Greetings to all,
Trying to clarify the difference (if any) and the reasoning behind the AWS D1.1 committees use of both terms.
This is from the Foreword of AWS D1.1
In 1988, AWS published its first edition of AASHTOIAWS D1.5, Bridge Welding Code; coincident with this, the D1.1 code changed references of buildings and bridges to statically loaded and dynamically loaded structures, respectively, in order to make the document applicable to a broader range of structural applications. This 2010 edition is the 22nd edition of D 1.1.
Table 6.1 lists Visual Acceptance criteria for Statically loaded and Cyclically loaded connections.
C5.23.6.1 Measurements. Allowable tolerances for
variations from flatness of dynamically loaded girder
webs are given in the code separately for interior and fascia
girders.
C5.23.6.2 Statically Loaded Nontubular Structures.
The flatness tolerances for webs with intermediate
stiffeners on both sides and subject to dynamic loading is
the same as that for interior bridge girders (see 5.23.6.3).
Would a dynamically loaded structure fall under the acceptance criteria of static or cyclic loading ?
Regards,
Shane
When I asked the question what's the difference between statically loaded and cyclically loaded I always got the answer Static is for buildings and cyclic is for bridges.
What never made sense was that D1.1 is for buildings but it still calls out both in Table 6.1.
Also I can't recall ever seeing any portions of the buildings weve done over the past 10 yrs ever saying a section was cyclic.
I've had sections with potential uplift, and other beams canterlevered out, but again the contract documents never said cyclic so I never had to use that portion of the code.
To answer your question, yes I would think dynamic is the same as cyclic, load on and off.
I agree, dynamic means changing AKA cycling.
disagree-cyclic stress implies frequency change, not dynamic!
Your definition is typically implememted through the use of tables that establish a relationship between the number of cycles and the specified limits.
Absent knowledge of the actual number of cycles or the frequency to use in a table like that, my opinion would be to take the "conservative" position, and treat it as cyclic.
But, at which point does it change from dynamic to cyclic? At what frequency? Jet engine welds are subject to high cycle fatigue, or high frequency stresses. (HCF). At my last employer, we invested $1.5 million dollars in computerized fatigue testing rigs that subjected our welded parts to triaxial stresses. All welds were tested to ultimate destruction, and the total number of fatigue cycles was then established and compared to the necessary service life requirement.
I am afraid this is becoming a philosophical debate.
A bridge under heavy traffic could be called cyclic, but under light or infrequent traffic, one might argue dynamic. I suppose "bridge codes" take this into account though. But, what about similar applications that don't have an applicable code? While I see your point, my inclination is to take the conservative position.
I would say it's one or the other, or a combination of both. It is an engineering call as to which type it is. My definition has nothing to do with tables, Dynamic loads and Cyclic loads are two different animals, but can co-exist. If an 18 wheeler is parked on a bridge for three days, is that dynamic load or cyclic? If an 18 wheeler crosses a bridge 10 time an hour for three days is that dynamic or cyclic? Cyclic load consideration SHOULD always take precedent, as this will cause failure before dynamic tolerance is reached. Conservative positions usually lose.
The way I understand it is:
Static - Joint is in compression from natural or gravitational weight loads (i.e - building column to base plate connections)
Dynamic - Joint is in compression from manufactured weight loads ( i.e - overhead crane column to base plate connections)
Cyclic - Joint loads alternate between compression and tension ( i.e. - building column to rafter, or moment connections where the load properties change every time the wind blows)
As such, each loading condition most often has separate strength calculation requirements and inspection criteria.
Tim
When we think of buildings in relation to this question we must ask ourselves: what is it that would make it one or the other? Does it make a difference when machinery moves across a floor? Does it make a difference as people come in and then leave? As with a bridge when traffic comes onto and then leaves?
But then, how about winds, earthquakes, and other forces that cause motion unrelated to normal loading/unloading? Do those change a building from strictly Static to at least occassionally Cyclic? Is this why some buildings have at least some sections that have Demand Critical members and D1.8 and AISC Seismic applications?
This is a topic that quite possibly could use the input of an engineer that could answer our questions from both a design as well as an inspection view point and do so in plain english for us lowly inspectors who don't speak legalese.
I have often thought this would be a good ariticle for the Welding Journal or even a topic for an AWS Seminar at FabTech.
Have a Great Day, Brent
I agree Brent, go for it!!
Sounds more like a mission for Al to me. With all those great articles he does in Inspection Trends he would be a natural candidate. Another good one I just finished reading in this month's edition: 'What You Should Know About Soundness Testing'.
I nominate Al Moore to the task.
Do I hear a 'second'?
Have a Great Day, Brent
I'll second that.
Will Forrest Gump make an appearance in this topic???
Dynamic loads are those that change in magnitude and in certain cases may include complete load reversals.
When the number of cycles exceeds a certain threshold, Fatigue must be considered when the changes in magnitude exceeds a certain number of cycles (cyclic loads). The structural welding codes have expanded the information on fatigue in recent years. It is interesting to compare D1.1 from the late 1970's to D1.1-2010.
The S-N curves included in D1.1 start at 10 000 cycles (changes in magnitude). The magnitude of change is dependent on the connection detail. Changes in cross section geometry has a significant influence on the magnitude of the change in stress, i.e., the threshold, required before fatigue is an issue and the life expectance before fatigue failure is an issue.
Typical wind loads, snow loads, etc. typically do reach the magnitude required to consider fatigue. The number of hurricanes or severe snow falls do not reach the number of cycles that requires fatigue consideration. The life span of most buildings do not reach the thresholds that necessitate fatigue calculations.
The same cannot be said for a bridge where the number of trucks passing over it justifies the need to consider fatigue, The same is true for a bridge crane lifting and transporting heavy loads. Many machines have a finite life expectancy because of cyclic loads. A good example of fatigue loading is a forging press. It exerts very high loads on a slug of metal which subject various components to high stresses repeatedly.
One press repair we designed had to withstand 6 million cycles at maximum load. A presses are self destruct machines. They have a life expectancy that can be extended with proper maintenance and if worn components are repaired or replaced as needed. The cost of rebuilding a press can be staggering. A large press in Michigan cost around 150 million dollars from what I heard.
Another example of fatigue is a tanker. The life expectancy of an oil tanker is only 15 years. Corrosion, wave action, etc. takes its toll.
Piping connected to a pump or compressor experiences fatigue. Special design considerations and low stress levels are used to improve the life expectancy of such systems.
Whether a structure is subjected to fatigue is not always obvious. AWS D1.1 requires the Engineer to specify the magnitude of the loads each connection is subjected to and whether or not they are cyclic or static loads.
Fun stuff.
Al
If I remember right, the auto frame plant tested frames & components for one million cycles.
The theory was that a poor design or manufacturing problem would usually give some indications or fail within the first million cycles. If the part was sufficient to last 1 million cycles there was a good chance it would last many million cycles and at some point You need to stop that test and move on to the next part, so one million was that point.
Al
I loath to link from Wikipedia but here goes:
'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.
Fatigue occurs when a material is subjected to repeated loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the stress concentrators such as the surface, persistent slip bands (PSBs), and grain interfaces.[1] Eventually a crack will reach a critical size, and the structure will suddenly fracture. The shape of the structure will significantly affect the fatigue life; square holes or sharp corners will lead to elevated local stresses where fatigue cracks can initiate. Round holes and smooth transitions or fillets are therefore important to increase the fatigue strength of the structure.'
You stated:
"Dynamic loads are those that change in magnitude and in certain cases may include complete load reversals."
I would argue Not Dynamic which are near constant, not changing or subject to frequency?
So, from this we can ascertain that fatigue is primarily generated by cyclic loading ?
You also stated:
"The life span of most buildings do not reach the thresholds that necessitate fatigue calculations" - Surely any type of building structure will be subject to cyclic loading forces from wind alone, much the same as any bridge? I know when I have been atop high buildings, I have felt sea sick due to wind deflection?
I agree that wind on a bridge can create a huge cyclic loading problem. When I was in engineering school, the welding structural design professor showed a very famous old black and white film of a long bridge in a wind storm. It began oscillating up and down, over and over with increasing amplitudes. Soon the asphalt cracked and the roadway broke up; shortly after that the whole bridge came apart and fell into river below it. The problem was caused by lift on the bridge as a result of the wind; the bridge was acting like the airfoil on an airplane wing. This problem is typically solved in modern bridge designs with the use of vent gratings in the middle of the bridge to "spill" the lift. I used to wonder why bridges that did not open for boats were vented and now I know.
Hey fellas, I don't claim to be an expert, so I'll defer to AWS D1.1-2010.
If you look at the S-N curves, figure 2.11, you'll notice they begin with 10 000 cycles. Each has several lines that have negative slope from left to right. At some point, each curve flattens to become a horizontal line. That horizontal line represents the Endurance Limit. If the magnitude of the change in stress is less than then that flat line, the item should never fail by fatigue. If the magnitude of the stress range is above the line, fatigue will occur after there are sufficient cycles of stress.
Table 2.5 depicts several pages of various connection details including a structural member with no holes or attachments, groove welds with or without backing, welds left as welded and welds ground flush. Each condition is assigned a letter "A" through "F" indicating the degree to which the connection is subject to fatigue. Along with being assigned a letter, each is assigned a Threshold value (Fth) which represents the minimum magnitude of the stress variation needed before fatigue is an issue.
Fatigue is essentially the accumulated damage sustained by a member due to loads that change over time. The magnitude is simply the fluctuation in the load; it does not necessarily require a load reversal, i.e., compression to tension to compression. Even members that are only loaded in compression can be subject to fatigue.
Small changes in load are not significant and most likely will not affect the usefulness of the component. That is, the part should not fail by fatigue. That is if the magnitude of stress change falls below the Endurance Limit (the horizontal line on the graphs in Figure 2.11 the member should not fail by fatigue.
Features such as changes in geometry, holes, notches, etc. are called stress risers. Stress risers can influence the longevity of the component by concentrating stresses at local features. It is at these features that fatigue failures usually initiate. The magnitude of the stress in the vicinity of the stress riser can be a multiple of the unit stress (load / area). While the unit stress may fall under the Endurance Limit thus indicating the component should last forever, the concentration of stress at the stress riser causes the magnitude of the stress at the feature to be much higher than the unit stress, thus the stress is actually above the Endurance Limit and fatigue failure is a distinct possibility if there are sufficient load cycles. This is the reason there are several lines on the S-N graph, each representing a connection detail that accounts for the severity of the stress riser the connection presents.
Consider the straight bar (Table 2.5) containing no holes, no changes in geometry, no welded attachments, etc. It is classified as Stress Category “A”. The Threshold “Fth” value assigned is rather high compared to the connection shown at the bottom of the page. The connection at the bottom of the page contains an access hole adjacent to the groove weld. The web containing the access hole is joined to the web of a member oriented transverse to the web containing the access hole. The sketch to the far right (B) depicts the probable location of the fatigue failure associated with the access hole. The member containing the access hole is assigned the Stress Category “C” which has a reduced Threshold “Fth” value that is only 42% of that assigned to Stress Category “A”. The reduction in Threshold value is indicative of the significance of the stress riser, i.e., the access hole. The engineer also has to consider the influence of the groove weld. Is the backing left in place or is it removed? Typically the design will require the backing to be removed, but if that is not the case, the Designer would have to use a Fth of 7 or 4.5 ksi depending on the location of th tack welds, inside the groove or outside the groove. The Stress Category could be either D or E depending on the location of the tack welds. So, each feature of the connection must be considered separately to determine which feature represents the "weak link" in the design detail. The "weak link" becomes the limiting component of the detail.
Compare the Threshold “Fth” value assigned to Stress Category “A” to that of Stress Category “F”. A slot weld or plug weld subjected to cyclic stress would be an example of Stress Category “F” if it is subjected to a sufficient number of load cycles. The Threshold value assigned to Stress Category “F” is only 1/3 of the Threshold value assigned to Stress Category “A”. Again, the difference is due to the influence of the stress riser In this case the cracks that will most likely initiate at the root of the welds at the faying surfaces between the members.
Once the Stress Category has been assumed and the Threshold value is determined, the designer has to determine if the magnitude of the change in stress requires the design to consider fatigue. If the magnitude of stress of the component indicates the conditions of Table 2.5 are exceeded, the designer has to look at the S-N graphs to determine whether fatigue is a possibility and whether the anticipated life of the component is sufficient to meet the customer’s requirements.
With regards to wind loads, snow loads, etc., their frequency is not high enough to be a design consideration. The design loads on a building are usually static, i.e., the dead loads of the structure do not change over time. The live loads, i.e., people walking the halls, lunch carts moving through the building, etc. are usually insignificant compared to the dead loads. Storms do present a challenge. The loads produced by high winds, heavy rains, snow, etc. are relatively high, but infrequent. Consider snowstorms; if the building is subjected to one snowstorm a week every week of the year, it only adds up to 52 storms a year. The building would have to be in use for 192 years before fatigue would be an issue.
That would not be the case for a bridge crane beam that is subjected to repeated loads every few minutes. Assume the bridge crane passes over a specific connection 2 times every hour. Further assume the crane is used two shifts per day. That is 32 load cycles per day or nearly 8 000 cycles per year. The bridge crane only has to be in service for 15 months to reach 10 000 cycles. Since we do not know the specifics of the connection details we cannot determine what the Threshold values are. However, it is safe to assume the live loads (loads under the crane hook and the moving crane) are much greater than the dead loads (weight of the crane beam). We can see that fatigue must be considered in our design.
There are some additional design considerations; for one, the design must be based on elastic design, i.e., the allowable stresses are below the allowable yield stress of the materials of construction and members and loads should be symmetrical.
The Engineer is responsible for determining whether the structure is categorized as static or cyclically loaded. It is not the inspector’s responsibility to determine whether the acceptance criteria are for static loading or cyclic loading. When in doubt, the inspector should ask the Engineer.
Summary:
Statically loaded structures are those that involve loads that do not change over time or where the magnitudes of the dead loads govern the design.
Cyclically loaded structures are those that involve live loads where the magnitude of the loads change over time and govern the design. The term cyclic load has largely replaced the term dynamic load. Dynamic is sometimes used to denote the load is moving.
Dynamic Load From McGraw-Hill: (civil engineering) A force exerted by a moving body on a resisting member, usually in a relatively short time interval.
Fatigue must be considered when the cyclic loads attain a threshold value as determined by the presence of or the influence of stress risers and the number of cycles exceed 10 000 cycles (per AWS D1.1).
If the magnitude of the cyclic load is low enough or if the sum of load cycles are low enough, fatigue failure is unlikely.
I hope I answered the question.
Best regards - Al
Now we're getting somewhere.
And, even with the engineer's structurals calling out what is static versus cyclic the fabricator's and/or erector's shop drawings are to also designate what connection points are what and how they will be handled and then be accepted/approved by the engineer prior to work commencing. That way, as we lowly inspectors, rather in house or TPI, are following the work and observing the completed members while comparing them with the approved plans, we know the issues have been dealt with and even if the shop wants to fabricate the connection point differently than the engineer called it out on the structurals it has been approved for fabrication.
This is one of those items that all too often inspectors get way too wrapped up in when it isn't our responsibility. We need to make sure plans have been approved by the engineer. Then, we need to make sure they are fabricated to those plans and according to the appropriate codes.
Many of the plans I have worked with have areas that are cyclic but not the whole project.
OOOOPPPPPSSS, gotta go, my grandkids are calling on 'scype' to see and talk to us.
Have a Great Day, Brent
Now, having lost my train of thought I will conclude with this:
Regardless of what I had just said, I still want to know as much as is practical to aid in my ability to properly discern what is being accomplished at various connection points on any job I am on. Information/knowledge is king when it comes to doing our job and protecting the public safety.
While I like to think I know a bit about the various states of the members this thread has clarified a couple of items for me. BUT, I would still like to see some of you continue further. I have been a little pre-occupied this afternoon with grandkids as well as preparing our annual church financial report for presentation this evening (this has actually taken all my free time for the past month). But, I would really like to look this over more and maybe ask some questions to see if 46.00, OBEWAN, Al, and others can't enlighten us further.
As I said previously, I think we need a seminar or article. ....AL??? 46.00??? Anyone??? Come'on guys, you know you want to. And the rest of us really need it.
I'll be back....
Have a Great Day, Brent
Al, this is not in the AWS D1.1 forum? I'll bite my tongue, but the words 'real' and 'world' comes to mind!
?
Let's just say I'm confused by your post Glyn.
Al
Al, you are quoting AWS D1.1 . This is not the engineering bible people proclaim, to be honest, it is full of flaw's as far as modern day engineering practices and methodology go, however, I grant you it is an excellent base for further investigation. It is, however, well dated when it comes to modern engineering practices?
Did anyone ask the EOR how they figured the loading?
Glyn, are you making a comment or are you asking a question?
AWS D1.1 provides a rational basis of determining whether the steel design needs to be investigated for fatigue. There are other methods of analyzing a design, but most are more involved and require more time and expense. FEA comes to mind, but the cost can be prohibitive. There is always a tradeoff between the degree of “precision” and the cost of analyzing the design.
There are also other forms of fatigue such as low cycle fatigue where the magnitude of cyclic stress is above the yield strength of the base metal and there is thermal fatigue, neither of which applies to steel structures that use ASD or LRFD designs. However, if designing piping systems or pressure vessels both modes may have to be considered. The applicable construction codes require the designer to make the necessary determinations.
Different base metals also respond differently to cyclic loads. Aluminum for instance does not have an endurance limit. Aluminum will fail if subjected to cyclic loads, thus many aircraft components fabricated from aluminum has a finite life expectancy.
There are many books written on the subject of fatigue. Codes, in general, have expanded the information and the analysis required in recent years for determining when designs must account for cyclic stress and fatigue.
As for structural steel design, AWS D1.1 is the accepted standard in the US for welded design and it provides the designer with an accepted rational for designing welded structures subject to cyclic stresses. That being said, D1.1 does give the Engineer the latitude to use an alternate design rationale.
As a welding inspector it is important to understand the influence of geometry and stress risers when inpecting welded structures subject to cyclic loads. Awareness is an important trait when working as an inspector.
Best regards - Al
Guys,
My initial posting was based on a question I saw on another internet forum - "What is the acceptable undercut allowance for a Dynamically Loaded Structure" ?.
Thought I would try to help and couldn't seem to get anywhere so thought I would try the most obvious place - here.
Basically, what is the difference between the two and why has something that was installed in the 1988 edition of AWS D1.1 now apparently been completely expunged from AWS D1.1 ?
In 1985 Section 9 of AWS D1.1 was titled "Design of New Bridges"
In 1988 this title was changed to "Dynamically Loaded Structures".
So, based on that you would assume there is some relationship between dynamic loading and bridges.
Now in the 2004 Edition Section 9 has disappeared and there is no reference at all (other than minor comments I have noted above) to Dynamic Loading in either AWS D1.1 or D1.5.
Something happened between 1988 and 2004 that involved the decision to replace dynamic with cyclic - was hoping some members with access to the older editions may have been able to solve my question.
Cheers,
Shane
I remember the post. It was interesting how someone bought ASME pressure piping codes into the discussion when the question was relative to AWS D1.1.
Maybe this will help differentiate between dynamic and cyclic loads. Dynamic loads can produce impact, impulse, or cyclic loads on a structure. All produce certain harmonics, but the dynamic load could be a onetime event, such as an explosion or a flood. The dynamic load can also produce loads that are considered to be cyclic wherein they repeat over and over again.
A bridge is subjected to dynamic loads; the loads produced by the traffic, consisting of moving cars and trucks, can be predicted over the course of time. Because the loads repeat, they are considered cyclic. Cyclic loads produce harmonics that can result in fatigue failures.
A onetime event such as a storm that occurs with a frequency of once in one hundred years or a single bomb blast do not produce fatigue failure. They can produce an overload where the structure is stressed beyond the elastic limit thereby resulting in permanent deformation or collapse, i.e., structural failure.
Cyclic loads that could result in fatigue failure are a concern to engineers because the magnitude of the loads involved can be well below the allowable unit stress used for design. Cyclic loads can result in accumulated damage leading to brittle fracture. Due to the danger associated with cyclic loads many codes require the designer to consider such things as the nature of the loads involved, surface conditions, geometry, and other attributes that could result in fatigue failure. The codes typically provide the designer with a means of assessing the potential for fatigue failure and parameters to mitigate the probability of fatigue failure.
There is a distinction between cyclic loads and dynamic loads, but the difference is probably lost on most people. It is interesting how the language used by our codes change with time as we learn more about the materials we use for construction. Most of us do not get our panties into a twist when the terms are used interchangeably. As an inspector it is important to be aware of those attributes that contribute to fatigue failure and be aware of when we need to apply either the acceptance criteria for static loads or cyclic loads. When in doubt, ask the Engineer for clarification.
Lack of fusion or incomplete fusion, dynamic load or cyclic load, circumcision or castration, what's the difference? It's only a word, not to worry! Ouch, cringe, oops!
Best regards - Al
I haven't seen any reference to ASME pressure piping codes in this thread, but it has been interesting and lively!
No, that was in a different forum.
Al
Shane
To answer your question "what happened between 1985 and 1988" the answer is AWS seperated the Bridge portion of the code out from D1.1 and created a seperate Bridge Code in 1988 for the first time.
I'm getting this from the forward of the current D1.5 Bridge Welding Code 2010 it says via my editing "In 1982 a subcomittee was formed jointly by AASHTO and AWS, with equal represatives from both organisations, to seek accomodation between the seperate and distinct requirements of Bridge Owners and existing provisions of AWS D1.1. The Bridge Code is the result of an agreement between AASHTO and AWS to produce a jointAASHTO/AWS Structural Welding Code for Steel highay bridges that addresses essential AASHTO needs and makes AASHTO revisoins mandatory. The 1988 version of the Bridge Welding COdde provided for the qualification of welding procedures and tests"...and goes on from their.
I only point this out as it appeared it wasn never addressed and I had to look up the definition of statically and cyclically again and I saw your question.
