Have you ever had conversations like this
ahh this makes no sense i've never done it like this before well let's call the engineer
hi Brendan speaking hi Brendan it's Bill and now i'm working on this job in Camberwell
look at these plans and then i've never done like this before it's
really confusing how come we can't do it the other way that I sent through to you
I've done it a hundred times it's so much easier yeah look I understand but you know sometimes
situations are slightly different so sometimes the answers don't come out as we expected
yes we do want to try and make it easier to build but not every situation is the same unfortunately
you're just being conservative whenever designing can you pull back some of those conservatisms and
actually get to something that I can actually build it doesn't matter which side of the fence
you're on i'll be going through why people have the feeling that engineers over designs things
why this is not the case and how you can deal with these situations my name is Brendan your
structural engineer now let's get into it there's a famous engineering quote that I absolutely love
and I think it completely sums up the situation about why people think we over design things and
this is from dr A.R Dykes which says engineering is the art of modeling materials we do not wholly
understand into shapes that we cannot precisely analyze to withstand forces that we cannot
properly assess in such a way that the public has no reason to suspect the extent of our ignorance
when we first break down this quote it starts to talk about how we cannot properly assess the
strength of materials as materials come in a range of strength it doesn't always come out of 500
grade steel or 32 npa concrete there'll be a range of strength of what we get this is when we're
first analyzing materials we're assessing down at the bottom end of the bell curve of the string as
we want to make sure that the majority of the material that we're assessing is higher than
that so it depends on the variation the material that you're looking at if you've got a higher
strength material which has got more control like steel potentially you'll be closer to that top
end median strength however when we're looking at concrete concrete is very well based on placement
how far it's coming have you put additional water in it this can all affect the strength of concrete
and it really just depends on the controls that you have in place to ensure that is the case
so that's where the first part of conservatism comes in as an engineer is not knowing precisely
of the material that we're putting on to site what the strength is that we're going to achieve
the next part of that quote is about analyzing shapes that we cannot precisely analyze because
when things come to sight it is not a factory so not everything is going to be in the same location
not every band beam is going to be the exact depth not every bit of reinforcement is going to be at
the height that you need to have it so you need to make sure you've made some allowance for this
so when we're designing we typically need a construction allowance of plus or minus 10 mils
so this means that the structure will potentially have a lot more strength in it because it's
potentially 10 mm lower but likewise it could have the other problems as well where it's 10 mm
higher so it could be slightly weaker it's not the fact that the builder doesn't want to put in
the right location it's the fact that sometimes site constraints make it hard as it's really
too hard to place the reinforcement precisely in the location that we need to have it so when we're
designing structures we need to make allowances for this analyzing structures is all about
statistics and to help the youtube algorithm don't forget to click the like button not only does it
help me out but also allows this content to get out to more people the next part of the quote is
about analyzing forces we cannot properly assess when we're assessing the loading of structure we
cannot control it after it's built so we need to put safety factors into the amount of force that
we're assessing so if we've got something that's really precise we know how big the slabs is we
know how big the columns are we know what type of screed and other services that are going into the
building these are considered dead loads and we can precisely know roughly how much of this dead
load is going to be in the structure so typically we'll have an amplification of that dead load
but it only be 1.2 so it's on the low end as we know how much that load is going to be but for
example if we've got live loads things that are moving in and out of the structure people and
other elements like that in in the building this is really just the statistical average of what we
think is going to be there so on these loads as we don't have as much confidence in them as we don't
want to underestimate the load that's going to be on the structure we put a higher load factor there
so we've got this load combination that's amplifying the load that we think is going to
be on the structure and these lead people to think that over designing structures there's potentially
done something easier before and it's worked in the past but not every situation is the same
the other area where potentially there's a little bit more conservatism is also based on
the mechanism of failure so if you're looking into the codes we have a material safety factor that
reduces the strength of the structure based on its failure mechanism so if we're first looking at
fletcher it potentially has a high material safety factor as the structure gives some warning signs
we're looking more at a sheer failure it has a lower factor of safety as we don't want it to
fail in this method as it will be brittle and sudden and potentially show no warning signs
so when we're thinking about it that first factor of safety is that bell curve of the strength of
materials where we're down one end so the majority of the material will be higher than that strength
then we're looking at loading it has a similar bell curve but we put it up the other end to make
sure that we don't underestimate the load that's on the structure then what happens is we then add
a material safety factor or a load amplification factor that moves these points further apart
and this is where we've got a bigger range between that strength of the material to
that failure mode so even though people may think we're over designing something we're potentially
only just meeting the code and this is where as engineers we need to make that judgment
call and talk people through why we need to do things certain ways and we can see there's also
an overlap so maybe sometimes everything was done perfectly the strength of materials there
however the structure just happened to see an over strength the material just happened to be weaker
so at some stages there will be statistical average which is in the hundreds of thousands
or even more potentially where nothing has been done wrong completely but the structure could
see a failure still just because it's worked in the past doesn't mean it will work in the future
now is there some engineers that over design things well yes and maybe this is where some
of the concerns from builders come from but when we're looking at that we've got that big range
from that strength of material to the actual load factor we realize that we can bring our
structures up to a 90% utilization or slightly more yes there is some areas where you want to have a
little bit of redundancy such as connections or other elements that can be super critical where
you may throw the materials but on your typical floors are areas where it's less critical you
potentially want to bring those utilizations up as hard as they can go so when you're talking
to builders this is where you're trying to take them on the journey about why you needed to design
something a certain way why things are that way sometimes you may have just had repetition across
the structure as you wanted to keep it simple to make it easier for the builder to build
and if the builder's complaining there may be some other reasons behind that complaint not just the
amount of money they're going to spend in the material that you're putting in the structure
maybe it's too hard so maybe by listening to them you may be able to come up with a different method
that both achieves the strength that you need to have but also achieves the ability of the
builder to build the building then sometimes you just get those people that just complain
and then really all you can do is say look this is the way it needs to be built we're not going
to take responsibility if you don't build it this way and give them the guidance that this is the
reasons why you need to be there but wherever possible trying to educate them so when they're
building a structure in the future they have some knowledge about why things are done a certain way
and gives them the confidence of you about how you've approached your design and as a builder
when you're looking at the structure just because something works some way doesn't mean it always
will and not every situation is the same as there will be some variation that maybe means the load
is flowing a different way well the structure is performing a slightly different purpose hence why
it's been designed in a certain way now when you're analyzing buildings it will also depend
on what stage of the construction work you're at if you're at scheme stage typically you will have
a little bit more conservatism in your design as there's things you do not completely understand
you don't know where everything's going to go are you going to add additional floors
where the loading of the structure is going to be as you don't want to make sure you're at that
100% utilization of the structure as any minor change will mean a variation where the people
you're working with always going I just changed one little thing and you're constantly changing
something so you want to make sure that you can allow for those changes and the further you move
through that project scheme the closer you will push the structure to being that 100% utilized
so when you get to that construction docks you'll push the structure as much as you're happy with
trying to push into that 90% range to make sure the structure is efficient as possible if you enjoy
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these many members here and as always stay safe keep learning and i'll see you next week bye