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“What are we building here, mate – the Sydney Harbour Bridge???”
It’s the most common question asked of every engineer when he or she gets to site. The moment the steel fixer believes the slab has too much reinforcement, or the fabricator thinks all the steel beams are bigger than they’re supposed to be, the engineer is instantly accused of designing the Harbour Bridge all over again!
Stereotypes aren’t always appropriate or deserved, but the general stereotype levelled against engineers is that we’re all a conservative bunch who over-design everything. Being over-conservative with our designs or using overly-conservative factors of safety, the presumption is that engineers are risk-averse and too busy covering their backsides than designing efficient structures.
Is there any truth in this? Is the criticism warranted, or is there a bigger picture at play here?
It’s a tough tightrope for engineers to walk: You’re criticised if a structure is too thick or too heavy – and yet you’re sued if the floor ends up being too bouncy, or – worse still – jailed for criminal negligence if the structure collapses and people are injured or killed.
More interestingly, there seems to be a recently evolving perception amongst some in the architectural and building community that engineers are more conservative than they used to be. Or that structures are bigger than they once previously might have been. A great example of this is with concrete slabs. Architects who were working with engineers in the 1980’s and 1990’s remember when 160mm thick slabs used to span four or five metres, and then question or criticise engineers when the same slabs today are now being designed as 200mm thick!
So what’s changed? Is it the materials? The design codes? The factors of safety? Is it the engineers’ skills and abilities? The answer might surprise you…
First of all, let’s come from the other angle and look at what’s improved in the construction industry in the last few years:
Quite simply, concrete is better today than it was in decades past. Higher strengths can be achieved, and technology exists in the form of admixtures and additives that improve the concrete’s workability, performance, and durability. Strong advancements have also been made in the field of waterproofing concrete. (Although be under no illusion – all the Xypex or Caltite in the world won’t make your concrete waterproof if it cracks excessively due to shrinkage or flexure).
Similarly, the yield strength of Australian-made reinforcing steel increased by 25% from 400MPa to 500MPa in 2002, allowing us to squeeze extra strength out of our concrete slabs, beams, and columns.
Furthermore, the concrete design codes have changed several times since the 1960’s and 70’s, and the code now stipulates higher strength concretes than what was previously the case. This is generally in response to durability and exposure issues but, in real terms, suspended slabs are generally 32MPa as a minimum these days, whereas 20MPa or 25MPa might have been permissible previously.
Steel and Timber
Australian-made steel is stronger and more consistent and efficient than it was in decades past. The yield strength of most hot-rolled products (i.e. Universal Beams, PFC’s, angles, etc) was increased from 250MPa up to 300MPa and 320MPa in the early 2000’s, allowing engineers to squeeze extra capacity out of their beams.
The stress grading of natural timber has also become more efficient and reliable since the late 1990’s. Visually graded pine used to be assigned either F5, F7 or occasionally F8, whereas the new machine grading technology (MGP = Machine Graded Pine) allows pine to be more reliably graded and for higher strength pines to be sourced for structural purposes. (e.g. MGP12 or MGP15).
The last 15 years have also seen the cost of engineered wood products (e.g. LVL) come down tremendously, and so LVL joists and beams are now being used extensively throughout the industry. Compared to natural pine joists, LVL’s are straighter, do not warp, and can have better performance characteristics for strength and deflection.
The computer and software tools available to the engineer today permit significantly more detailed, accurate, and advanced analyses than was ever possible in the past. Even in just the last 15 years, the development and introduction of affordable Finite Element Analysis (FEA) software and non-linear analysis programs into the humble consultant’s office have allowed engineers to analyse concrete slabs or plate elements far better than the old 2D strip analyses we worked with up until the late 1990’s and early 2000’s.
This means we can push the envelope with our structures and get results that assure us the beams and slabs can span further or deflect less than our hand calculations would otherwise have told us.
So, with the above as background, why is there a perception that engineers are over-designing or being more conservative than might previously have been the case? When it comes to concrete, the reason is pretty simple: The design codes we used in the past were……flawed!
Reinforced concrete has been around for over a century, but it wasn’t until the mid-1990’s that engineers started to notice and appreciate the long-term properties and characteristics of concrete structures.
To avoid a technical or verbose description, we’ll explain this in really simple terms: Suspended concrete slabs and beams effectively deflect twice. First, they deflect the moment that they are loaded up. (We call this the initial or short-term deflection). Then, they continue to deflect further under sustained loading over time. We call this the long-term or creep deflection. Creep deflection can continue for up to 30 years after the structure is poured, and its magnitude is usually greater than the initial deflection.
To put some simple numbers to this: A freshly-poured concrete slab for an office building might be fitted out with ceramic tiles on top, ceiling panels suspended underneath it, furniture, glass and wall partitions, and a compactus unit. The slab might deflect 15mm under this newly applied loading. Then, over the next 20 to 30 years – even though the load on the slab has not changed or increased – it might deflect a further 25mm! The final deflection of the structure (in this example) ends up being 40mm!
Engineers thus have to proportion their concrete slabs and beams to account for this long-term deflection. There’s no point having a roof slab that is flat for its first five years, but by year 20 it’s drooping and sagging in the middle, collecting water and causing serious ponding problems!
By the 1990’s, engineers noticed that many of the flat plate slabs designed and built in the 60’s and 70’s were deflecting excessively. Slabs were sagging and drooping significantly more than their calculations had predicted, causing all manner of problems for the building occupants. (Occupants felt themselves walking downhill and uphill when walking along corridors in office buildings; furniture and desks in office fitouts would not sit level; floor tiles and finishes cracked; flashings at slab and wall interfaces were torn; ponding occurred on roof slabs; cantilevered balcony slabs would no longer fall or drain in the right direction!).
Even today, Partridge gets approached three or four times each year to advise and assist with repairs and strengthening to concrete-framed buildings that were built as recently as the 1980’s and 1990’s but have suffered excessive deflections since their original construction.
The culprit here was the formulae for stiffness that engineers used to calculate and predict deflections. Engineers were designing correctly to the concrete code (AS3600 and its predecessors), but it turns out the formula in the code itself was inaccurate and could under-estimate the long-term creep deflection. The first attempt to correct this in Australia was in 2001, when the stiffness/deflection formula in the code was adjusted – resulting in slabs and beams becoming thicker or deeper than they might previously have been. The formula was tweaked and improved again in 2009 as the industry responded further to ongoing research, testing, and field observations.
Similarly, today’s concrete code stipulates significantly more reinforcement in slabs than was previously the case. Up until the early 1990’s, it was common for slabs to only feature top reinforcement in the negative bending regions over supports, and so large portions of a suspended slab would have reinforcement in the bottom layer only. However, with the benefit of hindsight and realising that this could lead to increased shrinkage strains and thus greater deflections, it’s now almost de rigueur to specify top reinforcement throughout the slab. It’s not a case of engineers being more conservative or simply adding more steel to a slab so that we’ve got something to walk across when we come to inspect! No, rather, today’s code has a better understanding of how to combat and resist concrete cracking and shrinkage issues, and it thus stipulates higher reinforcement rates than previous versions of the code used to require. And we haven’t even yet mentioned the more stringent requirements for fire rating and durability that exist today!
What this means is that – generally speaking – concrete slabs and beams being designed today will be thicker, deeper, and more heavily reinforced than what engineers would have designed them to be prior to 2001. (And then subsequently again in 2009 as the concrete code continued to become more conservative so that it dealt appropriately with long-term deflection).
The full benefit of this won’t be realised for another 10-20 years, when maintenance and remedial repairs on excessively deflecting slabs will hopefully start to become a thing of the past in Australia. In the meantime, don’t give your engineer too much grief if your slab is an inch or two thicker than you used to see 20 years ago!
[BTW…the above discussion does not imply that all concrete floors built before 2001 will have deflection problems. There are other parameters and variables that influence deflection (e.g. span, reinforcement, strength, and applied loads), and so not every floor will be susceptible.]
Of course, there are other factors at play here, and several other good reasons why an engineer might design and size a member larger than you’re expecting. A degree of robustness and flexibility is required to the design, and both structures and engineers have to roll with the punches as things unfold on site…after all, we frequently encounter design changes during the construction of projects, and these changes can compromise or impact the strength and performance of the structure. Here are just a few solid examples of change requests that we regularly encounter on projects once construction is underway…
- Floors that were to be carpeted are changed to being tiled, or steel-sheeted roofs are replaced with terracotta tiles (thus significantly increasing the weight on the structure);
- Lightweight stud walls get changed to brick;
- The client changes their mind for the bathroom and elects to add a huge (and deep!) spa bath;
- Supporting walls get moved by a few hundred millimetres, resulting in increases to spans;
- A fire-separation issue is identified, forcing the glazing to be fire rated and requiring the glass to change from being 6mm thick to 20mm thick;
- Architects elect during the build to recess pelmets or lighting tracks into the slab’s soffit, thus reducing the slab’s strength;
- Plumbers elect to chase or rout a pipe into the guts of the slab rather than suspend it under the slab;
- The client elects to add a screed or topping to the slab to accommodate hydronic heating;
- Ducts or risers not shown on the architectural plans have to be cut or penetrated through the floor structures, resulting in unforeseen voids and weaknesses;
- The builder asks if the slab can be reduced by 10mm because he got his levels wrong;
- The concrete gets poured on a hot day and the truck driver adds water to his mix, thus weakening the concrete;
- The architect requests a penetration through the beam so that services can pass through the beam rather than hang underneath it;
- And….. well, you get the idea.
At Partridge, we never endorse or encourage over-design, but a design should be robust and flexible enough to cope with all of the above compromises and manipulations that occur during construction. If the architect or builder wants to introduce a last-minute change on site, it’s a better outcome for everyone when the engineer says “Yes, I think we can accommodate that”, as opposed to “No, we’ll have to re-design everything” or “You’ll have to demolish this and start again”, and the Client then gets hit with a variation for the additional design and building costs. Not to mention the costs of delays to the build.
“Mankind’s construction history has to been to build something. And, if it falls down, we re-build it again, only this time a little bit stronger than we did before.”
Most of the above discussion has focussed on concrete, but similar developments and changes have occurred with other structures and materials. Mankind’s construction history has to been to build something. And, if it falls down, we re-build it again, only this time a little bit stronger than we did last time. That is essentially what has happened with Australia’s design codes as the industry responds to structural failures, collapses, and research. As some real examples, the code governing the design and construction of retaining walls now results in bigger footings and stronger walls than was the case twenty years ago – all because of older retaining walls either leaning or falling over in the last two decades. Design wind loads have generally also increased in response to more data being collected and researched, and peak storm events occurring more often. And strength and performance requirements for structures have become more stringent (i.e. conservative) in response to changes in the earthquake code, and for fire-resistance levels…..both as a result of disasters and tragedies that have occurred in recent decades. No, it’s not the engineer, but the codes we work to are dictating the play.
Structures (and hence engineering design) are also responding to changes in architectural trends. For example, open plan living means there are less supporting walls for suspended slabs and roofs to sit on than would once have been relied upon. The desire for higher ceilings in houses drives a need for floor zones to become thinner. Houses now typically feature more glass to their facades, triggering more stringent deflection requirements to the transoms or floor beams that support them. Concealed box gutters now cut into roof structures. Off-form concrete is flavour of the month again, as well as polished concrete floors, thus putting pressure on engineers to make sure the concrete doesn’t display shrinkage cracks. No one wants to see control joints or slab edges or have step-ups into bathrooms, etc, thus requiring more expensive detailing by the engineer. All of these architectural developments and trends require structures to be more robust to accommodate them, and thus structural depths and sizes may seem to be bigger than what the industry typically accepted 20, 30, 40 years ago.
Ultimately, engineers, architects and builders have to work together to achieve multiple common objectives: An efficient structure; an aesthetic and functional outcome; and an economical construction. Oh….and, it has to stand up for the next 50 years 😉 We’ll do our bit to help.
Hope this has been an interesting and informative read for you. Feel free to drop us a line or leave a comment down below if you’d like to discuss any aspect we’ve raised above.