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Spliced Beam Reactions

Spliced Beam Reactions

Spliced Beam Reactions

(OP)
Greetings:

I am designing a steel beam to support a 16" waterline crossing over a creek. The waterline will be supported by the I-Beam by hangers at 8' OC which I analyzed both ways - point loads and uniform load. The clear span is 72 ft. Because of limited space the contractor can only bring in a maximum 40' length beam so bolted splices will be required. I do not want to use a welded splice as I am not confident that the contractor's welder, although certified can provide the welds we need.

The bolted splices will carry 75% of the beam capacity as per my design. These splices are located at the 1/3 points from each end so the sections between supports are 16'-40'-16'. This was to avoid a splice at the mid-point of the spliced beam. In designing this beam I assumed that the splices can be considered as supports so that the max span under consideration is the middle 40' with the 16' sections at each end.

I referred to AISC Appendix J and the FHWA splice manual. My in-house peer reviewer says he does not believe my assumption is correct, but he does admit he has never designed or been involved in anything like this. I've designed for bridges and structures but never a waterline support, although I believe the principles are the same. Now he has me wondering if my assumption is correct.

Was hoping you structural gurus could chime in? Would hate to have a watermain rupture because of wrong assumptions!!

Thanks!!

RE: Spliced Beam Reactions

Your first paragraph states that the clear span is 72’, then in your second paragraph you state what appears to be a 3-span beam of 16’ 40’ and 16’ spans.

Are the proposed splice locations also where there will be vertical supports?

RE: Spliced Beam Reactions

Quote (These splices are located at the 1/3 points from each end so the sections between supports are 16'-40'-16'. This was to avoid a splice at the mid-point of the spliced beam. In designing this beam I assumed that the splices can be considered as supports so that the max span under consideration is the middle 40' with the 16' sections at each end.)


If the span 72' , and the splices are located at the 1/3 points from each end , the sections will be =24'+ 24'+24'.
If the sections 16'-40'-16', and for the assumption of the splices can be considered as supports can be valid if the splice locations are vertical supported , or the 72' span has fixed supports and the splices at 16' are shear connection.( in this case, the 16' sections will be cantilever ).

If the clear span 72' , you shall calculate the effects for this span and check the situation also at splice locations for reduced capacity.

What is the pipeline material and type of joint ? How did you consider the wind effects ?

A sketch and more specific info will be helpful for specific responds.

RE: Spliced Beam Reactions

Quote (Trillers)

In designing this beam I assumed that the splices can be considered as supports so that the max span under consideration is the middle 40' with the 16' sections at each end.
What sort of supports? Vertical supports? Lateral supports? Rotational supports?

Please elaborate on what type of support you are considering here because most people reading this are wondering what sort of magic you can perform that enables a splice mid way along a beam to function as a 'support'.

RE: Spliced Beam Reactions

Can you provide a sketch?

RE: Spliced Beam Reactions

The splice is there to stitch the beam segments together to be a whole. It can be a moment, or shear connection, but it's not a support point, as all forces are internal, unless you are going to provide external support, like piers. Otherwise, you have 72' design span, no matter the beam is made out of segments or not.

RE: Spliced Beam Reactions

and the splices are transferring the internal moments and shears (in case it wasn't obvious from the previous post).

another day in paradise, or is paradise one day closer ?

RE: Spliced Beam Reactions

I usually use something like the attached (from an SMath program):



Dik

RE: Spliced Beam Reactions

This also must be a pretty serious beam if it is a single beam spanning 72' with no bracing. Has lateral torsional buckling been considered? The comment of 75% capacity at the splices is unclear if its the LTB capacity of the beam, or braced capacity of the beam.

As other have said, I would not consider the splices as a type of brace or 'support'. unbraced length would be the full length unless you have other means to brace the member.

RE: Spliced Beam Reactions

I have seen 3D truss been used, but much less than 72'.

RE: Spliced Beam Reactions

Have you considered snow load and also the inevitable gang of kids using it as a bridge?

At 72 feet you’ll need to be well on top of your deflection control. What is the allowable deflection of the pipe?

Lateral torsional buckling will likely kill the idea of a single beam in any event. This requires due consideration from a structural engineer.

Also - Agree with others above. I have no idea why you would consider a splice as a support? What stops it falling to the ground? Your span is 72 ft regardless from the description above.

RE: Spliced Beam Reactions

Just a reminder, ice and corrosion prevention are also to be considered, as the beam is across the waterway. I guess you might need a middle pier (2-36' spans), otherwise, the structure will be huge, and the design of beam end supports would be very difficult.

RE: Spliced Beam Reactions

See below for a different interpretation of the OP.

BA

RE: Spliced Beam Reactions

"In designing this beam I assumed that the splices can be considered as support" … NO (as I think we've all said)

you could say that the central (40') section is doubly cantilevered and so distributes it's distributed load to the outer spans as SF and BM, but this would be misleading.

the spliced beam is a single beam (unless there are real supports we don't know about). The applied loads are reacted (to the rest of the world) at the two ends. This means the SF and BM of the central section is not a doubly cantilever beam in isolation, the internal loads on the central portion are increased because out the outer spans.

Your reviewer is correct.

another day in paradise, or is paradise one day closer ?

RE: Spliced Beam Reactions

16in dia water pipe, 72' span … 100 ft^3 = 6230 lbs of water (alone)
max moment, assuming doubly cantilevered, UDL = 87 lb/ft, moment = 56368 ft.lbs … conservative SS beam doubles this to 113000 ft.lbs.

another day in paradise, or is paradise one day closer ?

RE: Spliced Beam Reactions

Quote (Trillers)

The bolted splices will carry 75% of the beam capacity as per my design. These splices are located at the 1/3 points from each end so the sections between supports are 16'-40'-16'.

Try as I might, I cannot make sense out of the above. Would you mind clarifying what is meant? A sketch might help.

BA

RE: Spliced Beam Reactions

I'm not the OP, but the only way to make sense of this is that the total crossing length is 120 feet between abutments with 72ft between supports. If that's the case, then from abutment to first support is 40'-16'=24'. The first beam cantilevers off this support - 16' canti with 24' back span. Mirror this arrangement on the other side. Then in the middle, we have a 40' stick supported by the two cantilevers.

If this is the case, then you're good to go. Design the middle as a simple span, provide shear connections only, and apply the end reactions to the end of the cantilevers.

RE: Spliced Beam Reactions

Good guess, phamENG. Seems simple enough. I wonder why the reviewer didn't like that. It would be nice to have your sketch confirmed by the OP, who seems to have taken a leave of absence.

BA

RE: Spliced Beam Reactions

dik, that is a great reference - just wanted to note that reference doesn't cover a stiffness check. With (2) of these splices in the span I'd want that splice fairly stiff - or at least take a closer look at the effect of a "softer" splice at that location. I think I'd prefer a flange plate splice in this scenario, if forced into the end plate splice I wouldn't feel bad about bumping the end plate thickness up a bit from the minimum required for strength.

RE: Spliced Beam Reactions

Canpro... I've always been partial to stiffened end plate connections.

Dik

RE: Spliced Beam Reactions

(OP)
Sorry about the absence folks....not been abducted although some swear that UFO sightings are definitely on the rise in our region. Delay in responding is due to being out of office and the time zone difference.

Anyway I guess my post definitely confused a lot of you.

To clarify on Ingenuity's point - the clear span is 72', the 16-40-16 indicates the points at which I intend to utilize bolted splices to create in effect a continuous beam 72' long. I've attached a couple of drawings and a sketch which should help clarify things. I may have confused all with the description I gave. Essentially the pipe is buried until it reaches the pedestal footing at which point it crosses the creek with a clear span of 72' then down to the pedestal across the creek where it is underground again. The phamENG sketch is very close except that beyond the 16' points (end supports for the beam) the pipe is buried as shown in the first attachment.

In the second attachment you can see the original design which required a clear span of only 47 feet to be supported on the bridge superstructure. Then we were directed to avoid hanging the crossing section of pipe on the bridge. Additionally we were directed to widen the clear span so that the beam pedestals would be outside the line of the bridge, thus the 72' span. You can see this in the bottom part of the drawing at the detail marked "PROPOSED." For simplicity I am not showing the web stiffeners along the beam.

Hope this helps - I look forward to additional guidance, recommendations and critiques.

RE: Spliced Beam Reactions

You will need a lot of stiffness in the transverse direction. I suggest to use double girder with pipe support/maintenance platform, or a 3D truss. Is the 4' freeboard adequate?

RE: Spliced Beam Reactions

All this still begs the question on how would you imagine that a splice functions as a 'support' in this context. Everything you have shown shows a 72ft simply supported span. You can't just add a splice and call it a support.

RE: Spliced Beam Reactions

Ok, so single span of 72' with 2 splice locations for construction...but:



Is this a W12x65?

You use the singular wording of I-beam, so that implies a single W12.

The self weight deflection ONLY for a W12x65 for a 72 foot span is 2.5" at midspan!

Span-to-depth (L/D) = 72!

You cannot span a single W12 for 72' with an unbraced length of 72'.

EDIT: With a Lb >> Lr = 35.1' you are well into the domain of elastic LTB...your water pipe may be in the creek!

RE: Spliced Beam Reactions

Why is this not being designed by a structural engineer?

Your design assumptions and proposals are horribly wrong here and as far as I can see lateral torsional buckling has not been considered (web stiffeners wont fix this..) deflection has not been considered, snow/maintenance/accidental (kids using it as a bridge) loadings have all presumably not been considered since the beam under its own self weight alone deflects excessively.

Once the beam is in place, how do you hang the pipe? Surely you need a construction access load?

I don’t wish to sound harsh but this proposal is miles off the mark and may well land you in a lot of trouble when it goes wrong.

A starting point would be a triangular or box truss.

RE: Spliced Beam Reactions


Quote (a 16" waterline crossing over a creek. The waterline will be supported by the I-Beam by hangers at 8' OC which I analyzed both ways - point loads and uniform load. The clear span is 72 ft.)


The pipe is 16" waterline in original thread and supported by the hangers. However , the line is 6'' on the last released dwg. For an experienced eye,it is not a magic to say that , the proposed beam W 12 X 65 is not suitable.

The 2X 45 degr DI bends will exert an upward force ( 22.5 degr. with vertical). The bends shall be connected to the concrete abutment with pipe clamps.

My suggestion will be, consult an experienced structural engineer familiar with type of jobs.

RE: Spliced Beam Reactions

As noted above, this beam need to be stiff enough to not impart undue stress on the pipe. It also must be stable for the long expanse you have. Either two beams (likely 30+" deep) tied together with diaphragms, or some sort of box truss will be required to support this pipe. Also remember to consider thermal loads - I'm guessing the hangers have some sliding capability to them.

RE: Spliced Beam Reactions

I'd be curious to know what deflection a W12 would have on that span... I'd be thinking a W21 or maybe W24... If you fix it at the ends, it could become a catenary.

Dik

RE: Spliced Beam Reactions

that pic shows two splices, but not symmetrical ? Since both splices point to the same detail, it'll need to be designed for the loads nearer the CL.

fixing at the ends would obviously help (it'll 1/2reduce the moment in the beam.

I like retired's idea of using a small truss beam.

another day in paradise, or is paradise one day closer ?

RE: Spliced Beam Reactions

Trillers,

Your design is critical in weak axis bending. For unbraced length of 72ft, and a slenderness ratio of 200, ry=72*12/200 = 4.32 in3. This leaves you with the choice a rolled shape W14x398 and above, but then, the beam weight is almost double of the estimated beam load 330 plf. The break down of the estimated load is as below:

Wwater = 87 plf
Wpipe = 83 plf (sch 40)
Wice = 70 plf (td = 3")
W = 87+83+70 = 240 plf

Try W14x398, Ix = 6000 in4, Sx = 656 in3, Wdgn= 240+400 = 640 plf
- Stress check
M=0.125*0.64*722*12=4977 "-k
f = 4977/656=7.6 ksi, ok

- Check deflection
Δ=5*0.64*724*123/(384*29000*6000)=2.22", a span ratio of L/389, ok

Oh, well....!


RE: Spliced Beam Reactions

This is going to have to be a double wide flange laced together with plan bracing and pipe 'saddles' which act as diaphragms to brace the top flange (in a way). That eliminates LTB issues and weak axis bending issues.

There are loads of these types of pipe bridges in the Salt Lake City area. Spans are probably 150ft or more over a 8 lane interstate with a support in the center.





RE: Spliced Beam Reactions

Now that looks more like it

RE: Spliced Beam Reactions

Is 60 bolts in a web splice typical?

RE: Spliced Beam Reactions

Not in my experience Steve, i expect someone just went for a full capacity splice?

RE: Spliced Beam Reactions

For single beam solution, you can go down to W14x283, with 1"x20" cover plate at top and bottom.

RE: Spliced Beam Reactions

(OP)
Gentlemen:. Thanks all for the insight and wisdom. I will now go back to the drawing board bruised but wiseR.




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