Why does CAD hate CAM?: design vs. build

The posting  below came to me via an excellent CAD-focused ezine. It is reprinted with permission from upFront.eZine Publishing, Ltd..  Ralph’s focus in this post is normal manufacturing. Commercial construction is manufacturing, but we make large, complex prototypes (virtually no two buildings or other structures are identical in the way that more commonly manufactured items may be identical). The article was particularly striking because we are going through a large wave of start-ups, very often led by tech people who have only a very limited understanding how structures are built.  A core issue in this post is the (large and significant) difference between “designing” a thing and actually “making” it. The issue exists on steroids in construction. If these issues are not obvious to you, reading this may help you understand.

 

CAD ought to love CAM, but it doesn’t. The whole point to us designing things is so that things can be made. And so it’s kind of weird how much of CAD is hostile to CAM [computer-aided manufacturing]. While today’s MCAD software usually recognizes that the output of its designs is for manufacturing, drawings produced for BIM and civil projects do not. Prepping for manufacturing isn’t particularly innate to the CAD software we use. We can draw the entire solar system at 1:1 scale, right down to the lettering on a lunar lander’s plaque (as an early AutoCAD sample drawing showed), but most CAD programs can’t prepare the drawing to manufacture the plaque. When we send the drawing to a plotter or a PDF file, we drafters and engineers see our job as done. The Problem of Manufacturability The gulf between CAD and CAM exists because CAD works with mathematical certainty, while CAM works with tolerances, a technical word for “uncertainty.” We designers take pride in our consistent use of tools that are precise, as CAD vendors reassuringly remind us, placing geometry using object snaps and parametric constraints while working to 14 decimals places of accuracy. CAD is so precise that CAM operators cannot use the drawings we produce. CAM understands the real-world problem of tolerance, the plus-or-minus dimension in the order of 0.01mm or more that comes into play from the imprecision inherent in lathes running in machine shops, or the gears and hydraulics of graders guided by GPS while smoothing freeway expansion projects. So CAM operators need to tweak CAD drawings. This is one of the marketing pushes behind companies like SpaceClaim and BricsCAD. The direct editing found in these programs allows machine shop operators to import models from “any” CAD system, and then adjust the drawings to suit CNC [computer numeric controlled] output, such as removing unneeded details (defeaturing) and extracting geometry for fixtures and tooling. Most of us CAD users were never trained in manufacturability, and we so have no idea about it. Manufacturability is the extra information that ought to be included in CAD drawings so that parts can be manufactured without CAM operators needing to massage our output. Steel molds must include mild draft angles so that plastic parts pop out easily; sometimes the molds need pipes to deliver additional molten plastic to fill the mold properly. When the flanges of sheet metal are bent, allowance must made for the bend radius, known as the k-factor (or y-factor in PTC software). There is little manufacturability in regular CAD programs. AutoCAD, Microstation, and nearly all other CAD programs are ignorant that wood cut for kitchen cupboards needs to account for the width of the cutting blade or how to nest the parts to minimize waste. Nesting places parts optimally on plywood or sheet metal to minimize waste. Regular CAD doesn’t do this. MCAD Gets Acquainted with CAM The most basic manufacturability tools are GD&T [geometric dimensioning and tolerancing] and PMI [product manufacturing information] to tell manufacturers how much inaccuracy can be tolerated and the material to use. Only in the past coupleof years have translation packages boasted about being able to translate PMI. Regular CAD packages now offer STL[stereolithography], the universal format for 3D printers .These are baby steps, given that CAD and CAM have both been around since the late 1960s. So: what’s changing? CAD is a mature market, as the CEO of PTC is fond of pointing out. There are no new customers, only customers to be enticed from competitors with functions competitors don’t offer — such as better output to manufacturing devices. Once one MCAD system does it, competition ensures all the others quite suddenly find themselves quite congenial to the idea. We’ve seen the same rush in other areas, such as direct modeling, generative design, and lattice generation. How the Split Occurred The disconnect between CAD and CAM didn’t always exist. The first CAD systems were developed by McDonnell Douglas (Unigraphics, now NX from Siemens) and Dassault Aviation (now from Dassault Systemes) purposely to output to CAM. But most systems developed differently. CAD software is made by PhDs comfortable with multi-dimensional matrix transformations. CAM software is developed by practical users, whose software typically reads in a plain DXF file output from CAD, then adds tooling and path information, and outputs it as g-code (the DXF of CAM). Their concern is for roughing strategies, 2- through 6-axis milling, and turning-drilling — not in finding another way to draw a line or extrude a surface. G-code is not universal, unhappily. Pretty much every CNC machine offers a unique set of capabilities, and so the g-code needs to be tweaked for each one with post-processors — an annoying task for a CAD vendor that doesn’t particularly want to be in that end of the business in the first place. On the other hand, CAM software vendors like post-processors, as they make significant revenues selling machine-specific code. The split manifests itself in other ways, too. Nearly all MCAD software is owned by just a few corporations, but there are forty hundred companies offering CAM software. They have names that we designers may never have heard of, like Alphacam, BobCad-Cam, CamWorks, Dolphin CADCAM, Esprit, FeatureCAM, GibbsCAM, Hypermill, Mastercam OneCNC, PowerMILL, SolidCAM, TurboCAD/CAM, Visual Mill, and ZW3D CAM. While many are home-grown, others are repackaged from OEM providers, such as those made by MachineWorks of England. Here’s another split from CAD: CAM has its own language, with jargon like swarfing, flow cuts, toolpaths, cutters, multi-axes, point control, grooving, and inside roughing. Mold design is sufficiently complex that it is sold separately from CAM software. Nobody know why the CAM software industry never coalesced. Some attempts were made, the biggest by Vero Software. They raised over $10 million to own a dozen CAM packages; but then in 2014 Vero was taken over by Hexagon. If MCAD vendors haven’t already developed their own CAM software, then most of them purchased one or more firms, such as Autodesk acquiring HSM, and 3D Systems buying GibbsCAM, among others. If they don’t own one, then they embed CAM software from third parties, as Solidworks and OnShape do. The Horror of 3D Printing There two ways to make a part: remove material or add material. Originally the output from drawings was subtractive manufacturing (machining). The more difficult problem is adding material, but even additive manufacturing (3D printing) is more than twenty years old now. Awareness of 3D printing exploded a decade ago with those cheap 3D printers that were marketed for homes. Well, that ended as a bust. I recall one vendor stating they would place their 3D printer in every child’s bedroom, but then wouldn’t answer questions from us skeptical media about how children would be protected from fumes of melting plastic, the hot surfaces, and the post-print cleanup, which sometimes involve noxious chemicals. The samples that I collected from them still stank a year later. The collapse of home 3D printing should not have surprised, as it had everything going against it: small parts printed at poor resolution that might well collapse on themselves, and that printed slowly. Children watching in awe lasted a couple of prints. The cost of the plastic filament refill was where these companies made their profit — expensive, in other words, like printer ink. In contrast, the industrial version of 3D printing is booming, although here we are talking about machines costing $50,000 and up, that take up a lot of floor space, and requiring proper venting. I am fascinated by the new variations on materials announced every few weeks to output parts made of multi-colored plastics or sintered metals. And so we have an industry balancing between promise and frustration. The promises of AM are wondrous: · Outputting intricate parts that are impossible to make with subtractive manufacturing · Producing spare parts on demand to eliminate dedicated warehouses · Generating prototypes more quickly and avoid over-nighting to outside manufacturers · Minimizing material cost and weight, especially in surgical and aviation applications Sure, mistakes are made with subtractive manufacturing, but mistakes seem more precarious in additive manufacturing. AM works from STL files exported from CAD programs. STL is a simplistic format that consists purely of 3D coordinates for the myriad of triangles defining the surfaces of models; the format doesn’t even include units. It’s these surfaces and triangles that create problems for 3D printers: · Too thin walls lead to collapsed 3D prints · Non-watertight designs with one or more holes in surfaces, and edges that don’t match · Too many triangles used to define surfaces or ones that overlap unnecessarily · Triangles with inverted normals or ones that cut into each other Normals are vectors that indicate the inside or outside of each face of a 3D model. When a normal points the wrong way, the 3D printer thinks the inside of the model is the outside of it. Software firm Materialize reports that one in five parts fail during 3D printing, because walls were too thin. As a result, an entire sub-industry has sprung up to write software that fixes these problems and optimizes the 3D printing process, such as by shrink wrapping the CAD model to make a shell copy of the 3D solid. What Ralph Grabowski Thinks We see market inefficiencies in the disconnect between CAD and CAM that have gone on for 50 years. Small firms are successful in targeting niches ignored by bigCAD, especially for kitchen cabinets. The linkage between CAD and final product is pretty tight for MCAD, but completely lacking in other major disciplines, such as architectural and civil engineering. The complete lack of a CAM connection to BIM is a primary reason the construction industry is less digitized than most. [This article first appeared in Design Engineering, and is reprinted with permission.]

Who sees the complexity in commercial construction?

I recently retired (from 30+ years as a precaster and 6 years in IT before that). Later in the year I expect to complete a rework of a presentation on Knowledge Management, focused on possible industry-wide approaches to gathering and managing all the currently highly dispersed knowledge that underpins commercial construction and on why such an approach is a necessary precursor to any serious automation that reflects and incorporates all the deep real-life knowledge of materials, products, and interface issues required to actually build something. To this day I find that most commentators or thinkers (invariably from the software or design side, not contractors) grossly misunderstand those complexities. The main thread running through my writing is: how can we assemble, organize, and access the immense body of knowledge that underlies a building.

I specifically solicit your comment and disagreement on my assertion that virtually no one outside of the actual business of constructing (not designing) commercial buildings and structures grasps the actual complexity.

If you are interested in the earlier versions the 2002 early draft is at: http://caryconcrete.com/writings/CCP-Knowledge.pdf

A 2015 version, both more succinct and a little more focused on possible solutions than on explaining the issues is at: https://www.dropbox.com/s/i5drbam7xuvl704/Schlosberg%20Presentation%203-31%20notes%20reduced%20for%20speaking.pdf?dl=0

=============THOUGHTFUL COMMENT ON THE TOPIC=================

In working on that 2015 presentation I solicited comment from a young construction manager I knew and I thought his response was terrific. I have reproduced it below.

RE: Maybe a favor?
Morton, Leviticus
Jan 5, 2015, 3:51 PM Reply

to me

Leo –

Sorry this took so long – I decided to take my time and really reflect on everything. As with the construction industry itself, the topic of knowledge management is very complex as it applies to construction and can be taken down many avenues. It has been difficult to really outline all that I want to tell you. My initial thoughts are very much centered around two points you delivered in a conversation we had at the Schiffers’ house a couple of years ago (both of which also appear in your presentation) – Every construction project is a prototype, and, more importantly,  it is impossible to fully understand the complexities of any project, much less an individual trade or even a specific task. As this second point is part of the underlying theme of your presentation, I figured I’d outline the development of my understanding of the subject before moving onto other parts of my professional development.

If you remember our conversation, I was quite skeptical of this point. I was fresh blood at that time used to cramming useless facts and figures into my brain at maximum flow. I understood that there was a special language in the field with numerous techniques and mediums, and I thought that once you got that general grasp of the major trades – Concrete, Steel, Masonry, Finishes, Specialties, etc. – The rest would come naturally. I mean, how tough could it be? There are industry standards and everything.

Ignorance is bliss, am I right?

Things went pretty much according to plan throughout my time in our main office working estimating and marketing. Everything made sense, everything had an explanation, and everything seemed to be nice and organized. Then I hit the field and realized that everything was perfect because it was all theory. Application is a much different story. Not too far into my first job, I was surprised to run into constructability issues that perplexed even my senior superintendent and senior PM. In some situations our architect had omitted details or previous details were not applicable as new systems had been approved post design. One specific instance involved an approved roofing change (to standing seam metal roofing – cost benefit, go figure). This created an issue as we uncovered we had insufficient details at the interfaces with the roofing system, fascia, soffit, and building system. This was a large issue as this building was to have an above average rating for air barrier and building insulation.

The entire time I was in estimating, I was practicing building everything in my head. It was a useful tool I learned in the beginning, and certainly aided in my understanding of various building systems. One problem, however, is that you can only build what you see on the page or in the specs. If you’ve never built it in real life and never seen it, how are you supposed to know you’re missing something? How was I supposed to know whether a double pane window comes as an assembled unit or is assembled on site? Does it need an additional bead of sealant upon installation? The sealant isn’t shown in the details. Precast storm pipe is pretty easy, right? Just pair up the ends and connect it to the manholes. Or, have the manholes pre-cut to receive the pipe and hope everything lines up right. Oh yeah, and make sure to fill and grout the opening to create a water-tight enclosure. That wasn’t in the plans and specs?

Which brings me back to the problem at hand. The designer (or rather, the design) doesn’t know everything – and they shouldn’t have to – but sometimes they literally don’t know anything (because they didn’t design it). And that’s where you really run into trouble when you assume you will have to know everything. There are so many options and so many solutions. The standard industry practice when you hit a major problem with no solid direction is to frantically boil a ten gallon pot of information pasta, have everyone grab a handful, and throw it at the walls and ceiling of the office trailer. Whatever sticks the best with the lowest amount of effort at the best cost is usually what we frame and redline into our drawings. Senior management is the best at flinging pasta, but as long as you’ve got something to throw with, you always have the ability to find the best solution. Anyways, to get away from analogy, I was surprised at how freeform and chaotic the process of finding a solution with basically no starting point was. That’s not to say that our final selections weren’t reliable, proven, and up to code, but for example, there’s hundreds of ways to ensure a seal between roof and fascia. Multiply that by the number of field decisions you have to make throughout the lifespan of a project and you’ve got one heck of a ride. Instead of leading a horse, I found myself riding a bull and holding on for dear life.

The concept of knowledge management as you outlined in your presentation is certainly an intriguing one – the ability to, in essence, pick out the stickiest noodles from the bunch with little effort so that your starting point is much further along and at a much more civil point in the process of conflict resolution.

As a ‘newbie,’ though, the biggest problem that I’ve found is not finding the information I don’t have, it’s finding out what information I don’t have. It may take me days, weeks, or months to research a specific solution, but as long as I know I need it, I know I can get it. If I don’t know I need a specific piece of information for a process I’m preparing for, I won’t know until after I’ve made the mistake of not having it. And while this is a problem experienced more often by the inexperienced, it is something we all encounter on a regular basis. This, in my opinion, is where the current hurdle with knowledge management lies. Cataloguing and databasing information is all fine and dandy, but how do we express an issue we don’t understand to a system or service? How is this system going to fully comprehend all of the potential complexities? How is the information we don’t know we need going to find its way to us?

And more importantly to me as a GC, how does this system mitigate risk to the user?

I’ve passed this on to a couple of co-workers and am waiting for their response. Let me know if you have any questions, need me to expand, or would like to lead me in a different direction.

Thanks,
Levi Morton, LEED AP BD+C
The Walsh Group
Project Engineer
C: 931.588.1342
Marshall: 270.776.9306
UAS/UAV: 931.444.2053

From: Leo Schlosberg [mailto:leo@caryconcrete.com]
Sent: Monday, November 03, 2014 11:37 AM
To: Morton, Leviticus
Subject: Maybe a favor?

Levi,

I am contemplating writing a proposal having to do with knowledge management. I am interested in your experience if you are now on the build side (vs the estimating side). My specific interest is to what extent have you found things to be more complicated than schooling might have indicated? I can elaborate but there is no point unless 1) you are not simply estimating these days and 2) you are willing to write or speak to me on this.

If you want context, you can look at my early thoughts on this issue. http://caryconcrete.com/index.php?page=writings_industry

Leo

—————
Leo Schlosberg   cell: 847-226-0930   office: 815-338-2301            www.caryconcrete.com

Blog: www.planetcommercialconstruction.wordpress.com        https://www.facebook.com/CaryConcreteProducts

Wall problems or questions?  We can help with diagnosis and next steps. : send photos, get knowledgeable answers: :www.quick-look.us

Retirement reflections #2

I am now mostly retired and doing a lot of traveling to visit my far-flung grandchildren. Most of my days since the last posting in September 2017 (Retirement Reflections https://planetcommercialconstruction.wordpress.com/2017/09/05/retirement-reflections/) have been spent out of the country, but I still am managing a few projects. It is not unusual for folks to get crankier as they get older.  This business gives me plenty of reason to be annoyed as I am regularly interfacing with designers who have a clear concept of what they want in general, in the big picture, but have only limited understanding of either the materials they have specified or how to get them installed and functioning as they envisioned.  To this we have to add the fact that designers have no incentive to get the construction documents exactly right, as they are paid the same either way and contractors will eventually more or less get it all straightened out (if this is not clear to you as a reader, and you want to understand, contact me for elaboration). Another factor helping me to have no regrets about retiring, are bureaucratic resident engineers (RE’s — they do the construction administration on most public works).  Two of my few remaining active projects are public works, and neither involves  architectural precast, though the pieces do have architectural function (this translates to plain gray concrete, smooth form finish).  Each project has had minor bumps on the path from bid to booked job to getting approvals and starting fabrication. Those bumps are a reflection of  the issues discussed above.

The first project is a bunch of large blocks (2’x2’x4 and larger) stacked on the water’s edge, essentially a revetment, similar to large stone blocks that often line a river or coastline. There is a modestly intricate layout, and the contract documents left some (literal) holes, had some mis-counts of quantities (it was a unit price contract), and ignored tolerances (not unusual for designers to think that materials fit together perfectly with 0″ joints and no variation from piece to piece). Also, the documents reflected no understanding of reasonably efficient ways to pin pieces together. Not a big deal, not that unusual. But the documents also had bin blocks as their starting point (but without the keys and keyways that these have) and  called for these to be made of “return”concrete, of what is returned unused to the ready mix plant still in the truck .  Many ready mix companies do make bin blocks out of their returns but making them in different sizes and in a DOT-certified mix is not their cup of tea and so we got the project. Here are the notes from the contract drawing.

We dealt with all the drawing issues and standard submittals. A 4,000 psi mix was specified and we submitted cylinder breaks of 1-day (3500 psi) and 7-day (6500 psi) tests. The response was: “where are the required 28-day breaks” and could only have been uttered by someone who was either very ignorant of concrete (it gets stronger with age and by 7 days had greatly exceeded the specified 28-day strength) or was extremely bureaucratic (I see a box, it must be checked off no matter what) or maybe brain dead (for an engineer). [Side note: clearly  a portion of this person’s job is  ripe for  being handed over to a computer.] Oh well, write a letter and/or break another cylinder, keep them happy, no big deal, but somewhat annoying if you are a  mostly-retired, getting cranky 70-year who still thinks (with no empirical basis) that an industry should improve over time. Luckily, the note and picture below were sufficient to convince a professional engineer that we had met the contractual requirement that at 28 days the concrete had compressive strength of at least 4000 psi.

Another issue resolved. Then the RE (or another party, my communications come via the GC and so I don’t always know who initiates a particular nonsense) asked for a color chart and I had to write them a letter explaining that the contract clearly was based on “return” concrete and no way did that come in a range of color choices.    OK, next they asked for pictures of the concrete so they understood color range. Not 100% unreasonable, but very rookie-ish.  Then they asked for photos of the sand and stone, which makes no sense (if you know a little about concrete) since it is form finish and thus all you see is the cement paste, so the color of the sand and stone are not relevant.   A lot of futzing around for a bunch of blocks.  Maybe we will soon be cut loose to make the stuff.

The other job started with a cut sheet for a standard product – a simple tapered shape for a short bollard  from a precaster far enough away that it was uneconomical to ship, so we got the job.

Looked like the most straight forward item you could get in a plans and specs world .

We submitted a drawing and in return got a revised section showing a large void, with a note stating the new maximum weight of 620 pounds.

The original cut sheet listed the weight and nothing in the contract documents indicated that weight was an issue; you wonder if the designer ignored the weight or if the owner suddenly came up with a new requirement. Either way, what gets the old-cranky-guy juices flowing is when we do the 7th grade math and determine that the revised section they drew will weigh 795 lbs., well over their stated 620 lb. requirement. Why do they bother drawing the revised section if they cannot do the math? They could more easily simply have  given us the weight requirement and both our lives would have been easier.

Then it turns out that they want a stainless steel plate on the very top 8″ x 8″ surface and we are told they want Type 314 Stainless Steel.  My first response: Huh?! What is 314? Second response is to look it up and learn that it has high-temperature oxidation resistance and is “commonly used for furnace equipment, super heater suspensions, enameling grates etc.” My third thoughts: typo (they meant 304, the most common stainless in construction applications) or wacko? No way to tell, write another email and may as well also tell them that their note about gluing it is goofy – would likely result in reduced durability and that 3/8″ thick plate is overkill for a shiny ornament.  Other than being cranky I do not mind; having relevant knowledge is the job of a supplier or subcontractor in this business where the designers are neither required nor able to get to the level of detail needed to actually correctly construct anything. What amazes me is that there are people (they know software but not construction) who think they can easily automate procurement in this industry.

Retirement reflections

I have been learning about this business for 29 years now.  As I transition to retirement I get both reflective and annoyed. Annoyed because certain things do not get better, they may be getting worse, and of course I am crankier than I used to be.   I recently bid some “simple” precast panels for a retaining wall (aka “lagging” panels or “soldier piles”).  I have done a few; therefore I know a little, though not much, about them.

If you are unfamiliar with these structures, here is a primer, created by searching for “lagging panel” (you can try this at home).

The bid documents were created by a very large (well, maybe the largest) multinational design firm.  They showed a detail I had never seen before. Intent was clear: visually cover up the vertical H-beam. But to my limited knowledge the detail smelled of  serious installation/constructability problems and of likely durability problems as well. (If the basis of such thoughts interests you, ask and I will explain).

I called  a GC bidder I knew to be highly experienced with this type of wall.  estimator and he concurred that it was borderline nightmarish.  So I wrote up something for the pre-bid meeting. There were actually 5 separate confusions or errors pertaining to the precast in the bid documents (what about the rest of the project?).  I summarized the main one by asking: Has this use of a slot been fully engineered and is there a known example where it was both successfully installed and durable?

Subsequently, one of the many addenda issued to the bid documents featured a different detail. The revised detail was new to the project and obviously new to the original designers, but is not new to anyone with experience building these piles.

What lessons are implicit in this situation?

Designers sometimes “wing it”

They may have no idea of the best ways to solve a problem and they are under the gun to get the drawings out, so they just take a stab at it. Not uncommon in my experience.

Contractors are designer’s proofreaders and reviewers.

In effect the prebid questions and subsequent RFI’s are people with more knowledge checking and commenting on the work of those actually paid to design but with less knowledge of each specific material.  The cost of all these people-with-relevant-knowledge reviewing the work of those with very limited knowledge  never shows as a design cost; it is included in the overhead of every bidder.

Once-upon-a-time there were “master architects”

Everyone I know who has had to work from historic (100 years or older) drawings is very impressed with the level of detail, precision, and how it generally matches what  was built. There are a multitude of reasons it is no longer possible for an architect to have deep knowledge of what they are designing. Somewhat surprising is how little the process has changed given how much the reality has changed.

Big name implies big size + good marketing, little else.

Small boutique firms often bring much deeper knowledge to the table. In order to stay big, big firms have to be good at getting lots of client work. Owners are not in a position to assess expertise, and particularly the expertise of the people assigned to the project. The firm may have a world-class expert in one of it’s offices, but as in this case, that knowledge never made it to the project. This design firm is considered a leader in “knowledge management”; what must the laggards look like?

This  phenomena is not unique to construction. In custom corporate software, the large consultancies (IBM, PWC,  Accenture, etc.) are always the safe choice (decades ago when I worked in corporate software the mantra was “no one was ever fired for going with IBM).  Many small independents were much better, but not at marketing.

Why IPD is critical to the success of BIM

A model that does not reflect deep, usable, knowledge does not contribute to efficient construction. This was recognized in early experiences with BIM,  and Integrated Project Delivery is all about bringing expertise to the model earlier than design-bid-build does.

 

 

Lessons from small failures – Part II: Designing for durability

If you get to design and build generation after generation of essentially the same structure and you get to study the old problems, then you end up with a mature well-designed product. Over the decades automobiles have clearly become both more reliable and longer-lasting. Not necessarily so with structures, especially when they are not built repetitively, as might be a standardized overpass/bridge over an interstate; but put it in an urban area, jazz it up with architectural features and you may be adding problems. DOT’s have test tracks to learn more about particular pavements. No such luxury applies to anything with even a little bit of novelty. Lest you think the immense challenges of designing for durability apply only to the minor stuff Cary Concrete Products is normally involved with, here are snippets from a recent story.

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After Only Seven Years, New I-35W Bridge in Minneapolis Showing Signs of Age

07/27/2014

Star Tribune (Minneapolis, MN)

July 27 –Cracks emerged in concrete girders. A drainage hole on the bridge deck plugged up. Rust showed above piers. Seven years after the collapse of its predecessor, the new Interstate 35W bridge has been showing its age…Repairs to the system were eventually made under warranty, but not before years of finger-pointing between the state and the lead contractor. Taxpayers also covered some costs.

“Contractors are always very reluctant to give anything,” said Tom Styrbicki, bridge construction and maintenance engineer for the Minnesota Department of Transportation (MnDOT). “They always try to drag MnDOT into it: ‘We did it per your specifications. If it leaks, it leaks.’ ”

[My comments: 1) of course there is finger-pointing. Often problems arise from issues in both design and execution; 2) most contractors eventually fatigue from the battles over specifications that are poorly written (my own experience and informal polls peg error-free specs at 1%) and occasionally enforced to our surprise and detriment. The result of that fatigue is unfortunate but understandable to any subcontractor: ‘We did it per your specifications. If it leaks, it leaks.”]

Full story is at http://www.startribune.com/politics/statelocal/268746561.html or http://enr.construction.com/yb/enr/article.aspx?story_id=id:ZDAtwZvbDEgPAf81w4aA1jHIfPjJuLGNDcMwvg2Pfds8xYrzQnvP5LykSce0mHPo

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Our microcosmic story concerns a minor failure and then thinking in detail what it takes to avoid creating designs with inherent problems. The previous post in this series went into detail about the specific problem we recently analyzed and on how surprised we were by the cause of the failure: https://planetcommercialconstruction.wordpress.com/2014/07/21/lessons-from-small-failures-part-i-searching-for-truth/

To summarize: enough moisture entered an unusually large mortar-filled void. When  frozen it was able to generate enough force to crack concrete.  We say that the piece “failed” but in fact the piece performed exactly as it was designed to perform.  The curbs at the posts in this railing system were not durable because the design did not anticipate what occurred. With hindsight we can easily see what happened.

• The rail assembly sits on the sidewalk (and the sidewalk on the deck)
• Water on the back of the curb flows to a small overhang that is part of the sidewalk
• The sidewalk, including the overhang, slopes to the roadway for drainage.
• Water flows into the mortar that is the bed joint of the curb pieces.
• Moisture in the bed joint moves into the mortar-filled cavity (blockout)
• The mortar gets saturated
• The mortar  gets frozen and expands
• Frozen mortar in the bed joint is not a problem, too small to generate large forces
• Frozen mortar in the block out starts to crack the precast curb
• The crack takes in water, ice forms and enlarges the crack
• Visible large cracks form and the curb piece falls apart.
• In 6 years, about 15% of the pieces at this condition have already  failed.
• All other curb pieces, piers, etc., are in excellent condition.

This is the railing system. It’s on a roadway bridge.

Structural detail from contract drawings.

Architectural detail from contract drawing defines a blockout sized for real-world tolerances.

Early stage of failure.

Visible cracking has occurred in this piece.

Inspecting after removal of cracked pieces and cleaning the blockout. Shows crumbled mortar in foreground.

The foresight to anticipate this is highly improbably. For 25+ years I have been regularly involved with mortar-filled openings, and never before saw a failure of this type. Couldyou reasonably expect someone to anticipate this? Failure required both a large volume of mortar and a large volume of water (and a freezing climate). Rules simply about the size of mortar-filled blockouts would not help. (I am sure such rules exist in <0.05% of architectural firms and <0.5% of engineering firms.)  Avoiding this failure would require you to have imagined this scenario. It only is an issue at the pieces at the posts. If you were using this exact railing on many bridges you would be in a different business and in 5 years or less you would have learned that this particular detail does not work. And you would never use it again.

This can also be a reminder that a BIM with intelligent objects needs both software that can store and apply the knowledge and  an organization that can capture and organize little tidbits like what we learned from this minor failure.  We are not close to doing that in non-repetitive structures.

Lessons from small failures – Part I: Searching for truth

The “failure” here is simply a piece of architectural precast concrete that cracked open much earlier in its life than expected. The piece is part of a system of curbs and railings on a bridge.  Photos were sent to me and I was able to take a good guess as to what was going on. It seemed obvious;  I’d have given odds of at least 8:1 that my guess was right (spoiler: I would have lost the bet).  Here’s what the piece looked like on the plans, in real life, and as a failed piece.

 

The bridge is  7 years old; failures started showing up a few years ago. The problem is only at the intermediate piers; there are no significant problems anywhere else on the railing.

I knew that the steel post had an anchor plate and was bolted to hot-dipped galvanized anchor bolts cast into the bridge deck and that the tube steel post had some sort of serious coating.  The location and pattern of the cracking indicating a lot of force coming from the cavity where the curb piece surrounded the steel post.  My guess was that something was rusting – maybe some non-galvanized bolts were used or the paint on the post had failed or been abraded and was now rusting. In my experience I could imagine nothing else inside the piece that could generate enough force to crack a healthy piece of precast. The precast had epoxy coated bar and the crack did not follow the bar locations, ruling out rusting reinforcement. The Quick-Look report (www.quick-look.us is our service for fast, first-look inspections based on photos sent to us) identified some rusting object, probably the plate,  as the likely cause of failure.

After getting the report based on the photos the owner engaged me to make an in-person inspection. They wanted both a more careful look and some advice on remediation. We went, we looked, and we watched as they opened up one of the cracked curbs. To my surprise, all the metal was clean (the tiny bit of rust on the edge of the bolted-down plate was insignificant).  The mortar that was used to fill the void formed into the precast to go around the plate and bolts was so crumbled that it might have been just coarse damp sand. It was clear that rust was irrelevant and that freeze-thaw action had turned the mortar into a sponge that once saturated could become a large ice cube capable of generating enough force to crack the precast. Once the cracking started, freeze-thaw and chlorides (the piece is on the edge of a bridge that is in the middle of the snow-and-salt-belt) did the precast in.

After cleaning out the mortar fill

After opening and clearing out most of the crumbled mortar

This piece has a barely visible early stage crack.

Lesson One: Sometimes you have to open up a wall to be sure of what’s going on. Sometimes you will get surprises rather than confirmation.  We understand the limits of looking only at what is immediately visible, at what shows up in a photo. True in property inspection and in many other parts of life.  More lessons from this inspection in posts to follow.

Lovely spring day makes everything look better

Today was a lovely spring day and I even got to take a little bike ride. That puts me in a very good mood. So one of those constant screw-ups that occur here on planetcommercialconstruction struck me as funny rather than annoying. Viewing commercial construction as a comedy of errors is not as unreasonable as it might seem.

In other industries people build prototypes to discover problems before they build the real thing. Since we build large-scale prototypes (that is what a new building or road or park is) we are constantly finding new possibilities for errors. Because so little is “standard” in this business, even processes tend to be non-standard. Last month I was surprised when a plant submitted samples of two shades of red, got an order for some mockup pieces with the right mix number attached, and somehow, despite apparent good business process, managed to make and crate up 4 full size gray pieces. I did not laugh when that happened, but it wasn’t yet spring.

Today’s laugh came in the context of a job that is getting overripe. I sent samples to the GC and they took two weeks to get to the designer.  Those were rejected because they had the color and the finish that was specified  (rather than  what was “intended” –  that’s life on planetcommercialconstruction). We have run out of time. So this time I gave the plant a transmittal with the name and address of the designer so samples could go direct and we could get approval and get this into production. This AM I saw an email from Fed Ex saying they had delivered the package. Great!  I sent the designer an email so he would know it was in his mail room. So when I showed up at the office and saw that a carton about the size of two 12″ square samples had been delivered to me I was surprised.

I looked at the label and saw that the plant  (not the same one as switched red and gray) had put the designer’s name on the label, but not his address. I had failed to read and verify everything on the FedEx email; had I done what every sub is contractually obligated to do, which is to read every one of the gazillion pages of paper, email, drawings, etc. that get sent to us in a project and scour them for conflicts, then I would have detected this earlier and avoided the embarrassment of telling a designer the sample had arrived at his office when it was actually at mine.  The good news is that I took a short bike ride today and so it’s just part of the comedy of errors of commercial construction.