Bank Vault - What's going on here? 2

[JNEnginr]

Hey Everyone,

Take a look at these two photos and see if you can shed some light on what's going on here.

This is an old building, early 1900s I believe. Existing foundations are multi-wythe brick. Floor construction is concrete slab/beams, hollowed out with hollow terracotta block. See third image.

In this area, there is an old large bank vault above. What is the point of the dropped concrete beams if they don't rest on anything? The CMU wall looks just like infill. It looks like a dropped slab on dropped beams, I just don't get how it works.

Appreciate the thoughts!

 

[phamENG]

If there's a bank vault above, and the vault walls are concrete, could they be acting as deep beams supporting the concrete joist system?

 

[JNEnginr]

But the dropped beams aren't supported?

 

[JoelTXCive]

Interesting for sure...Maybe the beams were supported at some time in the past? But the wall or columns under them has been removed?

 

[JNEnginr]

In the 2nd picture, it's a short span across the corridor, about 8ft.

Maybe the concrete slab is strong enough to span that short distance, but since the corridor is 20ft long, the drop beams, while unsupported, make it stiff enough so it doesn't dip in the long span direction? Does it work like that?

 

[Tomfh]

Building feature? Part of a vault or cell?

 

[3DDave]

Is there a wall above that hides ties from these beams to a huge beam across the building that is holding the floor from above?

 

[JNEnginr]

Nope, No big beams on the 1st floor ceiling (these beams are all in the basement). Just the Vault walls

 

[3DDave]

The big beams would be in the attic/roof.

 

[JNEnginr]

The room above all this is completely open...i'm stumped

 

[phamENG]

Right. So the walls are likely the girders supporting your beams.

There is probably going from the wall down into this brand to act as a sort of 'hanger'. Probably not how I would do it, but it is plausible.

 

[milkshakelake]

It could possibly also be a bad design or lack of design. The terracotta infill adds some credence that they were just freestyling it. The thinking process probably went something like: "Hm, they said there will be a vault here. I got it, I'll add beams under it!"

I guess to figure out what's going on, you could run a GPR scan. It'll also show any voids, in case it's also filled with terracotta.

 

[kipfoot]

I agree with JoelTXcive. If the exterior foundation walls were constructed with multi-wythe brick, then my guess is that there were brick walls that were under the ends of the vault beams that were improperly removed. The concrete block interior wall is something that was added as part of a renovation.

The concrete joist system with terra cotta blocks was a common method of construction in the era you're talking about. The opening where you took that picture was improperly constructed or cut without adding reinforcement to the system, but it lets you see the square reinforcing bar that was used with these joists.

If you remove your assumption that they knew what they were doing, then what you're seeing doesn't seem so baffling.

 

[JNEnginr]

Thanks for everyone's feedback, though I'm still not sold one way or the other..

Here's a plan view:

Image#1. 1st floor plan. Red line is the extents of the vault. Dividing dashed lines are concrete walls. I'm wondering if the bottom portion of the vault is a later on addition.

Image #2. Basement Plan. Red line is extent of vault. Green lines are the dropped beams and the extent of the thickened slab in the above pictures.

 

[stevenal]

Looks to me like the beams are supported by the block wall and cantilevered into the space pictured.

 

[SlideRuleEra]

This is an old building, early 1900s I believe.

Maybe the concrete slab is strong enough to span that short distance, but since the corridor is 20ft long, the drop beams, while unsupported, make it stiff enough so it doesn't dip in the long span direction? Does it work like that?

Yes, it can work like... just as it works for notched wood beams and steel bridge girders with bottom flange cover plates (the top flange being continuously braced by the bridge deck). The concrete dropped beams increase moment of inertia to reduce both bending stress and deflection.

At that time, 1900's, wood, steel, and reinforced concrete were all designed by somewhat similar versions of what we would call "Allowable Stress Design". Using unsupported concrete dropped beams to increase moment of inertia would have seemed completely logical to engineers then.