Precast Parking Structure: Double Tee Production & Bearing Connection

[JohnRwals]

Hello!

These questions are related with precast production efficiency and member design detail.
Double tee floors are sloped everywhere in the precast concrete parking structures;
let's assume 60ft long 12DT34 and typical floor slope 2% for drainage.

1. Is it really necessary to make double tees with (1/2") batter like Double Tee-1
instead of straight vertical ends like Double Tee-2? (Typical end joint width is 3/4".)
This makes end formwork complicated, which will reduce production productivity.

2. Double Tee-1 shows bearing plates are flush with stem surface, which is good for production efficiency.
Is it really necessary to make bearing plates tilted like Det-C, D to make uniform bearing?
Does Double Tee-2 production method which will cause non-uniform bearing stress really matter?

 

[HTURKAK]

I would prefer sloping the bearing supports of the double tees.

 

[JohnRwals]

Sloped bearing corbels in the litewall and sloped ledge in the spandrel?
Are those special treatments worthy effort?
Economy and production efficiency due to non-rectangular forms should be considered.
I understand similar issue could be critical in the bridge industry with heavier loads.
What about in the typical parking structures with about 25kips service load per double tee stem?

 

[HTURKAK]

I understand similar issue could be critical in the bridge industry with heavier loads.
What about in the typical parking structures with about 25kips service load per double tee stem?

I totally agree with you . The use of bearing pad could be discussed for beam supports but double tee loads are not so much and IMO , bearing plate is OK. The bearing plates could be machined to have required slope or could be post installed using shims and grout.

 

[Brad805]

I suspect most precast manufactuer's have endless bulkheads for this problem. Slopes are normal, and that is not a lot of work. Sloping the bearing is more work given the reality of what one must do with the rebar and forming the bearing. If the bearing is 3" or 75mm, the slope distance over the length of the bearing 1/16" or 1.5mm. Most will have a bearing pad that will deal with that issue.

 

[JohnRwals]

Typical double tee bearing plate size is 4"(W)x8"(L).
Maximum 1/8" plate tilting tolerance is applicable according to PCI standard/manuals.

 

[Brad805]

Ok, I see that in Chapter 13. I would be curious if that happens all the time in industry. 3/32" vs 1/8"?

 

[JohnRwals]

Here is my personal solution/suggestion.
I am looking forward your comments.

1. All these double tee bearing connection movements are dynamic
due to vehicular impact load, thermal change, member cambers, etc.
2. Providing uniform loading conditions is expensive and hardly practical.
3. Therefore, a practical solution is up to bearing pad design and its optimal implementation.

Solution 1: Design/produce precast double tees with perpendicular, rectangular forms
with no special, skewed batters. (This will improve production efficiency.)
Solution 2: Design/select proper bearing pads with uniform thickness, not tapered.
(Ultimately, bearing pads will function as Det-A3 shows.)

The bridge industry tried to use tapered shim plates, Det-A2, to secure uniform bearing conditions.
(This goal cannot be achievable as temperature and load conditions change,
concrete structures should follow changing environment conditions.)
So, this experiment failed, I heard.

When bearing pads with uniform thickness are adopted,
I think the worst scenario related with this bearing issue is pad walkout, slippage, Det-A4;
bearing pad moves outward and loses bearing area as a supported concrete member moves/vibrates.
Or, available bearing area will be reduced significantly as pads are squeezed and worn out, Det-A5,
which can be adjusted with joint width (design issue).

 

[rlmengineer]

Salt water will rust any exposed steel bearing plates. I would recommend using elastomeric neoprene bearings to allow for some thermal movement

 

[SWComposites]

Why not just use the design that is in thousands of existed parking garages?

 

[JohnRwals]

This is a kind of improved design method or sophistication of existing knowledge.
Currently bearing pad design issue is not cleared completely yet.
I guess ROF is more popular than plain elastomer bearing in the parking structure double tees.