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An unusual feature (to me) on a steel truss bridge

An unusual feature (to me) on a steel truss bridge

An unusual feature (to me) on a steel truss bridge

(OP)
Below is a link to a photo of a railroad bridge (build date 1961) over Highway 101 in Ventura CA:

https://c1.staticflickr.com/3/2600/3924736355_282e...


I see that as being a two-span truss bridge.

I am wondering about the reason for the horizontal member along the top, immediately over the center pier. I believe usually this is empty space, in such a structure.

As in this delightful old bridge:

https://c1.staticflickr.com/1/422/31496755826_94d3...


Can someone explain?


spsalso

RE: An unusual feature (to me) on a steel truss bridge

I am not specifically a bridge engineer, but my suggestion would be that it is to achieve moment-continuity over the central pier.  This increases both the strength and the stiffness.

I would therefore reverse the question.  Why does your second bridge NOT have the additional members?

RE: An unusual feature (to me) on a steel truss bridge

(OP)
Because it doesn't need them?

Each span works perfectly well by itself.

There are thousands of multi-span bridges in the world designed like my latter sample, and they have generally worked for over a century. Why does THIS one have the extra piece?

Or, put another way: why bother to have the center pier? Belt and suspenders?


spsalso

RE: An unusual feature (to me) on a steel truss bridge

(OP)
I'll mention that the Ventura bridge is skewed, FWIW.

I do confess that I suspect the "extra members" are decorative. As in: someone said "The bridge looks so ugly with that big gap right in the middle. Isn't there something you can do?"


It could happen.



spsalso

RE: An unusual feature (to me) on a steel truss bridge

It does make the span stiffer across that joint. Otherwise when the train goes over there is a mid-span sag of some amount on each side of the center pier and a high point at the pier. Tying them together prevents a sharp change in deflection at that spot. It may be on other bridges they make the trusses extra deep so the deflection is less noticeable and tying them like this saved some money in material.

The other thing to notice is that when the trusses are extra deep they can use weaker steel while still benefiting from the higher bending resistance, but as steel quality/strength improved they didn't need the deep section to have enough strength but needed to account for the loss of stiffness by tying the two spans together like that.

RE: An unusual feature (to me) on a steel truss bridge

Considering it's in California, could the purpose be making it a continuous structure for seismic loading?

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. - http://xkcd.com/319/

RE: An unusual feature (to me) on a steel truss bridge

(OP)
"It does make the span stiffer across that joint."

Stiffer? Yes, adding connections where there normally are none will add stiffness. But to the improvement of the design?

And then one can ask whether the "span" IS made stiffer. "...when the train goes over there is a mid-span sag of some amount on each side of the center pier..." We would need to examine the case of the train being on only half of the bridge, compared to on the whole bridge.
For the former case, I don't see a benefit--the one span sags, which distorts the other one--gaining what?. For the latter case, a sag in both "halves" of the bridge would put compression on the "special member". Would that be better met by a more common design?


For steel truss bridges, I don't see where seismic loading enters into it. As long as the bridge shoes line up with the abutments, the bridge stands. Except for that, I don't see how seismic forces would affect a steel truss bridge.

I question a steel truss bridge ever failing due to seismic, aside from abutment support.


spsalso

RE: An unusual feature (to me) on a steel truss bridge

Quote (Spsalso)

For the former case, I don't see a benefit--the one span sags, which distorts the other one--gaining what?.
That's why structural engineers earn the big bucks. 'Distorting' the other span causes it to apply forces (reactions) to the loaded span that oppose its movement - a stiffening effect.

We can't know the reasons that design decisions were made from photos. The continuous bridge looks shallower relative to its span but we don't know if the loading is also less. The simply-supported spans would handle movement of the supports better, but trains don't like that so you plan on it not happening.

The additional top member would typically be in tension BTW.

RE: An unusual feature (to me) on a steel truss bridge

The sag puts the top member in tension.
Anyway the point of moment continuity is to provide curvature continuity.

RE: An unusual feature (to me) on a steel truss bridge

(OP)
I did find another bridge using the same concept (a "mid-span" support):

https://historicbridges.org/ohio/lorainhendersonme...

It's the Lofton Henderson Memorial Bridge. It's more complex, though.



spsalso

RE: An unusual feature (to me) on a steel truss bridge

It's hard to say why one is simply supported and the other is continuous. Although railroads typically prefer simple spans because of fatigue and deflection concerns.

Quote (I question a steel truss bridge ever failing due to seismic, aside from abutment support.)


Truss bridges aren't immune to seismic forces. Trusses typically have a high center of mass and the inertial forces can create havoc.

RE: An unusual feature (to me) on a steel truss bridge

(OP)
I would think seismic problems for steel through trusses would be from rocking laterally, while the members under discussion would have their effect longitudinally.


I found a bit of a detail shot of the midpoint of the Ventura bridge:

http://3.bp.blogspot.com/_IpNKtILWJPg/TOwrayJNw8I/...

Note that, at the lower part of the bridge center, it appears as if the two spans are separate--two pairs of bridge shoes, and a gap between the two adjacent gusset plates. The upper part is, however, continuous, with riveted gusset plates.


spsalso

RE: An unusual feature (to me) on a steel truss bridge

Quote (I would think seismic problems for steel through trusses would be from rocking laterally,)


It can rock laterally; it can rock longitudinally. Both conditions have to be accounted for in design.

Back to the original bridge. The top chord over the pier is a dummy member. There's pin clearly visible. Hard to see if there's one at the bottom of the vertical. It might have had something to do with erection stability during construction.



Dummy members are found on cantilever trusses. They carry load during erection (depending on the erection method. Sometimes the center span is hoisted into place rather than assembled piece by piece.) but once the center span is installed they don't carry any load. This is taken from the Silver Memorial Bridge between Point Pleasant WV & Gallipolis Ohio.

RE: An unusual feature (to me) on a steel truss bridge

It is a means to provide a ductile failure in the event of a loss of the center support, instead of a catastrophic failure.

RE: An unusual feature (to me) on a steel truss bridge

(OP)
No, Ron.

It is a means to NOT have an ugly visual dip in the middle of the bridge.

This is a bridge that EVERYBODY will see as they drive through Ventura. You leave that empty spot there, and everyone will think we're hicks from, oh, somewhere far away and backwards. THAT is unacceptable. Fill it in with some spackle, or something. Paint it shiny silver; they'll LOVE it!


spsalso

RE: An unusual feature (to me) on a steel truss bridge

Interesting point Ron. Perhaps there was a concern about a high-speed derailment taking out a member. This is the first time I've seen this condition on a multi-truss span.

RE: An unusual feature (to me) on a steel truss bridge

I wouldn't be so sure that it's not actually a continuous truss. Yes, it's on a double bearing pier, but the top member could still be a tension tie, I would think. If you guys have more information about that particular bridge and that top member, then I apologize.

I have seen 2 span continuous trusses, even here in Wyoming, so I know they exist.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: An unusual feature (to me) on a steel truss bridge

(OP)
Sorry guys 'n' gals, it appears to be a decorative feature.

In this first picture, we see the "T" shape of the unusual addition (note that this is a skewed bridge).



The top of the T has a pin in it. If you follow the darkish "I beam" over to the left, you will see another pin on the far side. TWO pins, one on each side! You will also likely note that, on the far side, the part of the T-top that goes to the right of the pin has a very curious termination. It sits on a little shelf. If you come back to the front plane, and also go to the right to the end of the T, and consider how far that end goes beyond the edge of the gusset plate, you will note that the end is attached to the rest of the bridge by FIVE rivets. NOT what one would expect in a major railroad bridge over a major freeway.

Now go over to the LEFT end of the nearer T. Note THERE that there are quite a few more rivet--upwards of 70. Now you could ask yourself: "If this top member is designed to accept either a compressive or a tension load, how come one end is so much more robustly joined that the other?" You could also ask yourself: "How come the lightly attached ends of these members are only attached to ONE gusset? Isn't that going to be a very nastily distorting input when/if a load arrives?"

Below is another shot of the "strange termination":



You can see the cute little shelf the end sits on, and you can see that the member is only attached to ONE gusset plate.

Now, examining the bottom connection of the vertical post of the T, we find that the left-hand vertical row of rivets do not actually join the post to the gusset. They miss the flange of the vertical post, and are only there for decoration.



Should you suspect that the visual offset is due to the angle of view, note the same type of rivet row in the TOP of the post, shown below. THAT one clearly DOES go through the post flange.



Also of note in the upper picture is the complete lack of reinforcement around the pin joint. Unusual for a load bearing point, isn't it?

And finally, we have an overhead view. You can see the shadows cast by the unusual connection of the western (left) end of the T-top. You can also see an interesting semi-V shaped bit of bracing connecting the two T-tops. Note where they attach.



It appears the two spans were installed separately, with the major part of the "T" structure attached to the eastern span. Once both were installed, the two remaining top pieces of the "T" were dropped in and attached by one pin and 5 rivets, each.


spsalso

RE: An unusual feature (to me) on a steel truss bridge

Good sleuthing, Spsalso.  The designers of that bridge have gone to a lot of trouble to achieve SOMETHING.  But what?  Pure cosmetics?  Accommodate some strange behavioural quirk that results from the extreme skewness of the supports?  (The amount of skewness only becomes obvious in your 07Feb22@22:33 post.)

RE: An unusual feature (to me) on a steel truss bridge

Quote (it appears to be a decorative feature.)


don't bet the house on it just yet. We need to see the plans. We (bridge engineers) are not in the habit of adding unnecessary members. I gt the sense you live near there. If so, maybe you can get the plans from VCTC; the Bridge ID is 52.0178

Quote (You can see the cute little shelf the end sits on, and you can see that the member is only attached to ONE gusset plate.)


In Street View the picture gets distorted depending on the viewing angle. Not even a half-assed engineer would connect a truss member with one gusset.

Quote (Also of note in the upper picture is the complete lack of reinforcement around the pin joint. Unusual for a load bearing pint, isn't it?)


Truss pins are designed like beams. The outer pin plate looks fairly robust. You don't know what's inside the box.

If you look at the upper connection of the vertical - a lot of rivets for a decoration. From the photos at the bottom it's not possible to tell how it's connected to the gusset. It might be connected through the out line of rivets in each gusset; can't say with certainty particularly since the connection at the bottom isn't symmetrical.

RE: An unusual feature (to me) on a steel truss bridge

(OP)
I live about 350 miles away--not what I call near. I have spent a bit of on-line time trying to get more construction info, but nothing yet. It would be swell to examine the plans, and/or to see the construction photos.

I wonder who would have plans. I can imagine the railroad (Union Pacific) since they own the trackage. Or perhaps state or federal government, since they paid for it. I don't think VCTC would be involved--I think they don't use the bridge.

I think there is only the single gusset at the joint in question because it carries no load. Adding another would only add needless expense. In the photo I posted (the second one), you can look right past the end of the member, and see the outside gusset. Therefore there is no INSIDE gusset. Hence: only one gusset.

Re: the quantity of rivets for the upper connection of the vertical. Note also the number of rivets on the OTHER end of the horizontal member--about 70, I said. Since this particular task was so unusual, it might have been a bit of a challenge to design the connections. The point of adding this bit of nonsense was to LOOK like a bridge. How many rivets does it take to LOOK like a bridge?

I fail to see how this "thing" could accept a real load: 5 rivets and a single gusset plate???????
Leaving out a connection between the bottom of the vertical and one of the two gusset plates it lands on? Non-symmetrical much?




spsalso

RE: An unusual feature (to me) on a steel truss bridge

(OP)
I've been thinking about the way the "add-on" pieces were attached to the rest of the bridge. It feels like the connections were designed intentionally NOT to transmit load.

Leaving out the 5 rivets for now, each piece is hinged at one end with a pin, which allows rotation. At the other, it rests on a plate, that allows sliding. There is a large gap at this latter end. And it's all done by intention. This is a flexible and movable connection.

Back to the 5 rivets. Those are a puzzle. On the one hand, none are needed for the above concept. On the other, if it's just for decoration (as the one vertical row of rivets appears to be at the bottom of the post), why not place a larger amount than only 5?

I'm pretty sure the reason for only 5 CONNECTING (as opposed to "fake") was that they would fail before having a significant effect on loading of the bridge members.


spsalso

RE: An unusual feature (to me) on a steel truss bridge

Quote:

I wonder who would have plans. I can imagine the railroad (Union Pacific) since they own the trackage. Or perhaps state or federal government, since they paid for it.

The railroad would have paid for it, but the the state would have the plans as well, since it crosses over a public roadway. not likely you could get the plans from either of them. The FHWA has tightened security for bridge structural plans since 9-11.

Quote:

I fail to see how this "thing" could accept a real load: 5 rivets and a single gusset plate???????
Leaving out a connection between the bottom of the vertical and one of the two gusset plates it lands on? Non-symmetrical much?

Some of those photos look strange; possibly composites that didn't sync? Google Street View? If that's what you're using, I wouldn't trust what you're seeing to be anything close to the reality. Street View warps the photos in an attempt to approximate what it would look like from an angle other than what the photo was taken at. It's usually only marginally successful.

It's also impossible to know what's cosmetic and what's structural. The structural connections could completely hidden by a facade.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: An unusual feature (to me) on a steel truss bridge

It shouldn't need to be said, but just in case it slipped by anyone, pinned connections can still be structural; they just don't transfer moment. Of course, outside of a few specific types, truss members are not designed to carry moment, anyway. A truss member with 2 pins, or even more, can still function as a tension member, which is all the top chord section in question would be carrying.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: An unusual feature (to me) on a steel truss bridge

(OP)
I doubt the railroad paid for the bridge, since the railroad predated the freeway. The railroad MAY have paid a small fraction, to the extent that various entities considered the new bridge an improvement over a previous one.

I am aware that Google Street View patches its photos together. And I'm aware that distortions can occur from the process. If there is something that looks strange/distorted in any of them, and it's relevant, I'd sure like to hear about it. The photos I posted aren't the only ones that show these details, and the details repeat between them. If necessary, I could post more pictures showing the same thing.

I disagree that it's "impossible" to know what's cosmetic. That statement would have to be true in all cases, which is a tough prove.

Yup, facades hide things. But sometimes you can look behind them.

I understand that pin-connections can transmit tension. There are some delightful trusses that use them as bottom members. Tell me about how that's done when only one end of the member is pinned and the other is free-floating on a support bench.


spsalso

RE: An unusual feature (to me) on a steel truss bridge

I just spoke to one of my colleagues, who informed me that riveted joints mostly went away in the mid-fifties, so it's possible, actually likely, that all of those rivet heads are cosmetic, and the structural members are rolled structural shapes, not built-up sections, and connections are actually welded.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: An unusual feature (to me) on a steel truss bridge

(OP)
Interesting.

So that bridge, built in 1961, is "likely" COVERED with thousands of cosmetic rivet heads.

I LIKE it!


spsalso

RE: An unusual feature (to me) on a steel truss bridge

According to Bridgehunter.com it is a "Continuous 2-span Warren through truss bridge", but that's just one source. The structure number is 52 0178. It's at 340 16' 37.4" N by 1190 17' 27" W. I wasn't able to find any info on the structure type, but perhaps others know where to look better than I.

Looking at the photos, particularly #25, it does look like that part of it, at least, is actually riveted.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: An unusual feature (to me) on a steel truss bridge

(OP)
Lots of people here thought/think the bridge is continuous.

Doesn't make them correct.

I also thought it MIGHT BE, at first. But I kept looking and questioning and wondering.......

Nope. It ain't.


Now, about those rivets:



The above photo shows a bit more of the bridge, and I'm pretty sure those are real rivets. And I'm pretty sure that they didn't just stop there, but did the whole damn thing with rivets.

Tell your buddy that you found a riveted truss bridge built in 1961--I think he owes you a couple of beers on that one!


spsalso

RE: An unusual feature (to me) on a steel truss bridge

Maybe California was just behind in their bridge construction techniques during that era.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: An unusual feature (to me) on a steel truss bridge

There are rived bridges in NYC constructed in the early/mid 60's. Bit of history: It was the ironworkers union that objected to bolts. A riveting gang was 4 men; and bolting gang only needs two.

RE: An unusual feature (to me) on a steel truss bridge

(OP)
Riveted bridges completed in 1961:



I thought six would be enough!

Rod,

Your friend is quite ignorant of the history of the trade. And yet he has opinions on the matter.

I would avoid him, if I were you.


spsalso

RE: An unusual feature (to me) on a steel truss bridge


Throgs-Neck Bridge, a friend of mine is running the deck replacement job over there.


Back to our bridge. I found something yesterday indicating it's owned by VCTC. Unfortunately I can't find the link because my work computer only saves the history for the current day.

Quote (I think there is only the single gusset at the joint in question because it carries no load. Adding another would only add needless expense. In the photo I posted (the second one), you can look right past the end of the member, and see the outside gusset. Therefore there is no INSIDE gusset. Hence: only one gusset.)


At the bottom of the post, the lower chords and end posts are connected with two gusset plates. It wouldn't make sense not to connect the vertical to both gussets.

RE: An unusual feature (to me) on a steel truss bridge

It looks like the laterals are along the skew, it may be something as simple as they needed a top node there for the top lateral to frame into.

IC

RE: An unusual feature (to me) on a steel truss bridge

(OP)
According to this article, UP owns the bridge:

Link

According to this article, UP owns the bridge and Caltrans owns the land under it:

Link


If these are two independent spans with "decoration" between them, it would serve no point to connect the two lower gussets via the flanges on the vertical.

And anyone still thinking this is a continuous bridge has a lot of splainin' to do about the connections for that top member. It cannot take a load.

spsalso

RE: An unusual feature (to me) on a steel truss bridge

Quote:

Your friend is quite ignorant of the history of the trade. And yet he has opinions on the matter.

I would avoid him, if I were you.

I didn't say friend; I said he is a colleague, as in someone I work with at the Wyoming DOT. He may not know what the 'state of practice' in other states was at the time, but having done more than his fair share of the load ratings for the bridges in the state, including most of the truss bridges, he knows what he is talking about with regard to Wyoming bridges. As bridgebuster noted, unions can have significant influence on progress, or the lack thereof, in the construction industry. Unions have never been very influential here, though.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: An unusual feature (to me) on a steel truss bridge

Quote:

If these are two independent spans with "decoration" between them, it would serve no point to connect the two lower gussets via the flanges on the vertical.

And anyone still thinking this is a continuous bridge has a lot of splainin' to do about the connections for that top member. It cannot take a load.

In your first sentence you seem to be arguing that these are not independent spans, but in the second, you seem to be asserting that you think they can't possibly be continuous.

I'm not sure how you came to the conclusion that the top member "cannot take a load". It still looks to me like it could very well take some tension. Maybe not a huge amount, but some. Given the height of the truss, it doesn't take alot of tension capacity to provide significant moment capacity.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: An unusual feature (to me) on a steel truss bridge

(OP)
QUOTE:
"In your first sentence you seem to be arguing that these are not independent spans,..."

I'm not arguing that they ARE. I'm saying IF they are, there is no point to connect the two adjoining lower gussets above the pier by using the bottom of the vertical part of the decorative T. I don't think anyone assembles a set of non-continuous spans on piers and then does any kind of structural connection between them.

QUOTE:
"...but in the second, you seem to be asserting that you think they can't possibly be continuous. "

I am asserting that IF the bridge is viewed as a continuous truss, then the members of the T must be designed to take significant loads, just like the other elements of the bridge. And I fail to see that this is the case.

And the whole point of what I said in the quote you used was that I do not believe that the T element under discussion has any structural reason for existing--not to connect those bottom two huge gusset plates "structurally", nor to connect the two top chord sections "structurally".


QUOTE:
"I'm not sure how you came to the conclusion that the top member "cannot take a load". It still looks to me like it could very well take some tension. Maybe not a huge amount, but some. Given the height of the truss, it doesn't take alot of tension capacity to provide significant moment capacity."

If we go with your "...some tension. Maybe not a huge amount...", then we must presume that each end of the T will have the same tension loading (small though it is). Now, if the end that I claim has only one gusset connection using 5 rivets, there is no reason for the other end to have more than that. And yet it does. So why put lotsa extra rivets where they're not needed--why not only 5?

spsalso



RE: An unusual feature (to me) on a steel truss bridge

(OP)
For those of you who are still a bit skeptical of the "artistic" interpretation of this bridge:


Below is a very good view of the western landing of the top member of the T, on the southern side of the bridge. You can see the landing pad, and you can see that the member stops way short of the next member. And you will note that there is no inside gusset for this piece. You can also see, peeking around on the left, the gusset that DOES cover the end of the member. I hesitate to use the word "attached" in this context.



Next we have just about the only view of the landing of the north member of the T, at its western end. Of particular note is the lovely blue sky showing up behind the member--no gusset, no NOTHING.



This next one is quite good at illlustrating the extent of the engagement of the member ends into the gusset area. From this view, I am convinced that the 5 rivets I was crediting for attaching to the end of the member do not actually do so. That means that there are NO rivets attaching the ends to the only gusset available.


Here we move down to the bottom of the vertical member, where it ties into the two gussets. I drew a red line to indicate the right edge of the foreground vertical post. The grey that is to its right is the member on the FAR side of the bridge. This indicates that we are looking very close to squarely at the side of the bridge. And those gussets.

See where the line of rivets is for the right side of the vertical post. They either do NOT engage to flange at all, or are ridiculously close to the flange edge, and obviously improperly place.




spsalso

RE: An unusual feature (to me) on a steel truss bridge

Here's one for you: this is an old bridge with a relatively complicated design when compared to typical railroad truss bridges. We're trying to interpret the load paths from some cobbled together google images and images of bridge 'enthusiasts' that have been collected. Why does it matter?

RE: An unusual feature (to me) on a steel truss bridge

It might not matter, but it is interesting.
(And participation is not compulsory.)

RE: An unusual feature (to me) on a steel truss bridge

(OP)
"Here's one for you: this is an old bridge with a relatively complicated design when compared to typical railroad truss bridges."

No, it's a two span through truss bridge. You think THAT'S a complicated design? Please explain why you think this.


"We're trying to interpret the load paths from some cobbled together google images and images of bridge 'enthusiasts' that have been collected."

Nope. Only one "enthusiast" here. I have no co-conspirators. Perhaps YOU can explain the load paths for members that have one end not connected to the structure. And rivets that don't rivet.

I am puzzled by your use of the term "cobbled together". Do you somehow think I built these images from scraps or something? They are there for the looking. And seeing.

"Why does it matter?"

It matters because some people who are interested in bridges like to know how they work (perhaps this idea is foreign to you). I asked my original question to that end. Thanks to the responses in this topic, I was inspired to keep examining this bridge until I had an answer.

It is unfortunate that you intended the term 'enthusiasts' as a pejorative (the quotation marks give it away). Perhaps you don't like people outside your professional field having an interest in it. For myself, I always welcome people from outside my field asking questions and showing an interest.


spsalso


RE: An unusual feature (to me) on a steel truss bridge

(OP)
I will mention that I do not have a negative opinion of anyone involved in the construction of the subject bridge.

As an 'enthusiast', it is one of my favorite bridges. Every time I travel on 101 down to LA or back, I always look forward to seeing it.

I am now quite convinced that there was input to the design from people not at all interested or knowledgeable in bridge design. And I also believe that the engineers who dealt with this input did a pretty good job of it.

I would VERY much like to know the details of the above. I bet it would make a great book. For those who read.


spsalso

RE: An unusual feature (to me) on a steel truss bridge


The latest of the two links is 2011; things may have changed. This might be the link where I found it is owned by the county but the local ownership link is down.

Link

I was looking at the bridge in Bing bird's eye view; the dark area seem to suggest the top chords at one end are only connected to the outside gusset plates. Another observation that hasn't come up: At the pin and the edge of the pin plate there's no sign of paint wear. I discussed this with a colleague, he agrees that we need plans to understand the function of these members. He also believes that the vertical is connected to two gussets at the bottom.

RE: An unusual feature (to me) on a steel truss bridge

(OP)
That's a very nice picture. For the area circled by the upper red, you can see the diagonal line of the exposed edge of the end post. And, obviously, the lack of top gussets at either location, unlike the rest of the bridge.

The paint wear point is interesting. If there is no sign of paint wear, I guess that would mean the joint did not move enough to cause it.

Did your colleague see the photo example above that included the red line? I note that only three rivets would make the connection between the vertical and the right-hand gusset. Did he have an opinion of why there were only three on the right while there were 16 making the same type of connection on the left? Surely a curious case of asymmetry.

And I still have a problem with those three either missing the flange on the vertical or being very near the edge of the flange.


spsalso

RE: An unusual feature (to me) on a steel truss bridge

Maybe architectural/aesthetics???

A proper looking road improves safety: diagonals, skews, angles and gaps make the roadway look confusing.

The infill will provide some continuity of the superstructure, which will improve seismic response, but my bet is that it was primarily done for the perception of the highway drivers.

The bridge looks weird to the driver traveling toward and under the bridge--and would look more bazaar without the infill. The bridge is heavily skewed to the highway. And the bridge is also heavily skewed to its pier supports. Skewed skewed skewed. The connection is for Bridge Aesthetics (for the drivers on the road), it attempts to look like a continuous box-girder.

The CA Division of Highways was not building many truss bridges by the 1960's. I've driven under that bridge thousands of times and have stayed in the Hotel "The Vagabond" right next to the truss-bridge several times.


RE: An unusual feature (to me) on a steel truss bridge

(OP)
I have real doubts that the impetus to add the "decorative structure" came from highway designers. This was 1961, and I do not believe such thoughts entered the minds of California highway and bridge designers.

I see it more as coming from some hissy-fit local politician who thought it looked stupid having a notch in the middle, and had enough pull to get his way. I will even concede that he could have been visually correct. But not structurally.

Again, I think there's a book in this. I'd be tempted, if I lived there.



spsalso

RE: An unusual feature (to me) on a steel truss bridge

I don't want to disillusion you, but, I don't think this bridge is worthy of a book. Besides, it wouldn't be much of a book as there's nothing extraordinary about the bridge.

RE: An unusual feature (to me) on a steel truss bridge

(OP)
Oh, it's not so much the bridge that's the subject, but the people.

Not that the former wouldn't be of some interest. Once the designers were instructed to add the decoration, they had to both design it and work it into the installation. I'm curious what options they went through. Since they most likely were used to only doing "real" structures, this was, in a way, new to them.

For example, why did they choose to pin one end of "the floppy piece", and use a landing at the other? Why didn't they make two pockets at each end and just drop the piece in? Are the pieces made from steel just as thick as the load bearing pieces?

Ah, but the people: I assume that originally it was designed as a simple two-span through truss bridge. And someone, almost surely not an engineer, said that it was ugly and stupid and needed that big gap filled up, in the middle. The designers said: "No it doesn't. That's silly." And it just grew from there.

Wasn't there a movie called "Chinatown"? Wasn't it about some boring old ditches that people dug, and the people who had input to that project?

spsalso

RE: An unusual feature (to me) on a steel truss bridge

It’s probably a zero force member and is purley aesthetic. I was involved with a bridge that had truss memebers once where they did this.

RE: An unusual feature (to me) on a steel truss bridge

(OP)
TheRick109,

Any chance of seeing that one?



spsalso

RE: An unusual feature (to me) on a steel truss bridge

Quote (For example, why did they choose to pin one end of "the floppy piece", and use a landing at the other?)


Pins are a typical method of creating dummy members; commonly used on cantilever trusses.

RE: An unusual feature (to me) on a steel truss bridge

(OP)
So then would the pins be a loose fit for those dummy members, so that they wouldn't accidentally take a load?

I've just read that they add dummy members to cantilever steel bridges so that they don't scare the public into thinking there's missing pieces.


spsalso

RE: An unusual feature (to me) on a steel truss bridge

In the detail shown, the pin is in a slotted hole and sits on a bronze angle so that the top chord can expand/contract.

Depending on the contractor's erection method, the dummy member could carry load if the center span is constructed by cantilevering all of the members. Once the closure is made it becomes a dummy. If the contractor elects to raise the suspended portion of the center span into place, then the dummy is always a dummy.

RE: An unusual feature (to me) on a steel truss bridge

maybe when they modified when they widened the 101 and off-ramp

RE: An unusual feature (to me) on a steel truss bridge

(OP)
I'm pretty sure the bridge was built all at once in 1961.

Unfortunately, the rather large amount of on-line time I spent trying to find out about the construction of this bridge revealed no original sources.


spsalso

RE: An unusual feature (to me) on a steel truss bridge

The 1st photo shows a 2-span continuous truss bridge.
The 2nd photo shows a 2 simply-supported truss bridge.

The span of continuous bridge generally is larger than simply-supported truss bridge.
For shor span bridge, simply-supported bridge is easy to built and costs less.

RE: An unusual feature (to me) on a steel truss bridge

(OP)
netsonicyxf,

"The 1st photo shows a 2-span continuous truss bridge."

And how are loads transmitted through the top member that is directly above the center pier, considering it is not attached at its western end?

How do you explain the radical asymmetry of the rivet patterns in the gussets in the area above the pier?

And, for extra points, you can explain the reasoning of having 4 bridge shoes on the pier, instead of two. How does the loading work through those four points, under a continuous truss bridge?


spsalso

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