Shoring - Beam continuity factor 3

[forming_eng]

Hello,

I'm working on a shoring project where I have multiple span joists, supported by multiple span stringers, and a conventional frames system.


Design load = 200 psf

joists spaced at 19.2" o.c .... load on joists = 320 plf
Stringers tributary area = 6' .. I'm having a debate with another engineer about the method of calculating the load on the stringers.

I'm assuming the load on stringers is 200psf x 6'=1200 plf, and the leg load will be multiplied by 1.25 (for beam continuity)

my colleague is applying the additional 25% on the stringer and eventually to the legs, which will make the load on the stringers 1200plf*1.25= 1500 plf

The SP4 is not so clear about the beam continuity issue and I know many shoring companies don't even apply it, since the safety factor could be enough to cover the load increase.

Moreover, the joists spacing is governed by the plywood capacity, which almost always make the joist way under loaded to cause any additional load on the center stringer.

Any thoughts?

Appreciate your help.
- Al

 

[Agent666]

I'm assuming you are just shoring the stringers.

Not sure what SP4 is specifically? Is it a standard for shoring?

The 1.25 factor is presumably a factor on the reaction, for example for two equal spans with pinned supports, the center reaction will be the load x half the adjacent spans x the tributary span width x 1.125. I imagine the 1.25 factor simply is to cover all cases of this effect for various mismatching spans + perhaps some safety margin for uneven loading or mobilisation of the propping due to not being snug enough to immediately take up the loads.

If you have a long continuous beam with multiple supports then this factor tends to 1.0 in the center of the member.

Either way, I believe you are multiplying the tributary area based reaction for a support point by the 1.25 factor, not the initial load by the factor for checking the stringers. However, in this case where you are only interested in the reaction isn't it going to ultimately be the same factored reaction irrespective of where in the process you apply the 1.25 factor (maybe I'm misunderstanding things).

There should of course be a load factor on the load according to the load case you are looking at which applies to everything upstream of determining the reaction due to the continuity (might be separate/different loadfactors for dead and live load for example according to whatever code you are dealing with). The load factored load you work out is then further factored by the 1.25 factor and the simple tributary area (i.e. spacing of props in this case).

 

[DTGT2002]

For many years, the debate on how shoring should be analyzed has gone on and on, with the general consensus at this point being that ignoring continuity is not reasonable (of course, I say). Then the conversations moves to just what do the minimum factor of safety required for various members cover - where they determined based on assumptions of area methods in design, on probability of failure for a member type without any consideration to the typical method of analysis, or something else entirely.


For me, the case you describe above is for a joist supported by on three supports and the middle stringer in this scenario has a continuous joist in a two span loading condition, thus the center stringer sees a load of 1.25 times the span, or 25% more than the tributary area due to the very real "stiffness" of the joist at the center support.

Further, when the stringer is supported on shores with "uniform load" or repetitive equal joists loads, the first interior reactions at each end of the stringer will be highest, approach 1.15 of the support spacing.


Using continuity follows the principles of structural engineering and allows you to find a worst case load for each of your elements, surely proving the adequacy of your system.


That said, if you do not have joists only spanning over three supports and all joists span over four or more supports, the maximum load increase due to continuity approaches 1.15 or could be 1.1 if all joists have exactly four supports. But these assumptions can be hard to prove true and frankly may require inspection - it is often a more overall efficient approach to cover a design with the assumption of a two span than to provide the inspection support or varied details to minimize material.


At my office/company, we believe more in labor optimization with material/system operation focused on the crews at hand.



ACI Committee 347 Formwork (keeper of SP-4, Formwork for Concrete) is in the process of developing a design specification that will help address this, but it still has quite a bit of time to go. The discussion of what is already covered by the ANSI support requirements for factor of safety has been a big discussion. It is a challenge to find the right balance of worker/project safety vs. economy to allow projects to exist.


For me and mine, we'll analyze using continuity, sometimes including lapping cantilevers where load sharing may make sense and consider the listed capacities of materials with their prescibed factor of safety. We likely put more material in place than some competitors, but we also know our field staff by name and a good, safe design that gets us all home comfortably at the end of the day is our goal.



Daniel Toon

 

[forming_eng]

Thank you guys.

Daniel:

I agree with you, it is a challenge. Applying continuity on stringers as well as on shores will definitely put more materials, and probably make us lose projects to the competition. I've talked to one of the members of the ACI Committee 347 about this issue and he said applying beam continuity is a conservative approach. I know that some of the major players in the shoring industry don't apply beam continuity in their designs.

My argument is that ignoring beam continuity dose not compromise the safety and/or the structural integrity of the shoring deck. It might slightly reduce the factor of safety, but will never cause a failure.



Can you recommend any publications that support beam continuity in formwork design?

 

[jjl317]

I have worked in the shoring industry in the past, and I agree that the use of continuity is inconsistent at best, and in most cases, probably a conservative approach.

In reality, I would expect that if you could measure true reactions, you would likely end up somewhere in between the values including continuity, and those without. This is especially true if the analysis is done with a typical 2d beam analysis program, which does not address the "soft" supports of a stringer, or even the compression of a frame/post leg. Running a 3D model might better reflect the anticipated behavior of the situation, and reduce the potentially conservative effects of continuity.

 

[DTGT2002]

Well, in this conversation, you have now spoken with another (voting) member of 347.

My thoughts on analysis for formwork and shoring as a licensed engineer is that regardless of convention of the industry or what the competition is doing, there is no rational basis to ignore structural mechanics. With that said, there is no reason to be pointlessly conservative - don't assume every condition is either a simple span or a two span to find overexaggerated deflections or moments higher than your reality. Model as accurately as your crews can execute and ensure there is follow up and inspection to verify your assumptions AND plainly state assumptions in your drawings and calculations.

The fun (read painful) of formwork is finding the right puzzle pieces to make your square frames fit your round building.


Over the last almost twenty years, one challenge that repeatedly comes up is the idea that there is a one size fits all set of rules for formwork, shoring and reshoring. There is not. There is nuance to how any given problem must be approached and there is still a lot of art left in this specialty, not just following examples in SP-4.


Load factors and factors of safety exist to cover the unexpected, the unsual and the unanticipatible - not load we know will occur, as well as appreciate known material properties to be designed around. Many are also seemingly arbitrary. That said, some folks use a higher than recommended FOS to account for the approach you describe. There have been yelling matches over this at committee meetings.

But coming back around, it is certainly a topic to speak on. ACI has a committee fully dedicated to structural safety - the statistical evaluation of load factors and the life safety consequence thereof that functions mainly in concert with 318 - it might be interesting to see if they have thoughts on formwork.


Do we believe the FOS for formwork material acounts for a little wear and tear and helps us through when things aren't perfect? Maybe, but we shouldn't plan to use broken equipment. Does our FOS come into play when the laborer pulls the handle all the way down on the concrete bucket and drops all three yards in one spot? Maybe.


ACI 347 Guide to Formwork for Concrete currently does not speak to what constitutes a "rational" design. Nor does SP-4 Formwork for Concrete. Nor ACI 301 Specifications for Structural Concrete. If you ask the EOR of the project you are working on, they'd likely respond "how could you get away with designing by area?" But it is not uncommon and is tradition and at some point, materials and componets were likely more overbuilt than what they are today. We have become stunning in our ability to take manufactured good to the razors edge of compliance with specifications. And with Southern Yellow Pine, our harvesting and growth methodologies caught up with us, leading a change in material rating such that SYP is no longer distiguishable from other pine species and dropping its capacity to that of mixed.

I'll try to look on my bookshelf for any textbooks that had a formwork section and see if they speak directly to formwork and design methods, continuity or otherwise.


We use basic structural analysis in each direction to arrive a solution that assumes continuity, many of the vendors we use employ fea methods with members resolved to unit loads for charts in their design manuals.


Most of the problems I have seen first hand come from lack of stability in a system vs. inadequate or overloaded components.

 

[dhengr]

DTGT2002:
You’ve given a really a nice thorough presentation on that issue. But, you are suggesting that the engineer has to think a little bit about his design and that he/she use std./basic, well established, engineering design principles to do their design, and then apply some good engineering experience and judgement to guide their design decisions. Unfortunately, that is not the direction things are going. It seems that today’s engineers want a code with an explicit paragraph on every possible construction detail and condition. They want a code formula to plug into, without having to do any engineering thinking, they want an accountant’s flow chart to show which numbers to crunch, in what order, for the exact answer. Can’t you just give them a few worked examples, on everything, that they can plug into, maybe a spreadsheet which does it for them, some expensive software which does the whole darn job with one click?

Forming_eng:
You don’t/shouldn’t really apply those multipliers to the basic loads. That’s screwed-up engineering thinking, caused by the (by too much) emphasis of the application of load factors in LRFD design approach. The load factors in the LRFD approach are used to account for our lack of knowledge/certainty of the actual magnitudes of the various types of loads on the system. Based on the issue you are trying to address, you apply the appropriate multiplier to std. (simple span) moments, shears and reactions, on any members on which they might apply/occur. This is an attempt to simplify a generalized design, but still account for the possible higher values, because you don’t always know which one of the shores will have the continuous beam over it. Some stringers will likely warrant a multiplier on their moments and shears too because the joists on that stringer are continuous over that stringer. If it can happen, it will, so don’t ignore it, you do so at your own peril.

 

[CANPRO]

I agree with the above - if its a continuous member design it as such. But in the interest of streamlining the design, you can assume the 25% additional load until it leads to what might look like an over-conservative design (this takes some experience) and then you can sharpen the pencil and see if there are any savings.

You kind of allude to this in your OP - joist spacing is governed by the plywood, but you don't reduce your joist size, you just use what is available. Same kind of logic can be applied to the stringers...if you already have an idea of what you want to use for stringer size and spacing (often dictated by what kind of system the formwork contractor is using anyway) then apply the additional 25% until it doesn't work and then take a closer look.

my colleague is applying the additional 25% on the stringer and eventually to the legs

There might be more to it than just increasing your load by 25% and carrying that through. You can have a stinger that has continuous joist (+25%) and then that stringer can be continuous over a series of posts, again increasing the effective tributary area of the post. As dhengr notes, you can't just use blanket factors and hope it covers everything.