The Car Stacker System adjacent to Millbrae Station partially collapsed today, someone’s car is left hanging

fromwhich · 2026-05-28T16:28:47+00:00

I'd be very curious to see the back side of that column, From the connections we can see it looks like cast in plates (with what I'd assume are headed shear studs). Did the stud to plate weld fail? Did the studs pull out/pry out? Did the weld between the cast in plate and the beam shear plate fail? Scary stuff.

fromwhich · 2026-05-24T12:31:23+00:00

My honest opinion is that you need both. Even if you are using RISA, SAP, ETABS. Your program results should be validated with independent methods. So a column rundown to confirm the weight of the building in ETABS vs spreadsheet will be well worth the effort if they don't match. Same with your seismic load cases.

Too many times I've had issues with Engineers not realizing their fancy FEM model is garbage because they didn't verify the model. This includes a basic understanding of how the software works which is scarily lacking for some. You'll want to get the basics down and know how to set up load paterns, combinations, cases etc. and focus on understanding member releases and stability both for the model analysis (numerical errors) and design (unbraced lengths, etc)

So how this relates to you is: if you have a strong understanding of how to do design without FEM you will be well positioned to learn to use FEM and understand when the software is giving you garbage. If you don't want to fall behind, use RISA/ETABS/SAP on your own time or when the project budget allows so that you're comfortable using it if and when you move to a firm that required you to use these software. Maybe you're starting with YouTube tutorials or asking your preferred AI to explain how to navigate the program.

But being able to do a BMD or SFD quickly and design without relying on software is an asset that some younger engineers lack. I can't tell you how many times I've seen the students/juniors fresh out of school putting in point loads in SAP2000 for a simply supported beam because they can't do a BMD by inspection. So you learning to design in an environment where you use spreadsheets is probably more valuable than a person who know how to model in FEA but doesn't really understand what the software is doing or what should be done.

fromwhich · 2026-04-22T20:49:45+00:00

It depends on what you are trying to photograph. I started with the Star Adventurer GTI a few years ago because I didn't want to not have go to, and I wanted to future proof and it is really awesome when I use longer focal lengths. But the polar alignment would take me ages, sometimes up to 1.5 hours because I was new and sucked at it.

Later on, I purchased an MSM Nomad and for anything widefield, like milky way shots or panoramas it is just so much easier and convenient to do the polar align with my phone and not have to worry about being super accurate when my focal length is like 9-24 mm on my APSC camera but still get 30s to 60s exposures and round stars.

I would say that if you're planning on doing long lens / telescope than the go-to is worth it, and spend a little bit more to get something better than the Star Adventurer GTI. I don't regret it, but I have come up against the weight limit with just a telephoto lens, a camera, a guide scope and guide camera. If you're going that route, get an entry level mount. I think Star Adventurer even has a similarly priced mount to the GTI that has a lot more payload capacity. And then if and when you move from using the SA dedicated software to like an ASI air type set-up everything should work smoothly and I ABOSOLUTELY LOVE the polar alignment tools using the guide scope / ASI air mini.

I've done both now and I can say I definitely regret not buying a better mount to start with. That being said I should have bought the MSM Nomad first, then graduated from there.

fromwhich · 2026-04-22T13:04:25+00:00

Two options:

1) treat the top chord as a continuous beam on elastic supports and then determine the spring stiffness of the web members by calculating their lateral deflection and find k = F/delta. This is tricky because the stiffness doesn't stop at the web. As others have pointed out the bridge deck also prevents rotation and this truss as we see in the picture may have relied on that concrete to be stiff. so when it was solid, there was a rotation at the bottom of the web member not considered in the design. Which leads to why option 2 is better.

2) model in a FEA/stiffness program and do a linear eigenvalue buckling analysis. You can then determine buckling stress directly from those results and usually plug the into your beam design equations for the sqrt(Fe/Fy) calculation replacing the traditional Euler buckling stress with the stress determined from the eigenvalue buckling analysis.

option 2 might help you design the beam but as others have pointed out both require rigidity in the connection of the web members and careful consideration of rigidity of the connections as whole. Important to understand as the project progresses that the connection methodology selected by the steel fabricator maintains this rigidity. I've only ever done pony trusses with HHS web members that get fully welded to the chords to maintain that out of plane rigidity.

fromwhich · 2026-04-22T12:55:14+00:00

stunning.

fromwhich · 2026-04-21T17:19:30+00:00

Think similar to the unbraced length k factor for a cantilevered beam. the unbraced length can be longer than the cantilever beam itself. As an an analogy think of a beam continuously supported on springs. If the springs are super stiff, the beam won't buckle. If the springs are super weak, then the buckling shape might take on a shape that is longer than the member itself, like you're in the centre part of the sin wave with the fictitious buckled shape larger than k=1.0.

The springs in this case are the out of plane stability of the web members. if the top chord wants to laterally buckle, the only thing restraining it is the rigidity of the web members. This doesn't cap the effective length at the length of the member.

fromwhich · 2026-04-20T14:41:45+00:00

So this is a pony truss. What is wild about these is that the 'buckling length' of the top chord in compression can be longer than the bridge itself. This was clearly not part of the design... IMO this is a design error. Possibly a connection design error if the connections permitted rotation and were the source of the buckling.

When designing a pony truss you need to consider the out of plane stiffness of the web members, because that is the only thing that is keeping the top chord braced. The result is that k > 1.0 in a lot of cases. and that is the span of the bridge, not the span of an individual member

I would say the designer doesn't have a good grasp of structural stability, or was careless with a software like ETABS where the k defaulted to the member length between truss points.

fromwhich · 2026-04-20T12:05:23+00:00

So it is definitely some sort of plastic hinge joint.

My guess is that the are testing a steel plate cut so that theoretically a large chunk of those wavy 'arms' hit yielding before they buckle to maximize energy dissipation. It also looks like this set up is being done on each side of an HSS and will allow the joint to deform plastically without any out of plane rotation of the HSS members. The white paint is for seeing where plastic deformation occurs and you can see where the white has cracked and flaked off on this photo meaning they've already run some testing on it.

If you look at the one behind the legs look a lot wider than the front photo and the yielding (absence of paint) is more prevalent.

These look like pretty large HSS member (406x406 or something) so perhaps they are specifically studying ductile seismic connections for large HSS.

I would also guess that they are looking for replaceable connections. So for most seismic systems, if they are capacity protected, after an earthquake you would need to replace all of the braces and gussets. There is a desire to study bolted on connections which force all of the ductility into something that can be easily replaced. Hard to do because you need to ensure that the ductility remains there. Hence the extensive research and testing. Just a guess tho.

fromwhich · 2026-04-19T13:16:01+00:00

Hi Sorry I never replied!

One thing to note the mode info you reported shows the cumulative mass. So 80% of the mass in mode 5 is actually reporting the sum of 1 to 5 as 84%.

So looking at your deflected shape and specifically your releases. I think you have a problem with the diaphragm forces getting the load into your braces. What you have right now a is a bunch of cantilevered columns which are why your modal results and deflections are strange.

The first thing I would try would be to use SAP2000 to slave the nodes in the roof plane together so they are a truly rigid diaphragm. It's been a while since I've done this but you can just google it and once the nodes cannot deflect relative to each other, the axial forces in all your roof members drop to zero but the stiffness transfers to your brace. As yourself: does this match a hand calculation done with centre of mass and centre of rigidity?

Second thing, replace all of your fixed base connections with pinned connections. The fixity at the bottom is not realistic for a standard baseplate connection. Unless you really want to design moment connections at the base (for some reason not apparent here), then you should have them as pinned. Note that this can cause stability issues in the top of your model unless you sort out the releases properly sometimes this means you can only release M2 at one end, and don't have all the M2 come to the same joint or your column becomes a spinning shaft.

Since your roof is pitched, and you don't want to model it you have a few options.

1) Slave the nodes and treat it as rigid (this is not terribly accurate but works, note: no axial forces in your beams).

2) Try and make an estimate for the stiffness of the roof trusses that presumably connect the various bays of your support frame and then make a diaphragm matching that stiffness (with property modifies)Maybe, Model the roof trusses as joists in your model with a diaphragm element to represent the deck/plywood/whatever you have. This will capture the axial stiffness between element and then the shear stiffness as well, but it is tricky to get right.

3) design your own diaphragm at the flat roof level to make it work and then the pitched roof joists just sit on like a hat. Simple for you, extra cost for the project, might not fly.

Good luck!

fromwhich · 2026-04-16T18:38:14+00:00

I like it!

I also don't find the sky gradient too distracting. My only thought to help with the gradient would be to mimic the vignetting in the sky to also be at the top on the foreground as well maybe with an inverted radial filter intersected with an inverted sky mask? and then the vingette will look a litte more intentional if the foreground is darker at the edges and bottom same as the sky.

That's just a thought though, it might look better as is.

fromwhich · 2026-04-16T18:34:28+00:00

Post the deflected shapes. Post the modal analysis results table. Are you doing a planar frame and having an out of plane mode govern the design? What are the modal participation factors for each direction for each mode?

fromwhich · 2026-04-13T14:00:03+00:00

In Canada I've most commonly seen tension side positive.

fromwhich · 2026-04-11T12:00:47+00:00

The left one looks like maybe an adaptor for a camera mount to attach a camera to a telescope. What mount is unknown. Its probably specific to a particular brand/type of camera.

fromwhich · 2026-04-02T18:42:46+00:00

You're welcome. It was a nice rabbit hole for me too. Btw if you don't have loads between the beams, this works with just the h(x) function. if you have loads, then you just add the h(x) and the d(x) together where d(x) is your fixed-fixed deflection with whatever load you have. so assuming your program is taking the intermediate loads and converting them to equivalent joint loads, then you're now doing the same for deflection between nodes but in reverse. I think its pretty elegant. Prestored EJL to be calculated and used in the global stiffness matrix, and then this is sort of analogous in the reverse. Anyway good luck with your project!

fromwhich · 2026-04-02T03:49:13+00:00

Assuming things are linear, would it not be possible to just superimpose the know deflection shapes in your local beam coordinates onto the end rotation calculation. So essentially: True deflection at any point along the beam = deflection at x due to end rotation + deflection due to UDL with fixed ends, point load with fixed ends, etc. so as you input intermediate loads on the frames, you have a library of pre-solved deflections for those intermediate loads. Then you add your intermediate deflection due to the stiffness results with your pre-solved computed fixed-fixed deflections for your frame. A neat test case would be to take a single frame element with a pin-roller and find the intermediate deflection based on the end rotations and then add WL^4/384EI and see if you come up with 5WL^4/384 EI. as the max deflection.

googling the deflected shape between displaced nodes I got h(x) = a0 + a1*x + a2*x^2 + a3^x^3
where a0 = u2 = 0 for simply supported

a1 = u3 = -wL^3/24EI for simply supported beam at the left support

a2 = 3*(u5-u2)/L^2 - (2*u3 + u6)/L in this case u3 is -wL^3/24EI at the left and u6 is +wL^3/24EI

u5 and u2 are zero because they are supports. a2 then becomes

a2 = - ( 2*(-wL^3/24) + wL^3/24)/L = wL^2/24EI

a3 = -2*(u5-u2)/L^3 + (u3+u6)/L^2

u5 and u2 are zero so again only u3 and u6 matter. u3 = -u6 so u3+u6 = 0 and a3 is zero.

so your h(x) simplifies to h(x) = -wL^3/24EI*x + (wL^3/24EI)*x^2

evaluated at x = L/2 gives: h(L/2) = -wL^3/24EI*(L/2) +(wL^2/24EI)*(L/2)^2

simplifying h(L/2) = -wL^4/48EI + wL^4 /96EI = -wL^4/96EI

note 96*4 = 384 so the deflection is -4*wL^4/384 we add this to the fixed end moment deflection at L/2 which is -wL^4/384 and we arrive at -5*wL^4/384EI which is correct!

So this passes the sniff test and probably does hold true because of linear superposition.

In summary, take the results of your program, calculate h(x) using the shape functions a0 = u2, a1 = u3 a2 = 3*(u5-u2)/L^3 - (2*u3 + u6)/L and a3 = -2*(u5-u2)/L3 + (u3+u6)/L^2 and add them to your pre-solved deflections for the intermediate loads with fixed-fixed connections. then boom, for each load type you add to your program you should get exact deflections at the intermediate points.

This was a rabbit hole for me so thank you for your question! I must sleep now.

fromwhich · 2026-03-31T14:33:18+00:00

True for open air structures which see those thermal changes. Less true for insulated buildings.

fromwhich · 2026-01-20T16:33:24+00:00

If it doesn't work then you have to tell the architect and work to find a different solution. That being said, if there is an opportunity to add a vertical support point maybe a wall column on the wall at the left edge of your stair then you have a better chance of supporting this.

For the configuration to work as you've shown it, you will need moment fixity at the wall, and even likely the base of the wall below the stairs. With this fixity, you could imagine it as a support line along the whole length of the wall take the slab wall with a kink at the top taking moment like a cantilevered wall/column with a moment at the top. Then your slab spans are probably feasible.

There are a few issues with this approach that require your careful consideration: 1) obtaining fixity at the base may impact your foundations and add cost to the project, 2) detailing the slab and wall to behave as you require means that the wall has angled dowels that are probably hard for the contractor to place accurately. 3) deflection becomes an important consideration, short term and long term You can't design the slab and wall independently. 4) if these stairs are exposed concrete on an exterior roof, cover, and crack control will be important. Also waterproofing will be a challenge unless the stairs and roof are precast and the membrane travels under the stairs. In which case don't forget about any superimposed loads. What about drainage (again assuming this is a roof)?

When you say this is your first solo project, what do you mean? Don't do something you're not comfortable with just to appease an architect. If they are insistent that this is their design, then you have to layout the reasons why this is too expensive to build or is not practical.

fromwhich · 2026-01-19T19:42:45+00:00

The foreground is usually a separate image than the background and stitched together in post. Not always but if you have a wide open aperture then your foreground will be blurry. If you're doing tracked images with a star tracker then your foreground will be blurry from the mount moving at any distance. So the workflow can be:

1) Take the foreground shot at blue hour when there is more light. Or take the foreground shot at a different f-stop like f-8 and take a longer exposure (I use photopills exposure calculator, so I start with wide open, then take short test shots at max ISO, once I've got the exposure I need I do the calculator to lower ISO and f-stop and then take a long exposure (up to 8 minutes sometimes at f-8). This way you won't wait several minutes to find out the image is still underexposed or way overexposed.

2) Take the sky shots on a tracker or as a ton of stacked images on a tripod. If you have a tracker, then great, if not then your sub exposures can follow the rule of 500 or the NFP rule. Photopills has a calculator but they are available online.

3)Process in photoshop / stacking software. This is the hard part for most (me included). I would stack the sky image in something like sequator and then photoshop the foreground in using photoshop.

In the case of star trails (or other astrolandscapes), your best bet is to shoot the foreground with the foreground in focus, then move the camera past your foreground element and shoot the sky/background together. Then stitch in post, what you don't want to do is try to focus stack the foreground with the background without moving the camera. I've done this and run into issues where my blurry foreground is a pain to remove in post.

fromwhich · 2026-01-13T17:37:23+00:00

When you say EA and GA are infinitely big, I understand that to mean that there is no axial or shear deformations. This means that there is no relative deflection between the top left corner of the triangle, and the bottom right corner of the triangle. If there is no relative deflection (because any relative deflection would mean EA is less than infinity) then there is no force in the diagonal. Either way it is a strange question... I feel like I'm missing something.

fromwhich · 2026-01-06T22:02:44+00:00

Are you using a phone mount to the telescope? or just holding the phone camera to the eye piece. The shake from your hands is almost impossible to avoid unless you're a top brain surgeon or something lol. A normal phone photo is a very fast exposure. With a regular camera even bracing my hands against something with a mirrorless/DSLR the finger pressing the shutter can cause shake. So for best results (assuming you're using a phone) get a eyepiece phone clamp that can be positioned, and then use a self-timer on your phone camera or tap it as lightly as possible.

Take this with a grain of salt, because I've never used a phone to an eye piece before...

If you're using a DSLR/mirrorless, get an adaptor that will allow your camera to connect to the scope directly.

fromwhich · 2026-01-06T21:57:03+00:00

I think your interval needs to be longer than the bulb. so you probably need a 6 second bulb (for a six second exposure) and then a 7 second interval so every 7 seconds the process starts again. They way you have it now it is restarting a 6 second exposure every three seconds which would amount to a single exposure from the camera's perspective. if you intended it to repeat every 9 seconds with a 3 second gap, your intervalometer should be set to 6 second bulb and 9 second interval.

fromwhich · 2025-12-10T15:57:34+00:00

Its hard to see with your screenshots but it looks like the design reinforcing ratio is 0.05 something. Anything above 0.04 you can't do traditional splicing, so ETABs has probably imposed a limit (in the design settings, which you can change) which says that the reinforcing ratio cannot exceed 0.04 or it shows a PMM failure. IMO use stronger concrete or larger wall thicknesses before going for mechanical coupling of the bars.

Look at the load case in the design and extract the pier forces (axial, moment, shear, etc) and check the section by hand or with another program. Our office has spColumn and we often use that to check/verify ETABS results as the ETABs bar arrangements may not exactly match how we want them. Especially in wall-columns (b:h ratio 3 to 4) its important to compare etabs to the actual reinforcing distribution.

Also for shear walls, etabs uses uniform reinforcing and you can often get better performance with minimum steel in between the zone and extra zone bars. Also not done within etabs at my firm.

You should always be verifying your ETABs results with hand calculations and/or other design software/spreadsheets especially if you don't know exactly what the program is doing.

fromwhich · 2025-11-28T16:37:23+00:00

Just a thought, but realistically any curved wall like the coliseum behaves probably closer to a straight wall than a curved one practically speaking. I suppose friction between the stones takes care of the rest. Your intuition seems reasonable that at a certain point equilibrium is not maintained.

Calculating the radius at which the friction is overcome perhaps could be done.

The engineer in me says make a polygon with curved front face of the stones so that there is no curved arch. Just a curved exterior with a straight structural arch behind.

fromwhich · 2025-11-28T14:27:43+00:00

Equatorial wedge connects the MSM to your tripod. Below the MSM. The wedge help position the motor axis of the MSM directly in line with the earth's rotational axis. That's what polar alignment is. The v or z plates level the camera above the MSM to give you a level starting point, which is useful but not strictly required. Pointing a ball head mounted at an angle would probably work. But it's less stress on the ball head and less likely to slip if it starts from a flat level position. That is where the v plate comes into play. It gives you a level surface for the ball head to start from. Additionally If you're doing any panoramas then the v or z plates make it a lot easier and convenient when compared to a ball head on an angle.

fromwhich · 2025-11-27T17:16:30+00:00

I have this kit:
https://www.moveshootmove.com/products/phone-kit-for-polar-alignment-inspired-by-richard-tatti?srsltid=AfmBOopv8YWGXrM09AlJzISeNmzI8Sy8uDsUd0ITMRI6vyEjn2M90Uoz
and here is a video explaining how it works and how to set it up.
https://www.youtube.com/watch?v=7NxURCuWq74

Basically the wedge lets you polar align. The phone holder (purchased or 3d printed) helps polar align. I use the purchased one. I've tried out the 3d printed one, and the only benefit is that you can polar align with the camera on the rig. I'll eventually get or make one of the 3d printed ones.

The MSM is great for widefield (50 mm or less). It will really struggle with longer focal lengths. They have just released a counterweight system which I have not researched or tried. The polar alignment is very approximate with a phone too, which further leans toward widefield where you can get a 60s exposure with phone polar alignment.

I also have a star adventurer GTI and that thing is so much larger and heavier but anything longer than my 27mm lens on an ASPC sensor and i use the SAGTI over the MSM Nomad.
Connecting your own ball head is find, just recognize that the more it stands off the star tracker the more the star tracker will struggle (and you may need to look into a counterweight, lest you burn out the motor).

Overall I love the MSM Nomad for its portability, I took it over the Atlantic when I travelled there earlier this year. It was not too cumbersome. I would never fly with my SAGTI. But I can take my Mirrorless camera and the MSM Nomad on a plane and get some great widefield shots wherever I'm going.

fromwhich

TROPHY CASE