Wind on Shoring/Bracing for tiltwall panels during construction

[steeledan]

Project is in 120mph basic wind speed zone. Are there any Code-allowed reductions to that design wind speed for the shoring/bracing of the tiltwall panels while they are being constructed? Tiltwall Contractor is claiming they "always" use 90mph since "temporary condition". I'm inclined to make them design the shoring/bracing for the full 120mph, but want to make sure I'm not missing something... Anyone know?

 

[csd72]

The code should have a clause for short duration or temporary works check in the back of the code. There also may be a seasonal factor.

If the job is going on during hurricane season then you may need to consider the consequence of failure.

 

[fancypants]

Take a look at ASCE 37-02 Design loads on structures during construction.

 

[spats]

See TCA's GUIDELINE for TEMPORARY WIND BRACING of TILT-UP CONCRETE PANELS DURING CONSTRUCTION, available at tilt-up.org.

 

[msquared48]

Not to be fececious here, but is this project in Indiana?

Seems like we have covered this wind gust reduction/mitigation scenario very recently.

I could see a reduction outside of the June to September hurricane season (October to May), but not during it. So what if the contractor has to use super struts instead of the normal ones. Include it in the bid. Failure is not an option.

Mike McCann
MMC Engineering

 

[WillisV]

As stated by FancyPants, ASCE 37-02 Design Loads on Structures During Construction is the governing document in this case. This document clearly spells out the wind loads to be used on temporary structures, including several options for handling hurricane season.

 

[frv]

We do a lot of tiltwall and yes, it is standard practice to reduce wind loads to some lower return period (don't know it off the top of my head).

When we receive shop drawings, the design wind speed is 72 mph (we are located in a 90 mph area), so it seems reasonable to me that the reduction from 120 is 90.

Titlwall needs to be braced until you get the roof structure (and, obviously the LFRS, if not entirely tiltwall) in. If you get a major hurricane during construction, well that's just really bad luck, but it's not like anyone will be at the job site during construction. I don't think it's reasonable or cost effective to design every stage of construction for the maximum possible loading.

 

[csd72]

Yes that was the type of thing that I was trying to infer.

If there was a hurricane then people wouldnt be hanging out at the construction site and most of these sites are in the middle of a vacant lot. So very unlikely to be alife safety risk.

BUT if the panel could fall on say a public highway, a school, a hospital or similar then I would design the propping for the full wind load.

 

[msquared48]

Ever try to keep a kid out of a construction site?

Mike McCann
MMC Engineering

 

[rowingengineer]

Ever try to re-lift a tilt panel once the bracing has failed? You are exposing you client to a lot of risk of damages if the bracing was to fail during a cyclonic event that his insurance dose not cover. I recommend getting your client involved before making this change of wind speed decision.

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[csd72]

msquared48,

by that logic we should reduce all highway speeds to 30mph to allow for the occasional kid that may try to cross it!

Rowingengineer,

This reduction of wind speed is standard practice for temporary works. The cost of the additional bracing would far outweigh the risk. I think you will find that the onus is on the construction insurance policy rather than the owners.

Lets make this clear, we cannot completely eliminate risk! We can only allow for those reasonably foreseen risks and do our best to reduce them to an acceptable level.

Even if we designed for a hurricane, there is always the chance that nature will hit us with an even bigger hurricane.

This is not just a matter of bigger props, the footing or structure at the bottom of the prop needs to be designed for uplift and would need to be twice as big if the higher pressures were used. If these anchors are into the building structure then it may not be possible to make them work without completely rechanging the whole structure.

 

[frv]

M^2..

I almost always agree with you. However, a kid in a construction site during a hurricane?!

csd72 hit the nail on the head. We cannot eliminate risk. We can only try to manage it in a reasonable and, frankly, conservative way. 90 mph is already at the upper end of a category 1 hurricane.

 

[spats]

Everybody seems to forget the fact that tilt-up bracing design, like everything else, is done using safety/load factors. Designing for 90 mph means it shouldn't fail until about 115 mph using a 1.6 factor.

 

[csd72]

good point spats, although much of that factor of safety is allowing for wear and abuse of the bracings.

 

[rowingengineer]

csd72,
this maybe true for us law, but in Australia if the propping is design below the code requirements it will fail the insurance requirements and the onus will fall on the owner.

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[csd72]

rowingengineer,

I know exactly what you are talking about but AS1170 does have provision for seasonal and short term factors (I think this is in the importance factor now) in the wind section. This is also acknowledged in much of the correspondence from workcover, worksafe e.t.c.

By the way, I think the regulations in Australia regarding tilt-up and precast are some of the best in the world. Many things that you would never get away with in Australia are still regular practice in the US and UK as well as most other parts of the world.

A few good australian references:

http://www.commerce.wa.gov.au/worksafe/PDF/Codes_of_Practice/Code_Tiltup_Precast_.pdf

http://www.concrete.net.au/publications/pdf/briefing08.pdf

http://www.worksafe.vic.gov.au/wps/...1f586a814fee1fb554c40/tilt_up.pdf?MOD=AJPERES

 

[a2mfk]

Agree with CSD, this all boils down to actuarial probabilities and economics. During the 2004 hurricane season in central FL there were plenty of failures of CMU and tilt panels under construction. Like was said above, we know about hurricanes for several days out and nobody is anywhere near a construction site. This is why the GC's have insurance. Sure, the BEST thing to do would be design for full design wind loads, though that would ignore the probability and economic factors.

Since a braced tilt wall during construction usually has no axial load other than dead load, the failure mode we all agree on is pullout failure of the bolts at the slab level?

 

[frv]

Or buckling of the braces.

 

[csd72]

Usually a combination of pullout and shear on the anchor bolts.

The panel is unlikely to fail as the span is much less and the critical bending stress is usually during the lift from the casting bed.

Actually, thats one more point, I expect you will find that at 120mph wind most panel propping arrangements will fail due to uplift of the panel!

 

[peal724]

72 mph is the tilt-up industry standard wind load for temporary bracing.