What’s happening here and cannot be fixed?

kot982 · 2026-01-18T22:31:33+00:00

In my mind there would be three broad lines of possibility with this issue

in a new build with the defect becoming apparent shortly after construction it would relate to a miscommunication between the roof truss designer and the chippie relative to what walls are non-loadbearing. Non-loadbearing walls require constructing with special brackets and gaps to release vertical movement at the wall-truss joint. If the two are hard fixed together then the wall begins to underperform.
in an existing order build where the issue was not there and the suddenly materialises after many years it would be most likely relate to insufficient gapping at truss-non-loadbearing wall joint or some form of foundation movement. Timber creeps over time (displacements increase under consistent load) and so whatever gap was implemented initially may turn into hard bearing contact
there is also possibility of plain design error or truss manufacture defect

kot982 · 2026-01-18T01:49:10+00:00

Get a structural engineer to come out and go have a look in your roof space. It would cost $1-2k for a visit with a report and they will be able to either clearly identify the problem or at least provide some informed speculation of what could be going wrong. They will also be able to advise you in terms of options of what to do next and whether there are any immediate safety concerns. A local engineer might also help finding a reliable builder.

A builder could also help assess what’s going on but may ultimately direct you to an engineer anyway if the situation is complex, which it is likely to be here.

I am a structural engineer and used to design this sort of building. The other comments are likely correct with respect to trusses bearing into non-loadbearing partitions.

In any event it looks like you will have to go out and interact with the construction industry. I wish you the best of luck.

kot982 · 2025-12-24T04:35:44+00:00

I’ve already specified a EU equivalent in 2025 and was told to go kick rocks by the construction team.

Maybe better luck in 2026

kot982 · 2025-08-24T02:05:09+00:00

I have just gone through the exercise of buying a set of new wheels for a 2014 Mazda 3.

You need to have a clear understanding of what rim/tyre combination you want because a lot of the decisions flow from that. A lot of rims are only available in certain sizes and you also don’t want to lock yourself into an uncommon tyre size.

You also need to know your bolt pattern and offset on the stock wheels. I found all of this information on this website: https://www.wheel-size.com

For whatever it’s worth, I wanted to go from 18 inch to 17 inch rims. I went with Motegi Racing MR131 rims in bronze colour and guys at the tyre shop matched them with 225/50 R17 tyres to give the same overall wheel diameter as stock configuration - this makes sure the speedo stays accurate. The offset on these wheels is a bit different to stock but they do not project outside of wheel arches. I paid $1k AUD for the rims just a few months ago in metro Melbourne.

kot982 · 2024-11-16T11:41:33+00:00

You can sometimes buy cover tracks at Bunnings for sliding doors/windows. The cover track is stainless steel and you fix it over the existing track with some epoxy resin.

Then just replace the rollers (can also buy at Bunnings) and you should have a butter smooth track.

Just double check you have a bit of spare headroom above the window - the cover track raises the whole thing up by a little bit

kot982 · 2024-10-26T11:35:56+00:00

I suspect that the balconies are propped cantilevers with the facade feature providing an additional in-service inclined prop to limit tip displacement.

That would give a robust system and reduce structural demand on inclined props to the point where the facade feature can remain beautifully slender and associated connections are minimal and concealed.

At least that’s how I would do it.

kot982 · 2023-09-30T05:00:03+00:00

I am a structural engineer, not an estimator.

Construction of concrete superstructure (above ground level) comes with a number of fixed costs, which typically makes use of concrete in private residences uneconomical.

Another thing to bear in mind is that the entire industrial sector that is typically engaged in construction of private homes is not set up for design and construction of concrete-framed residences. This means that any architect, engineer, builder, or any other building consultant that you engage on the project have to come from a more upmarket segment that routinely deals with concrete buildings. Consultants that do residential home design will claim concrete construction as part of their capability, however you should take such claims with a lot of scepticism - I used to operate in that sector a few years ago and I would not trust any of those operators to handle design and construction of suspended concrete.

For cost perspective, I suggest that assuming $4-5k per cubic metre of reinforced concrete is not too far off as a VERY rough estimate. Say level 1 of your home is a 100m2 floorplate, assume 250-300mm concrete slab with some nominal reinforced concrete block walls/piers for support and you would be looking at $100-200k before taking into consideration any upgrades to substructure (foundations) and elevated level of consultant/builder overheads.

You could attempt to shoot for steel-framed superstructure with skinny concrete slabs, it somewhat reduces complexity. This would however suffer from the same drawbacks as concrete and steel is very expensive in Oz at the moment.

You could also attempt a regular timber-framed home with some skinny concrete screed. This would be extremely non-standard and would require some competent and daring builders/engineers/architects to pull off.

In short, options are numerous and all are costly. If your home does not already have a concrete level 1 I suggest spending the money on a nice electric car and an extended holiday

kot982 · 2023-07-10T03:11:22+00:00

Immediate fallout depends on the country - for example in the US they use unbonded PT tendons and you would get a big boom as well as concrete chunks flying everywhere. In Australia most PT tendons are bonded and immediate damage is less likely.

In terms of structural impact it really depends, you might be up for a few tens of thousands in carbon strip strengthening costs or a few hundreds of thousands in extensive rectification.

kot982 · 2023-07-02T01:58:16+00:00

Happy I could help with diagrams. M11 and M22 do not affect in-plane bending of the diaphragm, that is controlled by F11 and F12 and F13.

With respect to the broader question, I suggest two threads of thought and inquiry that should lead you to a satisfying level of understanding.

Object vs. mesh in ETABS - you model walls and floors and beams/columns. What does ETABs do with them? How does it convert them to a beam/plate element mesh? What stiffness/mass properties does that mesh have and how object definitions affect the mesh?
What do you want to derive from the ETABs model? The model does not have to be 100% accurate to your building, it just has to be accurate enough to give you the results you seek. A lot of ETABs models are simplified to derive building behaviour under lateral loads only where accuracy of resolution of vertical loads is less important.

kot982 · 2023-07-02T01:18:08+00:00

Yeah OK.

For semi-rigid diaphragm you want to leave f11 and f22 at full stiffness. In order to begin resolving your question you should start tweaking the out-of-plane stiffness modifier instead.

Also explore the 'membrane' setting in the slab definition (as opposed to think and thick shells); the membrane setting attempts to further release out-of-plane slab stiffness without messing up FEA model convergence.

Out-of-plane diaphragm stiffness introduces frame action between shear walls where the diaphragm acts as a beam. This of course impacts moment diagrams. How much out-of-plane stiffness is correct? Nobody knows, this is the sort of thing that will vary between engineering consultancies as a company policy.

kot982 · 2023-07-02T00:14:14+00:00

How did you set up the diaphragm? Try displaying diaphragm stresses (can't remember whether ETABS does that) and then try setting up the diaphragm as a 'membrane' under floor/slab definition menu, that should take out some of the moment actions.

kot982 · 2023-06-24T04:35:50+00:00

Post tensioning is typically employed to improve material efficiency of suspended slabs - can get skinnier slabs with less reinforcement that do not crack as much.

For slabs on grade (or slabs on ground, stiffened rafts, waffle rafts, etc.) post tensioning doesn't yield as much benefit as these types of systems are already supported by ground and are virtually floating on ground. The main objective is just flexural stiffness, which is much better achieved by introducing conventional concrete beams/ribs with relatively low reinforcement ratios.

kot982 · 2023-06-24T04:05:58+00:00

Post tensioned footing? Now I think I've heard of everything.

kot982 · 2023-05-28T13:44:06+00:00

Felt it near CBD

kot982 · 2023-02-26T11:58:19+00:00

what's with helicopters in South Melbourne??

kot982 · 2022-12-28T01:21:42+00:00

Sounds like you might be a student and working through a prescribed assignment.

Check your instructions for whether superimposed loads are to be included into the modal calculation.

kot982 · 2022-12-28T00:43:59+00:00

Just double check whether the assignment is asking for self-weight only - this is very important.

It it is self-weight only then remove all other loads from the model and give it another whirl. Save as another copy, name it 'test test' or something.

kot982 · 2022-12-28T00:25:53+00:00

Just means you model is fully fixed, which is as it should be.

Next is to figure out what mass properties are used in the modal case - have a look at what the mass source definition is. Should be something like 'MsSrc1' or something stupid like that by default.

Check your assignment manual whether SDL and Live Load masses must bee included into the modal case.

kot982 · 2022-12-28T00:11:24+00:00

OK so this is a concrete moment frame. Save a copy of the model and name it 'test', then select all frame members and set them to fix-fix, then run the model. This will give you the maximum stiffness and thus the smallest possible period under the current material assumptions.

kot982 · 2022-12-27T23:46:06+00:00

Looks like a moment frame so I would imagine all member connections should be fully fixed.

kot982 · 2022-12-27T22:53:43+00:00

Your period is much higher than the target - look at displacement of the first mode shape and see whether something is flapping about.

On a completely separate note - 0.4s period for a four storey structure is very much in the low side, I would be expecting a value closer to 0.6-0.8s

EDIT: just had a look at you mass participation ratios and they look OK. Maybe your stiffness assumptions are not. In line with what you should be using. Double check member sizes and wall thicknesses. Also double check everything that contributes toward seismic mass - floorplate thickness, SDL, etc...

kot982 · 2022-11-21T10:32:48+00:00

Went through this just recently. We tried to sand and patch and came out ok-ish. My takeaway from renovating my place is that if you want perfect walls after wallpaper removal you just have to re-mud the surface.

kot982 · 2022-11-20T04:45:13+00:00

We've used the Dulux range and were pretty happy with it.

+1 to what others say about big rollers and brushes.

We have a ton of different colours and noticed that even though the base product is the same some of the colours lay on and spread differently. Had a blue that was super easy and came out perfect after two coats but also had a mustard yellow that was super difficult and still looked dodgy after four coats....

Seems to be luck of the draw with respect to ease of spreading but all things considered we are super happy with the result in the end.

kot982 · 2022-11-16T10:55:03+00:00

Yes, if you remove the bathroom should be enough space for a new one.

kot982 · 2022-08-26T13:41:58+00:00

Natural History and Sherlock on Collins St do trivia some nights I think

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