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Shear flow ?

Shear flow ?

Shear flow ?

(OP)
Folks,

This may be a really simple, stupid question....

I have a situation where I need to connect 2 single angles (toe-toe) to create a U shape.

I think the weld should just be to keep the two members together, preventing them from separating. A skip weld maybe.

A single angle in bending unrestrained)....a case of two drunks leaning on one another? How do you design this?

RE: Shear flow ?

hi slickdeals

Whats the orientation of the two angles once welded in relation to the load path.
If it was me I would choose a continuous weld.

desertfox

RE: Shear flow ?

(OP)
Orientation of the angles is such that the two legs are pointing upwards (legs in compression)

Imagine a U shape, comprised of 2 angles. As if a Channel is bending in weak axis.

Loading is downwards (connected legs - tension flange)

One of the things I will have to check is local buckling in the outstanding legs.

RE: Shear flow ?

(OP)
This U shape will be carrying blockwork.

RE: Shear flow ?

I don't think shear flow is an issue in the weld design.  I agree with DF...make it continuous, but watch for warping from weld heat.

There is apparently a reason that a channel can't be used here?

Be aware of a strong potential for rust staining on the exterior (assuming exposed)with this design...don't let that come back to bite you in the a$$.

RE: Shear flow ?

(OP)
Thanks fellas, is there a "design" to this or just minimal "feel good" welding?

RE: Shear flow ?

Assuming steel, the weld just extends the material to complete the section (considering near CJP weld)....can consider as if the weld did not exist and you had a rolled shape.

RE: Shear flow ?

Why not consider a bent plate?

To answer the question directly, though, as everyone else has said, there is no shear flow.  You're just joining the sections to make a single U.
 
One thing I have done in the past is have the edges prepped for a CJP for the entire length.  I provide a skip weld and then the spaces in between the welds get filled in with something like bondo or a metal filler with SS chips if you are using SS.  The CJP should be ground flush so that when the filler is used you can apply a coating for rustproofing and you never know it isn't a single, solid CJP.

RE: Shear flow ?

If there are wind loads on the wall, there would be shear flow in the weld, in the form of complimentary shear stress and at the maximum location.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Shear flow ?

paddingtongreen is correct of course in that with a lateral wind (or an internal 5 psf nominal load) you would have shear flow in the welds and a biaxial bending condition in the "U".

The wall/masonry itself may take a good portion of the lateral force as it might be significantly more stiff than the "U".

I always hate to have welds in bending so if there is any chance that the vertical legs of the "U" would move relative to one another, I'd also suggest occasional vertical stiffeners to tie the two vertical legs together a bit.
 

RE: Shear flow ?

(OP)
Thanks for your responses.

In India, elevator shafts require hoistways to be separated by 2 hour fire rated walls. There will be 100mm thick AAC block separation wall.

The contractor wants to use this U shaped steel member that will be set on the slab formwork on one side and bear on the shear wall (either in a pocket) or directly if the wall above is not cast.

This will also enable to attach the elevator guide rails to the sides of the U shape. Why a channel is not being used is beyond me.

The way I am designing this is such that the steel member will act only as formwork and there will be a 8" wide x 8" deep concrete beam spanning the width of the elevator. That way, the steel member will not need to be fire-proofed.

 

RE: Shear flow ?

For lateral load, the weld will be at the neutral axis; therefore, the shear flow offset would only be the weld width, which will likely be less than 1/4".

I agree with JAE that the wall will take up most of the lateral loading, so the shear flow would be negligible and perhaps nonexistent.

Not a fan of using fillers for weld gaps.  They tend to hold moisture and promote corrosion.  Just use a seal (cap) weld if you only do a skip CJP weld.

RE: Shear flow ?

if you assume these to angles are loaded independently and loaded the same, the load on the individual angles is likely not through the shear center. The angles will want to twist when loaded.  

RE: Shear flow ?

Use a bent plate or channel.  It will be much cheaper than two angles joined with a CJP.  Also, if it is not a CJP, legs welded in that manner are reminiscent of the Hyatt Regency failure.

RE: Shear flow ?

Ron,
Have you seen a lot of problems come from that type of detail?  We've done that detail after getting push back from fabricators about the amount of weld and the architect wants to see what looks like a continuous weld.  When we do that, we typically coat the entire thing with tnemec paint.

If that's a bad detail, though, it's definitely good to know.

RE: Shear flow ?

slickdeals,

I don't like continuous welds on angle legs, as the whole thing is likely to distort.  And it is a moot point in your case, as the angles are just for temporary support.  Intermittent welds would be fine.

Don't you have concrete or masonry walls at both ends of this beam?  Why not precast it and eliminate the need for temporary support?

RE: Shear flow ?

RE: Shear flow ?

The shear center of the combined shape occurs below the weld between the horizontal legs.  For gravity load, shear flow varies linearly from 0 at the top of the vertical legs to a maximum at the corners, then linearly to 0 at the weld location.

The weld carries a continuous moment directed parallel to the member. This is necessary in order to prevent the combined section acting as two separate angles as suggested by Toad Jones.

Under lateral load, the shape has a tendency to rotate because the load is applied above the shear center.
 

BA

RE: Shear flow ?

Lion06...if you prepare the surfaces and coat with a good coating like Tnemec to a reasonable dry film thickness ( > 5 or 6 mils)it should be OK.  At one time, the practice was prohibited by code (AWS D1.1).  I haven't checked it in a long time, so you might want to look into that.

RE: Shear flow ?

This wouldn't be subject to lateral loading, as it only supports the beam above until cast.  The section is symmetrical for gravity loading, so I don't see how shear flow is a concern.

RE: Shear flow ?

(OP)
The following summarizes the problem:

1. There is no lateral loading, since this member is only acting as formwork.

2. The member is symmetrical once made composite by joining the legs.

My concerns are:
What are the forces acting along this weld that will make this member composite. Once composite, this is akin to channel in weak axis bending.

Taking 2 angles, which want to rotate and trying to connect them together is the issue. What are the forces acting at the interface?

RE: Shear flow ?

(OP)
To simplify:
Assume two angles L4x4x3/8 created to form a 8" wide x 4" deep U shaped member.
Assume that the 2 angles are connected by a 3/16" thick x 0.5" wide member to mimic a weld.

Assume a loading of 0.5 klf and span of 10 ft.

RE: Shear flow ?

I wouldn't call it a composite member.  It is just two angles side by side, joined by enough weld to prevent rotation.  But really, if you are just using it to cast a 200 x 200 beam across a lift shaft, how much rotation is that?  A few short welds will suffice.  If rotation remains a concern, weld a few spacers across the top.

RE: Shear flow ?

@slick, This "channel" would look like a structural member to many of the code officials that I banged heads with.

 

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Shear flow ?

For a section having one axis of symmetry and loaded in the plane of symmetry, the shear center would coincide with the center of gravity of the section. We could then use formula q = (V.Q) / I
where  V = Shear at the support, I = M.I. of the two angles about minor axis of the (overall) section, and Q = moment of area of one angle about the c.g. of the (overall)section, i.e. about the weld axis.

RE: Shear flow ?

Quote:

Assume two angles L4x4x3/8 created to form a 8" wide x 4" deep U shaped member.
Assume that the 2 angles are connected by a 3/16" thick x 0.5" wide member to mimic a weld.

Assume a loading of 0.5 klf and span of 10 ft.

Assume:
.1  Vertical legs carry all of the load.
.2  Load is uniformly distributed over the horizontal legs.
.3  Span is 8"

M per foot = 500 * 8/8 = 500"#/'

S = 12(3/16)^2/6 = 0.0703 in^3

f = M/S = 7,111 psi in 3/16 plate.
 

BA

RE: Shear flow ?

Correction to my previous post .......  The shear center would be that for a channel and not at the c.g. of the (overall)section.
Q = moment of area of one angle about the shear center / weld axis.

RE: Shear flow ?

As BA has repeatedly hinted, the angles will burst apart sideways, and twist, in the absence of 'sufficient' weld. The vertical load is only being applied through the shear centre if the section is integral. It is not, in practice you will have two vertical loads applied, one to each angle, and it is not at the angle's shear centre.

Quantifying exactly how much weld is required depends on your definition of acceptable spread in the joint.
 

Cheers

Greg Locock


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RE: Shear flow ?

(OP)
@Greg,BA
I understand what you are saying, which is what I am concerned about too. In my OP, you will see the reference to two drunks leaning on each other.

RE: Shear flow ?

(OP)
And Happy New Year to all your wonderful folks and your families. Hope 2012 turns out to be better than 2011.

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