Base Plate Stiffener 2

[Kozzybear]

I am attempting to design a base plate connection for a HSS 3.5"x3.5"x3/16" Column. It is a cantilevered awning that is creating a rather large moment about the base. I calc'd that I would need a 3/8" fillet weld around the member to resist the moment, but the 3/16" thickness of the HSS column limits this.

Because of this limitation I am thinking of adding stiffeners to that connect the HSS member to the Base Plate, but do not know how to go about calculating out the strength of the stiffener and having trouble finding resources that describe this process.

My best guess would involve treating the stiffener plate as a triangle and making it resist the moment a length 1/3 of the way up (so if it is a 3"x3" right triangle, apply the force at 1"). Then to use the area at the midpoint of the hypotenuse (essentially 0.707x3"x Thickness of stiffener) and treat it as though its in compression. Will this give an accurate answer or am I thinking about this the wrong way?

Any help is appreciated!

 

[driftLimiter]

You can certainly try there is a paper by Shakya and Vinnakota called Design Aid for Triangular Bracket Plates Using AISC Specifications. I guess you would resolve a couple at the centroid of the stiffener down to the baseplate of the column, then apply the resolved moment as a bracket load and check min thickness of plate.

I say you can try because the 3/16" wall of the column is not going to like that point load. You can check that using AISC Ch. K or AISC DG 24.

You could also consider a knife plate detail, or consider a CJP weld to a stiff baseplate.

You can check your assembly with something like Hilti Profis or Enercalcs Baseplate module, the hand calcs can be difficult because there are many limit states.

 

[bugbus]

I don't like stiffeners welded to the flexible part of hollow sections. It doesn't make sense intuitively to me, and I think you would risk crushing the face of the tube.

The limitation on fillet weld size normally only applies to fillet welds along an edge of a plate, not at a T junction. Check the wording in your code more carefully, but I'm not sure which code you are using.

Alternatively, specify a butt weld. I think it would be easy to achieve in practice just by grinding the edge off the tube.

 

[dik]

Can you use a 3/4" base plate with no gussets? It's likely less costly.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[driftLimiter]

@dik I don't think he has reasoned with baseplate flexure yet hes just trying to get the moment off the column.

@Kozzybear if you CJP it straight to the baseplate, and use 70 ksi filler, the weld is stronger than the tube, so you have developed the moment from the HSS to the baseplate. In order to have a 'fixed base' you want to also make sure that the baseplate itself doesn't add significant rotational displacement to the column. Generally the best approach is to make it thicker, stiffeners generally cost more to detail and fabricate than just making a thicker b.p.

 

[dik]

The moment off the column to the baseplate?... also 3/8 fillet weld seems excessive for 3/16 tube wall; based on CIDECT info, a weld of 20% greater than the tube wall thickness nearly develops a CJP equivalent.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[Tomfh]

Consider a thicker baseplate with no stiffeners.

If using stiffeners you should have two stiffeners each side, aligned with the column web.

 

[Kozzybear]

@dik
It is excessive, and forbidden according to good practices, hence the post because I was hitting a wall with this base connection.

@driftLimiter
I guess I didn't think CJP was an option at the base plate because I had never seen it done. But if allowed it would solve my problem. And yes, already designed the base plate based on the common approach of m&n and using 0.95*width of HSS. The calc'd moment is below the flexural limit and the base plate can receive the moment, but the connection itself is the problem.

Thanks for the feedback everyone. When I get back to the office in the morning I will be looking into the use of CJP's for the connection.

 

[BridgeSmith]

The AASHTO sign spec. has quite a bit of guidance on design for reinforced connections. The tricky part is getting those type of connections to work for fatigue. The fatigue stress limits are very low, and usually control over the design wind loading for the pole bases we design.

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

 

[dik]

I'm missing something here; can you elaborate? I've often used a simple fillet weld at a baseplate to an HSS section for transferring moment. What good practice is forbidden?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[T_Bat]

The moment off the column to the baseplate?... also 3/8 fillet weld seems excessive for 3/16 tube wall; based on CIDECT info, a weld of 20% greater than the tube wall thickness nearly develops a CJP equivalent.

I'm with Dik here - Intuitively, it seems interesting you need more than double your wall thickness to make the weld work - the weld is not half as strong as the tube right? Seems to me you need to take advantage of the directional loading increase for a fillet weld (if using AISC). In the past, I have also calculated the minimum weld needed to develop the yield strength of the connected element then just specified that weld for the connection of the tube to the plate. That ensures your weld is strong enough to allow for redistribution of moments due to yielding in the tube.

 

[dik]

Are you limiting your fillet weld based on your HSS wall thickness? You don't have to...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[Kozzybear]

AWS states that maximum fillet weld for thin walled material is a) the size of the material if it is less than 1/4" or b) 1/16" less than the size of the fillet weld for welds equal to or greater that 1/4". On the weld the thinner member dictates, so for this specific weld, the maximum fillet weld you could use to follow AWS practices with the HSS 3.5"x3.5"x3/16" is 3/16". Right?

My calcs show that 3/16" fillet with E70xx in a square that measures 3.5"x3.5" will only resist a moment up to 2850 lb-ft while the HSS section itself can resist 6330 lb-ft (from table 3-13 in AISC).

Would using a complete penetration with E70xx would allow me to use the 6330 lb-ft as the limiting factor? The weldment filler Fy=60ksi and the steel Fy=46ksi so I believe that defers to the member material, even though it would technically be stronger.

 

[T_Bat]

AWS states that maximum fillet weld for thin walled material is a) the size of the material if it is less than 1/4" or b) 1/16" less than the size of the fillet weld for welds equal to or greater that 1/4".

I believe that requirement is specific to welds against a plate edge in a lapped type joint. This is to accommodate the potential for the corner edge of the plate to melt and reduce the effective throat of your weld. See below.

Your case is more of a butted joist where the weld is not applied on the thin edge of a plate. As an exercise - I would look at the yield strength of a 1" wide strip of HSS, then calculate the equivalent weld thickness needed to give you a greater or equal strength. Make sure you take advantage of the directional increase from equation J2.7 (13th Ed. AISC Spec). I would anticipate a weld that is a little larger than your wall thickness as Dik previously stated.

 

[Kozzybear]

Oh man, thanks for clarifying T Bat. You and dik are exactly correct. I misunderstood the constraint.

Thanks for all your help @dik & @T_Bat

 

[dik]

My prelim calcs indicate you can use 1/4" fillet weld and 3/4" thick base plate with no stiffeners. Fastener forces assuming a shear of 1K and a moment of 66K-in are: tension 3.88K and shear 0.5K per fastener. Once I check my SMath sheet, I'll post it. I have to modify it to include eccentricity of weld to wall thickness outlined by Packard. Do you use Sx or Zx to determine tube strength?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik