Andrew Derbidge, Director at Partridge, Sydney-Australia
As industry and technology march on in an increasingly accelerated fashion, there are numerous professions, skills, trades, and crafts that fall by the wayside. There’s an old joke, “Which jobs don’t exist anymore?” (The answer is “Steve”).
There are two other things that, over time, can fall by the wayside: Knowledge and experience. When it comes to structural engineering and design, the developments and advancements in technology – as well as social, cultural, and economic changes – have meant that some of the fundamental knowledge and principles of design begin to fall away or get forgotten. Or, more accurately: They don’t get passed on. And one area that has particularly suffered is the efficient detailing and scheduling of reinforcement.
For many structural engineers and their approach to materials, reinforced concrete is the odd one out and often viewed with trepidation or dislike. Steel beams can be designed with the benefit of design capacity tables; timber members can be sized by reading off span tables, and even masonry can be sized and proportioned with the benefit of safe load graphs and charts. But reinforced concrete is trickier and more demanding – chiefly because it has a creative component: In addition to the physical size of the element, you can choose how much reinforcement to put in the section; you can choose where to put that reinforcement, and you can manipulate the section repeatedly to tweak both its ultimate strength and its effective stiffness. And with this choice comes the power to create efficiency and economy. or to lose it.
As such, more than any other construction material, designing reinforced concrete is a skill, bordering on a craft. You need to spend years at the design coal face to understand its intricacies and to get a feel for which parameters to tweak to achieve efficient and effective outcomes. And also to realize the economy in designs. It is the one material where good training, education, and mentoring is needed if young engineers and draftspeople are to truly master the art and craft of laying out reinforcement. And, sadly, with the way modern design offices work and the commercial pressures of business, this mentoring is being lost.
Sustainability and consideration of a structure’s carbon footprint are very much part of the construction industry’s zeitgeist at the moment, and (hopefully) every consulting engineering firm is looking at ways to specify and generate solutions that make their structures more environmentally friendly. However, there is an irony – nay, hypocrisy – with the number of firms that espouse sustainability values on their website and marketing collateral, yet churn out concrete designs that waste tonnes of steel and feature unnecessary embodied carbon.
“Old school” is a term that gets thrown around a bit in the construction industry, and it’s not always clear if the sentiment is one of respect and acknowledgment for what was once “tried and true”, or whether it’s a backhanded compliment for being stuck in the past. Regardless, old school reinforcement detailing offers significant benefits, cost-savings, and design efficiencies that provide so many positives, yet the techniques and tricks are often overlooked.
Why are these benefits unknown, ignored, or being forgotten? There are several reasons: Some are industry-driven; some are a byproduct or side-effect of technology, and some of it is what happens when consulting fees get squeezed and engineering offices are pressured to produce more for less. Let’s explore some of these, before looking at some tricks and techniques that can improve your next design…
Thinking about and laying out a full reinforcement plan for a deck takes time. Time costs money, or ties up staff on one job/task when there are many tasks to be done. Consulting fees are being increasingly squeezed (for a number of different reasons), and many consultants cannot afford the time required to properly draw and produce fully detailed, individual, top and bottom reinforcing plans, or to elevate the beams for the low fee they quoted. And so some consultants choose to put the top and bottom reinforcement on the same plan; often also putting beam reinforcement on the slab plan, rather than elevating them individually. Which, in order to be understood and read, means instantly that they have to simplify and generalize the design, robbing themselves of the ability to change bar size or spacing where better efficiency can be achieved. Worse still are the firms that elect to dispense with reinforcing plans altogether and to simply call up (for example), “N16-200 (16mm diameter rebar spaced at 200 mm) each way, top, and bottom, throughout”. Again, the opportunity to realize the economy and efficiency in the slab zones with lower stresses is lost. Not to mention the difficulties that then arise on-site when less knowledgeable steel fixers put laps and splices in inappropriate locations.
A drawing from the late 1960's by consulting firm Rankine and Hill - a time when all beams were drawn and elevated in full, with designs tailored and optimized, such as relaxing the spacing of the shear stirrups in midspans where shear loads are less.
The skill and know-how of designing concrete by hand is fast disappearing. Even simple spreadsheets are being eschewed in preference for software that analyses the sections or slabs in a fraction of the time and eliminates the iterative process of design. The over-reliance on software is seeing the mentoring and training processes slowly disappear from the engineering design office. “Black box” design is seeing a “dumbing down” take place across the industry as engineers get the worst-case zone and maximum moment solution identified for them, without necessarily understanding the levers and mechanics taking place in the background. More critically, the design of the cross-section is done for them, leading to a lack of understanding and gut-feel for how the solutions are achieved.
Tied in closely with increasing operating expenses and businesses needing to cut costs, the reinforcement companies and suppliers no longer employ qualified engineers/draftspeople to undertake the bar scheduling. Up until the 1990’s and early 2000’s, both BHP and Smorgon had full-time engineers or structural draftspeople who would take the engineering drawings produced by the consultants and designers, and work hand-in-hand with the designer to produce fully scheduled drawings and plans for the production line and the steel fixers on the deck. It’s a distant memory now, but I remember I used to get two or three phone calls a week from the schedulers, who would phone to discuss the design; discuss the bar solutions; discuss difficulties they may have identified with, say, spacing or penetrations, or achieving adequate development and anchorage; and they would offer suggestions and improvements, or simply head-off problems that might occur on-site. This has been lost from the industry, and it is a loss to both the designing engineers (who would learn a trick or two through such dialogues) and it is a loss on-site, where steel fixers now receive jigsaw puzzles they struggle to interpret.
Even the days of the engineering firms themselves partially scheduling the reinforcing have gone. Look back at slab plans from the 1970’s - 1990’s and you’ll see call-ups like “46 N12’s at 200 ctrs (46-12 mm rebars spaces at 200 mm)”, i.e. the designer/draftsperson would determine the actual number of bars, thus minimizing over-stocking and wastage.
Another industry change is that, in the case of the larger buildings and developments (particularly high-rise), the solutions are increasingly adopting post-tensioned designs, rather than conventionally-reinforced concrete. With such projects being sub-contracted out to design-and-construct post-tensioning firms, the daily practice and know-how of reinforcing slabs and beams for large scale decks and flat plates in many consulting offices are slowly disappearing.
So what can be done, and how can we improve? More importantly, what can you do to save your client money and contribute to more environmentally-friendly concrete solutions?
1. Know your trade
It sounds obvious, but it behooves all structural engineers and draftspeople to invest in and know their trade. Know your governing design code inside and out. Learn and memorize all the little clauses and fine print that define spacing, lap lengths, development lengths, covers, and so on. Importantly, learn the minimum requirements. All engineers are taught where they need to put steel; they often ignore or miss the parts of the code that tell you when and where you don’t need steel.
2. Know and understand where slab and beam reinforcement is not required
Too many engineers/draftees draw every bottom bar spanning from supporting wall to supporting wall (or beam to beam), ignoring or overlooking the concessions that allow you to curtail a percentage of the bars shy of the support.
A classic detail, sadly falling into dis-use, is to specify bottom bars shorter than the span, and then stagger them alternately – illustrated below.
Used correctly, this detail complies with the design codes, satisfies strength in positive bending, and has the potential to save tonnes of steel on large jobs with multiple spans and multiple storeys.
One of the worst and most frequent examples of over-specifying unnecessary steel is when it comes to shear stirrup reinforcement in beams. Shear is an issue near the supports; usually less so in the middle of spans…and yet beams frequently get specified with just the one, single stirrup size and spacing being called up universally along the entire length of the beam. Surely a diligent engineer would specify closer centres near the supports to deal with the higher shears, and then relax the spacing in the middle spans where the economy can be realized? Again, the tendency of many firms to save fees and costs internally by leaving off beam elevations robs them of the opportunity to realize such economies and to actually deliver on what they promised their clients, i.e. to save them money.
It’s both ironic and concerning that developers and clients will engage a cheaper engineer to save a few thousand dollars on consultants’ fees, then accept designs that cost them many times more than that due to inefficient design and detailing.
3. Make your designs fit for purpose
Engineers tend to get their back up when accused of being conservative or over-designing, but there’s blatant hypocrisy in such defensiveness when you look at some of the reinforcing plans being produced out there. This was touched on earlier, but putting a blanket specification of N16-200 across the entire slab when it could be better tailored is just being lazy and inefficient. Surely there will be zones where a smaller bar or greater spacing could be employed to good effect? Yes, it’s probably insignificant on a single house slab or a small pour. However, on larger projects with multiple bays or multiple storeys, the savings being wasted or thrown away can be massive.
There are some ridiculous lines peddled to justify such specifications. “N16-200 is easier for everyone to walk across” is a common one. “Switching to N12’s would have made it more confusing for the steel fixer” is another. "The savings in tonnage will be lost with increased fixing time" also gets trotted out. (An argument which holds little water if we're serious about reducing embodied carbon). But such dubious justifications ignore an inconvenient truth: You’re being paid by your client to do your job, which is to optimise the design and to devise the most efficient solution. Let’s do our jobs.
The old school technique of staggering alternating, off-set N12-150 same-length top reinforcing bars over supports to then achieve N12-300 in the midspans is a classic method of simultaneously achieving design efficiency; bar length efficiency; steel-fixing simplicity; and drafting simplicity. Better still, if possible, take advantage of stock lengths and call up the bar length on the plan.
4. Take advantage of stock lengths
Steel reinforcement bars come off the production mills in long, continuous lengths, and are then cut and supplied in shorter stock lengths. In Australia, standard stock lengths for most bars are 6m and 9m. 12m and 15m lengths are available as stock, but there are transportation issues with these.
Many engineers will mark-up a reinforcing plan “by eye” and – particularly with top reinforcement – draw the bar lengths to suit naked eye judgement of where the points of contraflexure are, or where the “0.3 x Ln” point sits, relative to the supports. This can result in seemingly random bar lengths of (say, for example) 5.65m being sketched on the drawing. This gets scaled off the sketch by the CAD draftsperson or scaled at the scheduling plant at 5.7m long; the specification goes through to the production line for the required quantity of 5.7m long bars; these get cut from 6.0m stock lengths, and all of the 0.3m off-cuts go to waste! Had the designer drawn and specified the bar at 6.0m long originally, there would be no waste and no additional carbon footprint with the off-cuts being melted down and re-rolled.
5. Know your materials-handling limit
On large reinforcing decks with large spans or large footprints, there is a tendency for engineers/draftees to draw long bars – often completely oblivious to the length of bar that can realistically be handled and delivered to the site. I consistently see drawings showing bar lengths of 10m or more for projects in domestic suburbia, which overlook the fact that 9m is often the maximum length bar that can be put on a standard delivery truck that can navigate around the streets of the suburbs. The schedulers or steel fixers will subsequently take it upon themselves to splice and lap two shorter bars to achieve the 10m bar drawn on plan, and the odds of that splice being located in a sympathetic location or an area of low stress are….slim.
Yes, there are recent changes to the design codes that, in certain circumstances, require us to put more steel in our slabs than was the case, say, 20 or 30 years ago. A better understanding of creep and long-term deflections, plus a nod to combating shrinkage and crack control has also led to codes stipulating higher reinforcing rates than was the case in decades past. But that’s not to say we can’t be smarter about how we detail, nor still achieve efficiencies and economy where valid. So let’s do it. Please.