New Loadings Standard

It’s not readily apparent from the outside, but the building industry is going through a few changes at the moment. For the last couple of years there’s been a bit of focus on the so-called “leaky house” problem, and how to fix it. We’ve had a new Building Act to cope with this, which has included such measures as registration for builders. Now, it’s been the turn of the engineers to try and set their house in order with the release of AS/NZS1170, the Loadings Standard.

This is vitally important. Forget a bit of dry rot, or some piddling energy inefficiency, the Loadings Standard is the basic chasis of the building industry. It is ultimately the basis for day-to-day performance standards and worst-case design. The previous code was published in 1992, a hefty amount of time ago. Work began on the new one in about 2000, and I was already adopting provisions from the replacement in 2002, and now the full thing is out in mid 2005. But it’s not being adopted as the basis for design until concomitant changes have been made to all the other structural standards.

As with all matters of subtle importance, I doubt this will be mentioned in any newspaper or TV news or on the radio at all. Maybe a 5 second interview on Morning Report, but I doubt it. The reason for this is that the Loadings Standard is buried deep in the heart of the machine, and ordinary people just don’t ever need to directly see its workings. The other reason is that it is a bit of news that everyone who needs to know, does know; through industry representations and by fulfilling their ethical obligations to keep abrest of developments in the field.

Anyone who’s gone through the process of building a house in Wellington (or even conducting significant alterations) will know that the process has changed a bit, become more onerous. Mostly due to the Leaky Houses things. Some of you may have read my rant about it a few weeks ago. When I started out of university, I would provide about 5-10 pages of calculations for a 200 sq m house. Now I tend to send out about 50 odd. That’s additional cost right there, and the benefits are a bit hard to see. So far as I know only 1 of my houses has needed any kind of significant remedial works, and I churned out an average of one every couple of working days for 3 years.

So, now that I’ve piqued your curiosity, you’re doubtless asking: What’s Changed?

Well, there are three different things we worry about as structural engineers: Vertical Loads, Horizontal Loads and Architects. Now one of the major complaints we get from architects, and builders for that matter is “why does this have to be so big/expensive”. They’re talking about steel beams, they’re talking about reinforcing, they’re talking about bracing panels, they’re talking about everything which cuts their profit margin compared to a simpler house. I haven’t yet met a builder who doesn’t have a “the engineer said we needed steel/whatever, and I knew better, and it works”. The difference in our perspective comes from the Loadings Standard.

Above I talked about the day-to-day performance, and the worst case. The day-to-day performance is all anyone ever sees, and the worst case is what the Loadings Standard is all about above and beyond the experience you get as a builder.

For a while now we’ve been comparing the results of the present code, and the new code, and what we’re finding is that the new code is producing wind loads significantly higher than the old code, and so despite being available since 2002, nobody in wellington has adopted the wind provisions. The question everyone asked themselves was: why are the new wind loads so much higher? The adoption of the larger new loads carries with it the implication that everything we did in the past was wrong. Now, unlike Builders, Engineers are governed by a code of ethics which puts as its core focus that we get things right. A mistake for us is a hazard to other people. Who wants to look back at a body of work and say “so, all wrong”. Nobody. Okay, there’s no legal implication, because we conducted ourselves using the best information we had at the time; but there is a personal implication there.

But, I’d like to think that the new wind standard is not in fact “more correct” than the old one. I’d like to think that in large part its more stringent provisions are a response to wider building industry issues beyond the control of one component. To an extent, that would require the new wind provisions were prescient, because they predate the “Scarry letter”. John Scarry is an auckland structural engineer who wrote an open letter to IPENZ about 3 or so years ago with dire warnings about the standard of work being undertaken in many firms around the country. In fact, attacking structural design practises generally. It was an influential letter, and tied into the whole Leaky Building thing. How true each of his particular allegations is I’m not personally sure, but some of the stories you hear about some designs in Auckland do cause concern.

Well, the new code is here, albeit an unwanted bastard child. It is accompanied by a paranoia in the building industry that your house will leak, and you’ll be sued. Only today I was checking some wind loading stuff for a house which is leaking, potentially because of an inadequate provision for wind loading. But, my concern is that this new provision for wind is the wrong tool for fixing what are not really structural defects in houses. The causes of Leaky Buildings are numerous, but a lot boils down to someone looking at the design and construction of the house with care and an eye for problems – not something you can regulate with a design standard.

New standards always raise issues when you start to compare the results with previous designs. I designed the structural refit of a 1930s reinforced concrete building to replace some lost strength from extensive alterations, and bring it as close to current code as possible. As part of the assessment in such work you start by evaluating the present performance of the building, and I was surprised to see just how poor its chances were in an earthquake because it had actually survived several which should have pushed it close to its capabilities.

Edward Hoover said: The role of an engineer is to build structures with shapes we cannot accurately model, out of materials we cannot precisely control to resist loads we cannot accurately predict, while concealing from the public the extent of our ignorance. 1170.4 – the Wind section talks about design return periods for wind of 500 years, but what this really means is a big-ol’-guess with an impressive number stuck in front.

Returning then to my opening statement, that this was an opportunity, or even an onus, to sort out our ideas on one of the three Hooverite entities, my considered opinion is that we need an entirely new approach to design. The stocastic approach adopted at the moment is great for the day-to-day loading which we can pretty well confirm with experience, but for the worst-case, we tend to come up with designs which are never proven to be either correct or inadequate. Many structural failures, such as the dicore floor failures at Northridge in ’95 are as much due to detailing issues as a lack of understanding of earthquakes; most other failures are due to non-structural issues. In terms of design, it seems like you need to think about the structure’s potential failure mechanisms, and detail to avoid them irrespective of the load which may be applied. The failures in Kobe, for example, were usually obvious to the untrained mind: big heavy rooves without much wall below. Turkey’s damage was largely due soil issues which are impossible to perfectly generalise, or a blatant disregard for commonsense building practise.

Largely then, we’ve missed the boat this time in terms of getting towards engineering better homes, all we’re going to do is carry on the way we’ve been going and hope it works out better in the future than it did in the past. From this flawed basic foundation we build our entire approach to building.

Advertisements
This entry was posted in Engineering and tagged . Bookmark the permalink.

3 Responses to New Loadings Standard

  1. jezebellion says:

    Hi! Remember me?

    • mashugenah says:

      Of course. πŸ™‚ Wow, the internet is sure handy for things like that. πŸ™‚ What are you up to these days?

      • jezebellion says:

        Working seems to be most of it…I’m working on the 2006 Census, it should be a good one πŸ˜‰ Um, singing in choir with Ivan still, or again, really. Involved with someone else now though! Shayne and I are going to be on currently screening quiz show “Cash Battle”, I think on July 28th, look out for us πŸ™‚ Still see Shayne a lot, and Jeff every now and then.

        How are things with you? You quiz engineering for English? It’s good to “catch up” with you πŸ™‚

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s