precast stadium bleachers 5

[pepperoni]

Dear all,
my firm has a project where we are assessing the condition of the existing stadium structure. The stadium consists of precast bleachers supported by CMU walls. The building, in general, has a lot of issues due to the age and exposure to weather but the main concern is the excessive, noticeable deflection of the planks. The worst deflection due to self weight is approx. 1.5in and it varies throughout the structure. The plank spans 28'. I'm concerned that the planks are losing the prestress force. All planks are exposed from below, and not a single crack was observed during the inspection.
From your experience, is loss of prestress possible and would the planks deflect beyond the limit without cracking of the concrete cover? What would be the other reason for a 1.5in deflection due to self weight.
How would you reinforce or repair those planks?

 

[hokie66]

Good post, Ingenuity. But it looks like pepperoni's riser is only 12", so the PCI table would give a maximum span of 24'.

 

[Ingenuity]

But it looks like pepperoni's riser is only 12", so the PCI table would give a maximum span of 24'.

Exactly - I was taking the alternative route by highlighting that for a 28’ span a 14” deep section is required, based upon PCI.

 

[pepperoni]

BUT, the significant SW of the section adds another 1000 psi of compression to the top fibre, and with such a small compression area, creep is significant

can you explain, do you mean the weight from the upturn 6x6 section?

So creep after 30 years, applied to a downward deflecting section, would be say 4 to 5 x elastic deflection and 0.34" becomes 1.5" over time.

is it likely that creep will progress resulting in larger deflections and ultimately failure? if 4 to 5 x elastic deflection is reasonable assumption, what would be the deflection before failure and would the potential failure be due to creep only? is there a limit for the creep due to sustained loads (SW)?

Assuming TME is correct about the section being compression controlled, I retract my previous assertion that a lack of cracking indicates it is not in danger of failing. As TME points out, in a compression controlled section, brittle failure without warning is a possibility.

what would be the failure mode for this structure then?


Would you also consider the water infiltration and possible corrosion of the strands/anchorage resulting in steel relaxation and loss of prestress? The grout and concrete around strand anchorage is cracked and spalled, in some locations. Since the whole section is in compression, loss of prestress would cause the tension in the bottom fibers of the section and ultimately tension failure?

 

[TehMightyEngineer]

So creep after 30 years, applied to a downward deflecting section, would be say 4 to 5 x elastic deflection and 0.34" becomes 1.5" over time.

Good point. I assumed a long-term deflection factor of 2 but obviously with that small of a compression area you definitely will get higher long-term creep than normal. That would definitely make me feel better about this not losing too much of the prestress but obviously wouldn't rule that out.

I also calculated similar numbers to yours.

Also, PCI 7th edition design aid table 3.12.11 on stadium risers, may be of some use:

Good double check of my math, I was showing an overstress of about 110% - 120% depending on the concrete compressive capacity.

Without any info on the design or reinforcing of the sections, doing a load test as JAE suggested, would be a prudent move.

Agreed.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[TehMightyEngineer]

is it likely that creep will progress resulting in larger deflections and ultimately failure? if 4 to 5 x elastic deflection is reasonable assumption, what would be the deflection before failure and would the potential failure be due to creep only? is there a limit for the creep due to sustained loads (SW)?

Concrete creep due to long-term loading is self-limiting. My understanding is you will not have failure (except for very weird situations) due to it. However, if the deflection is caused by a loss of prestress or by overloading then it is indicative of potential failure as you're relying on the prestressing strands for reinforcement.

what would be the failure mode for this structure then?

Crushing at the top of the 6" vertical stem of the riser. There will be little to no warning of a imminent compressive failure. Check for cracking at the top and especially spalling near midspan. Due to the asymmetrical shape and prestressing offset horizontally from the CG the indicated region will be under the highest compression:

Would you also consider the water infiltration and possible corrosion of the strands/anchorage resulting in steel relaxation and loss of prestress? The grout and concrete around strand anchorage is cracked and spalled, in some locations. Since the whole section is in compression, loss of prestress would cause the tension in the bottom fibers of the section and ultimately tension failure?

This could cause a loss of anchorage or a loss of cross-section in the strands; both could cause a loss of prestress. Loss of prestress would cause the section to essentially become a normally reinforced beam. In which case it would likely still be a compression failure just with much higher elastic deflection.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[BridgeSmith]

"can you explain, do you mean the weight from the upturn 6x6 section?"

It's not the weight of that portion in particular, but the entire section, producing substantial compression in that region because it's a small area and the furthest from the neutral axis.

"what would be the failure mode for this structure then?"

Most likely, local crushing or spalling of the concrete at the extreme compression fiber that progresses down through the section, with the pressure becoming higher as the effective depth of the section decreases. There's a video here: Link Because your section is not rectangular, the failure could be more abrupt.

Of course, the worst part is that if it were to fail, it would be under its maximum loading, which would be when the bleachers were full. If the system is to be left in place as is, I strongly recommend load testing a bottom plank (one that is not supported by a section below it).

 

[IDS]

Is there any significant trend in the deflections around the stadium?

I would expect differential shrinkage to increase deflections, and this would be greatest in the South facing risers.

The deflection predictions should include differential shrinkage anyway.



Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[TehMightyEngineer]

Same producer as the video posted by HotRod, but this time a non-prestressed beam:

https://www.youtube.com/watch?v=3xw9_33uNJA

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[pepperoni]

So it may be logical to say that the risers were underdesigned from day one and because of creep, shrinkage and loss of prestress the structure is experiencing significant deflections and basically reached its useful lifespan?

Another question though, do you think that the type of connection between planks and the wall may have any impact? The risers are connected to walls with horizontally and vertically slotted steel angles (vertical slots at riser). The same type of connection exists on both ends. See picture. My concern is that the planks are restrained in a longitudinal direction on both ends but allowed to moved vertically and horizontally. The bleachers are experiencing uncontrolled differential movement that is causing cracks in joint sealants and significant water leaks.

 

[TehMightyEngineer]

Doubt those connections are causing your problem as the risers get their vertical support from bearing on the brick wall.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[structSU10]

This is exposed to the elements right? Is it in a region where large temperature swings happen?

 

[TehMightyEngineer]

So it may be logical to say that the risers were underdesigned from day one and because of creep, shrinkage and loss of prestress the structure is experiencing significant deflections and basically reached its useful lifespan?

No, I'd say it's logical to say that the risers were minimally reinforced only. We don't yet know if the design was truly deficient and/or if there is a structural problem with the risers.

If I had to guess I would agree that the most likely conclusion is that the risers are structurally sufficient but their serviceability life has reached it's useful lifespan. I wouldn't make this conclusion officially until more work was done.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[pepperoni]

This is exposed to the elements right? Is it in a region where large temperature swings happen?

North-east us climate. The stadium seating is exposed to direct sunlight and the space below is used as storage. During temperature change, especially in the morning, when exposed to sun, you can hear the planks "pop".

If I had to guess I would agree that the most likely conclusion is that the risers are structurally sufficient but their serviceability life has reached it's useful lifespan.

We will recommend a load test to confirm that the prestressed risers can deflect that much and still be structurally sound. We will also scan the surface and will try to find the reinforcement so the proper analysis can be performed.



The risers should be designed for 100psf as requested by code, in my understanding in order for the planks to fail, they really need to be designed for much less load or shorter span (referencing the PCI table for recommended span and riser) or the structure is experiencing significant prestress losses due to exposure to weather and corrosion.

 

[Brad805]

These pieces are cast in either single, double or triple rise configurations. Unless you have a very large precaster within say an 8hr drive, I would have my doubts they are pre-stressed. You need a dedicated casting bed for this that is not usable for projects other than this. That requires a large production volume to justify. They could be post-tensioned quite easily by many that have the appropriate equipment and skills. The most difficult pieces to design are the single risers due to the reduction in cross sectional area. I agree with TME that the connections identified are not doing much to create composite action between itself and its neighbor. You may be able to create such a connections to reduce the problems with these pieces.

Before starting down the load testing path I suggest you rent a scanner and start investigating how they are reinforced. Provided your raker beams are uniformly spaced it will not take very long to do so because it is common to standardize the rebar for production purposes. We have x-rayed concrete parts too. Xray works very well in a case like this where you have good access to both sides and you do not need a long exposure time since the pieces are thin.

Load testing is a wonderful catch all to suggest, but keep in mind that at some point you will be asked how, and to what level. At the end of this you will also have to give them a simple good/bad answer, and I doubt that will be as simple as you might think.

 

[TehMightyEngineer]

Brad: Did you look at the picture OP posted in a pdf? These are almost definitely prestressed, you can see the sealed caps where the two strands are. They're also a single riser so a dedicated bed isn't required. Also, OP cropped the image to hide the location but it looks like a large stadium so worth it from a precasters standpoint.

Agree 100% about your load testing comments.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[Brad805]

I hadn't, now that I have I see they are stressed. The single image posted suggests they are all single rises, but that makes very little sense from an erection or production standpoint. That could triple your part count, and that would have a significant impact on the crane, handling and install time. Anyway, good fun for the OP.

 

[pepperoni]

Brad: We will perform scanning and analysis and most likely load test too. We will be looking for a sub who can take the lead in the precast analysis portion.

 

[TehMightyEngineer]

but that makes very little sense from an erection or production standpoint

I think you answered your own question. A single riser can be made on a normal stressing bed by just blocking out the 6" riser stem; whereas a two or three riser likely requires a custom stressing bed or other considerations. Plus while the amount of handling is obviously greatly increased the crane required is much smaller and shipping is easier. Still, I agree it would quickly become more cost effective to make these double or triple risers.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[Brad805]

I get why and how one would cast single rises. We have done the seating for four arena's and the amount of singles on any project was minimal. The company I work with does not have the capacity for stressing pieces, so we either use stems or the spans were adjusted for our purposes prior to the EOR finalizing the design. I hated singles because they were always more difficult to make work. I suppose in the era these were built the access to 100t+ cranes was less.