Reinforced Steel Yield Stress

[ddavis187]

I work with steel light poles on which we cut out hand holes so there is access to the wiring near the bottom. On poles with span lighting connected, which produces a large bending stress, this produces an issue because the removed material decreases the section modulus. As a result we reinforce the hand hole with welded steel of a lesser grade. What should I use as the yield stress of the section modulus, as the section includes materials with two different yield stresses? I would normally just use the smaller yield stress, but this can sometimes lead to a weaker section despite the added material (and why would we reinforce in a way that weakens the section?). My initial thought is to take a weighted average of the yield stresses. Is there a standard way of going about this? For reference we abide by the AASHTO standards for pole design.

 

[phamENG]

The yield stress and the section modulus are unrelated. When trying to combine different materials into a single section with a unified section modulus, we use the transformed area method. This uses the modulus of elasticity of the materials. This is because it's all based on stiffness. The load will be shared by the various based on stiffness - not capacity.

In your situation, it's still steel. The modulus of elasticity will be essentially the same. So there's no transformed area. It's just the modulus of the geometry of the pole.

Your calcs aren't telling you that the entire section is weaker - it's telling you that the weaker material will fail earlier than the stronger material. Once it has failed, you're essentially back to the stronger material with a hole.

So your paths forward as I see them
1) Use stronger steel for your patch.
2) Ensure you have a sufficiently robust analysis to know peak stresses everywhere on the pole for all load case, not just looking at the worst one and applying that everywhere. Once you're confident that you have accurately captured every eventuality of loading, look for the location with the lowest stress and see if you can place your hole in that spot.
3) Move the hole up. The erectors and electricians may have to alter some of their procedures to make it work, but it'll get it up and out of the highest stress area. In high seismic zones you have to adhere to 'protected zones' anyway that would prevent you from putting these very close to the ground.

 

[rb1957]

Is the reinforcement on the side of the pole ? (I suspect so) Then it'll see higher (bending) stress than the parent pole, and should have a higher yield allowable.
The only "offsetting minor" is does the added material reduce the bending stress (compared with the parent pole) ?

Personally (in my ignorance) I'd worry about the HAZ from the welding, does this reduce the allowable stress in the parent pole ?

You're also being conservative (I expect) by saying that the cover for the hole doesn't take load. I'm sure in an ultimate load case it would, but it is conservative (and I assume common practice) not to take this into account.

 

[jhnblgr]

Look at fatigue area in AASHTO for signs and luminaries. NYS DOT has standard details for hand holes in their BD sheets.

 

[HTURKAK]

What should I use as the yield stress of the section modulus, as the section includes materials with two different yield stresses? I would normally just use the smaller yield stress, but this can sometimes lead to a weaker section despite the added material (and why would we reinforce in a way that weakens the section?). My initial thought is to take a weighted average of the yield stresses. Is there a standard way of going about this?

The use of two different steel complicates the situation . Using basic principles of bending theory "Plane section remains plane" assumption, the strain profile is linear , you can solve the problem,
Steps should be ,
- Find the MOI of hybrid section , ( both materials are steel )
- Find the max. stresses developing . If the max .stress less than yield stress of reinforcement , the problem is solved.
- If reinforcement yields, you are expected to calculate the increase of pole stresses caused by yielding of the reinforcement which must be taken into account by limiting the stresses at reinf to fypl.