not a labour has to install, due to the use of a torque wrench.
When in doubt, just take the next small step.
(Electrical)
8 Aug 09 18:18Thank you rowingengineer,
We use these on heavy vibrating equipment such as generators and fire pumps, specifically for the seismic bracing of the equipment with the vibration isolation mounts and seismic restraints. msquared48's post specifically states that these are the types of loads he would not recommend these anchors for. Does this concern apply to both epoxy and mechanical type after-set anchors?
Regards,
EEJaime
(Structural)
8 Aug 09 18:51Hilti Kwik-bolt III is the incorrect bolt for taking seismic forces; from the ICC report "The Hilit Kwik Bolt 3 Concrete Anchor (KB3) is used to resist static and wind, tension and shear loads in uncracked normal-weight concrete". There are certain anchors that can take seismic forces, Hilti KB-TZ ICC ESR-, is one anchor; I remember there is an issue about vibration loading. Epoxy anchors can also be used to resist seismic forces but again only certain types, Hilti HIT-RE 500-SD Adhesive Anchors can be used. Help the SE out change the detail to a bolt that can take seismic forces in cracked concrete, these details always show up the day before the project gets resubmitted.
(Structural)
8 Aug 09 18:58Kwik bolt:consider:
When in doubt, just take the next small step.
(Structural)
8 Aug 09 19:08going to fast make a mistake, heer is your boltKwik III
When in doubt, just take the next small step.
(Electrical)
8 Aug 09 19:38Gentlemen,
First, my apologies to the OP for hijacking this thread. It was an excellent question that brought another important question to mind. rowinengineer and sandman21, you have been extremely helpful and I appreciate it. I am going to revise my specifications and details to incorporate the HSL anchor, it is clearly a superior selection for my applications. I am sure our SE's will approve.
My co-workers often limit their participation in these fora to their own discipline, but it is these forays into semi-related or peripherally related fields that often, as today, prove most useful.
Again thank you and regards,
EEJaime
(Structural)
8 Aug 09 20:22I nearly missed Hokie66 reference to the Big Dig Boston ceiling collapse after taking a look at some articles on the web and a previous thread thread507-: Big Dig Boston ceiling collapse I think the Big Dig Bosten has made me rethink my saftey factor for design, and the amount of tests required.
When in doubt, just take the next small step.
(Civil/Environmental)
9 Aug 09 20:22I have used mechanical and epoxy bolts for many years. I prefer hilti becuse they have the best engineering data availble and have very good quality and consistent product. They will also answer any technical questions you have for your application. Although hilti quick bolts are not reccomended for siesmic, in many areas and applications that is not an issue.
Hilti anchors are not cheap, and generally it is cheaper to set traditional bolts than use hilti bolts.
Finally the perception that epoxy is an inferior product due to the big dig is not true. The epoxy used was a temporary epoxy not suited to long term use due to a tendency to creep. Further the material was used in an overhead application which is not the best use for epoxy. (a good application for cast in place bolts) Finally the panel loads were supported by five bolts and the loads were distributed by diagonals that were tightened or lossened by turnbukles to adjust the panels, so the load in the bolts was indeterminate. In the end the epoxy manufacterer who supplied the epoxy to a reseller and sold it under a different label took the hit. However I believe this had more to do with a ability to pay rather than truth or justice.
(Structural)
10 Aug 09 16:53Had a Hilti Rep. in today asked about which anchor he would choose for a typical mech. unit slab on grade for seismic loads, the TZ is the preferred, then HSL.
(Civil/Environmental)
11 Aug 09 23:19SlickdealsIn regards to pull tests, our general notes have a section on post installed anchors that specifies pull tests consistent with OSHPD (California Hospitals) requirements. OSHPD has a Code Apllication Notice (CAN) for field testing post installed anchors ( http://www.oshpd.state.ca.us/FDD/Regulations/CANs//2-A.8.pdf
(Structural)
12 Aug 09 13:36I work for a homebuilder and since we ship all over the world, we have historically specified epoxy anchors for sill plate and holddown anchors. However, we're finding more and more jurisdictions are demanding special inspections on all epoxy use and this has become a bit intense for a straightforward house. We're planning to switch to a mechanical screw anchor for standard sill plate anchorage but will likely stick with epoxy anchors for the special hardware as required.
(and as a side note - as a woman, am I allowed to read EEJaime's posts? lol)
(Electrical)
12 Aug 09 14:59slta,
My apologies. I am used to my home forum, (electrical), where as far I can recall, at least in the past two years, we have not had a female member that is a regular participant. I should know that this is different in other disciplines. Even in the last 30 years of working in this business, I can probably still count on one hand the number of women that I've met whom are electrical engineers. Many Architects, Lighting designers, product representatives, engineers of varied disciplines, Mechanical, but not electrical.
I guess I just fell into an industry that is not to the liking of many women. Thank you for lighthearted response. Apparently you are familiar with us that write without always thinking things all the way through.
Have a pleasant day,
EEJaime
(Structural)
12 Aug 09 17:03slta,when you say mechanical "screw" anchor do you mean like the "hilti Hus" anchor or equivalent? I have to be honest in all my years of practice i have never spec'd this type of anchor, I have always been put off by because they have to drill a hole and clean, but if the hole is oversized at all, the anchor capacity is reduced immensely.
When in doubt, just take the next small step.
(Structural)
12 Aug 09 17:24I have specified and used the HUS-H and Simpson Titen anchors many times successfully over the past couple of years. Not for column anchor bolts problems, but other requirements where an anchor was needed in an existing concrete structure.
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