Formwork Shoring Design Loads 1

[MrPete70]

I was curious if there is any standard in designing of formwork shoring.

I am currently reviewing some shoring that is what I consider a typical shoring configuration. A plywood form supported on runners spaced 12” to 19.2” on center, which in turn are supported by stringers spaced 4’ to 5’ on center, which in turn are supported by scaffolding or doka shores at 4’ to 5’ on center.

The contractor has utilized some design software that only takes into account tributary area when determining the design loads, however, the members are two or more spans in both directions. From and engineering standpoint, when designing the supporting members the maximum load is at the center support whose reactions are 25% greater than that of the tributary area. When you finally get down to the scaffolding the reaction is 56% more than just tributary areas.

Is utilizing the tributary area only the correct analysis procedure for this type of design? Older text books that I have found do, however, my instincts tell me that this in incorrect and that this older procedure has creeped into the design software. Any comments?

 

[Ingy]

MrPete70,

Check out

http://www.dot.ca.gov/hq/esc/construction/manuals/Falsework/

This is the Caltrans falsework manual which we have to use follow for all their projects. It is the industry standard here in California.

 

[DTGT2002]

MrPete70,

ACI Document 347, Guide to Formwork for Concrete, is considered to be the standard for concrete formwork as governed by ACI 318, the building code for concrete, in the US.

The caltrans falsework specs and guides are very useful though somewhat specialized in their nature and scope.

Design by tributary area is very common in many circles of formwork. However, it is often accompanied by reduced capacities of members to help offset the actual loadings seen by members due to beam penalites.

Further, analyzing the shoring system with a respect for the the differential deflections based on load application will show sufficient shortening in many common wood members to distribute the increased loads to adjacent shores. It is possible that the design software accounts for this or at least was written for the shoring system in questions and can provide a justification for the loading appearing to be tributary area only.

In my company, we take into account beam penalties for span loadings in both direction and use our tested values for member capacities or use the capacities as allowed by the appropriate member codes. Other firms use the tributary area and reduce the allowable loadings of their members.

With appropriate judgement from the shoring engineer of record, either method can provide satisfactory results.

If you would like to list out the basic specs of the drawings, I would do a gut check look at it and we'll see if I find it to be reasonable.

One good resource for general formwork design is ACI puplication SP-4 "Formwork for Concrete". It is older and uses tables to describe acceptable spacings. It is not the easiest or most efficient way to design or check, but it does provide a time tested methodology and resource for guidance.

I have never seen or used a generic spreadsheet or design program for formwork that I thought was all encompassing or perfect(or even very good). I have made plenty of them, but usually for very specific tasks. Some manufacturers do provide programs to help spec their proprietary members and such, but we usually design in house using appropriate sectional data or have the engineering rep of the system design and stamp the details.

please provide more details if you can and lets see if there is a simple solution.

Thanks,

Daniel Toon