Supporting existing precast plank 2

[zstructural]

I have an existing precast plank roof, and we're adding a taller addition, so it will cause snow drift on the existing plank. If I install a new beam line to break up the existing span into two smaller spans, do I need to saw-cut the precast over the beam so I don't produce any negative moment? (essentially creating two simple spans) Has anyone done this before? Do you have any direction? Do I need to cut all the way through the plank, or can I just saw cut the top of it down to the core?

Thanks!

 

[Ron]

Hollow core panels are usually cast symmetrically, so they should be OK. I would not bother to cut them. Reverse moment would be max 1.25x at center.

 

[JAE]

Ron, I thought that PC Hollow core planks had tendons in the bottom of the plank - so with a new beam, you'd have negative moment, with no top reinforcement, and thus a tension crack from the flexure over the beam.

With that situation - seems like there'd be a cause for concern as to the shear capacity just off the new beam edge if the crack didn't "happen" right over the beam.

Thoughts?

 

[Ron]

JAE...most of the ones I've inspected have been symmetric...top and bottom reinf. the same.

 

[zstructural]

That was my thought, TAE.

I don't know if this complicates matters, but there's about a 4' cantilever on the far end. I've attached a sketch of my situation.

Thanks for your input, everyone!

Another question I have is what is the best way to fasten the top flange to the precast for lateral support for the beam. Sleeve anchors through the top flange of the beam into the bottom of the plank?

 

[msquared48]

If it is as JAE says, just add a 2-3" topping slab and put in the negative steel in the topping slab. Should work with the shorter spans.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[zstructural]

I'm used to seeing the steel in the bottom only, but I'm sure it could be either way, depending on the situation.

For my case, let's assume a worst case. Is it worst case to assume only steel in the bottom? If so, is it necessary to saw cut the plank over the new beam?

Are there other considerations I'm not considering?

Thanks!

 

[zstructural]

That's a thought, Mike. I'm not sure if it's feasible to add 2-3" topping, as it is an existing roof, and they would have to rip up the entire roof insulation, etc. to do that. I'm also concerned with the ramifications of adding that much extra dead load. But, if that's what's required to do it, I'm sure I could make it work.

Any other thoughts?

 

[BAretired]

In my neck of the woods, the planks would be prestressed on the bottom only. They might have nominal strands on the top for camber control but usually not.

It would be prudent to sawcut directly over the beam to ensure that the slabs are adequately supported each side of the sawcut. The sawcut does not need to go through the entire depth of plank. In fact it should not because you want to keep the bottom strands intact. A sawcut depth equal to the thickness of top flange is adequate.

So far as the 4' cantilever is concerned, it was there before and you are not making it any worse, so leave it alone. Hopefully, the original engineer reinforced it adequately on top.

BA

 

[hokie66]

Agree with BA. Just saw as deep as the top of the cores.

As to bracing the beam, I would do it outside the flange. Maybe bolt sections of angle or plate to the planks. That way gives better access for drilling. Some sort of drilled in anchors, maybe into the cores to miss the strands.

 

[zstructural]

Thanks a lot everybody. I will go ahead and have them saw-cut the top of the plank. I like your idea hokie about fastening outside the flange. I think I'll have a wide continuous plate over the top flange, that way I can increase my bearing area for the plank as well.

 

[hokie66]

That's a better idea.