Transfer structure design..... key element design method. 3

[Agent666]

We are looking at getting involved in a structure with a considerable vertical load transfer beam structure at the lowest level.

This consists of proposed long span PT beams supporting up to 5-6 levels of building structure above, typical ~8.5m grids, transfer beams supporting two columns worth of structure above at approx quarter points (transfer beam spans ~17m between supports and is continuous over multiple grids. The details aren't too important in terms of the questions, but provided for background.

We are starting to think about what additional load factor to apply to the ULS design to allow for added robustness in the transfer level. There's some local advice (not codified) suggesting design for 1.5 x full ULS gravity loadcase (in this case a 1.5(1.2G+1.5Q) loadcase) and a similar design factor that is codified for amplifying seismic forces to accommodate compatibility actions right up to MCE 2500 year event.

We also have a requirement to design for vertical earthquake actions, similar to above we have some localised advice on combining this with horizontal and gravity actions (a suggested rational design procedure using combined vertical seismic loadcase with a Gravity+horizontal seismic case using SRSS combination). I also found some advice in Eurocode 8 regarding this particular aspect... question on this below

Much of the literature I've found/reviewed suggests using a risk based approach, and in the next breath says there is currently limited recognised guidance on doing this... happy to use engineering judgement, but there's nothing like backing it up with some best practice approaches.

I feel like we have a good handle on the beam design itself and performance objectives, apart from settling on the exact loading scenarios to be adopted.

So ....

1.....
Basically sounding out others regarding any codified advice from around the world related to the gravity load 'extra' factor to ensure sufficient robustness at the ULS for critical gravity transfer structures. One word/phrase that popped up in some searches regarding critical elements like the transfer beam and columns supporting it is the concept of 'key element design'. I believe it comes from the UK/Eurocodes, but for the life of me I cannot find anything in any of the structural Eurocodes related to this. But it seems to thrown in there in a lot of UK literature when talking about disproportionate collapse and methods for dealing with this aspect.

2.....
Regarding in Eurocode 8 CL 4.3.3.5.2, the combination of vertical and horizontal seismic forces, does someone know exactly what the '"+" implies "to be combined with"' means in the following context. For combining two horizontal components the referenced 4.3.3.5.1(2) clause seems to imply SRSS combination between 100/30% cases in each orthogonal direction. Once including vertical actions is it a simple linear superposition/addition, or a similar SRSS combination method or other method as the normal interpretation/usage of this clause? (not a regular Eurocode user unfortunately, maybe I'm overthinking the "+" to be more than a "+" since they singled it out for explanation?)

3.....
Anything related to this in American standards that someone can direct me to?

 

[Guest262653]

1 - Never done it myself, but I suggest the following standards and guides:
- EN 1991–1–7, Eurocode 1: Actions on structures – Part 1–7: General actions – accidental actions
Look for "key elements", points 3.3(2)a) and section A.8.
- "How to design concrete buildings to satisfy disproportionate collapse requirements" - Concrete Centre guide Link
- "Practical guide to structural robustness and disproportionate collapse in buildings" - The Institution of Structural Engineers Link
- "Structural Analysis and Design to Prevent Disproportionate Collapse" - Feng Fu, CRC Press Link
- "Progressive Collapse of Structures" - ICE Link

2 - It envolves an additional SRSS combination group and another term in the usual two plan direction combination groups. Take note that the design spectra and behaviour factor for the vertical direction are different.

 

[JLNJ]

It sounds like you are also dealing with an “irregularity” as defined in ASCE 7 and all the provisions which come with it.

Is it to be built in the US? What is the seismic design category?

 

[Agent666]

EN 1991–1–7, Eurocode 1: Actions on structures – Part 1–7: General actions – accidental actions
Look for "key elements", points 3.3(2)a) and section A.8

Thanks for that. I'll have a read of those reference and see how it's suggested to apply this notional 34kPa load in the case of a transfer structure. Having read just the clause at this stage, I'm not immediately clear on whether the 34kPa is just applied in a vertical direction to the immediate tributary area of the transfer beam at the level of the transfer beam, or if its to the entire supported area on all levels?

It envolves an additional SRSS combination group

Again thanks. Nice and easy.

It sounds like you are also dealing with an “irregularity” as defined in ASCE 7 and all the provisions which come with it.

Is it to be built in the US? What is the seismic design category?

Yes the structure is irregular in several ways (vertical, torsional, mass), I'm based in New Zealand. Its in a region of low seismicity for us, but no idea how this compares with US categories. A typical fully elastic design in this region would have a base shear coefficient of approx 0.3 for comparison.

Irregularity in our seismic codes doesn't come with any additional load to be considered apart from needing to scale any response spectrum cases to 100% of equivalent static, if regular there is the opportunity to only scale up to 80% unless you're already over this level of base shear. There are also limits on the degree of irregularity to ensure an acceptable configuration.

Can you direct me to the appropriate clause in ASCE if you're suggesting there is any additional loads vertical or horizontal prescribed in US for a similar scenario.

 

[JLNJ]

It sounds like you might be in the equivalent of Seismic Design Category C for the US.

I believe you would be carefully looking at using the overstrength factor (Omega) for the elements associated with the irregularity.

Check out Tables 12.3-1 and 12.3-2 and all of Chapter 12.3 "Diaphragm Flexibility, Configuration Irregularities, and Redundancy".

 

[rapt]

Agent666,

I thought that was a no-no in your earthquake zones!

We have been discussing this for a while for Australian codes but have not resolved it all yet. The 34MPa logic does not work for transfer beams in our opinion. It was meant more for members like columns affected by blast loading.

When looking at transfer beams you need to consider continuity and the ability to redistribute actions. A multi span transfer beam will normally be able to redistribute actions partially through catenary action for the beam itself and partly through redistribution through vertical ties and the catenary action in the floors above. It will also depend on how many columns are transferring and how many are direct to the ground. Your earthquake detailing will help a lot with this anyway.

For simply supported transfer beams, and cantilevers, you have a lot more problems and the over strength logic is really the only solution. Options are to either increase the load by a factor, or reduce the capacity by a factor. So either 1.5 times the load effects or an extra importance/key element factor on the capacity reduction factor of .67.

Or design it for the full earthquake load, no reduction factors for ductility!

 

[miningman]

Ive got absolutely zero experience with earthquakes / seismic design but my reading of the OP original post suggests to me this might be a structure that will have to withstand excessive daily shock loading...... some kind of mining operation perhaps ?? If I'm correct, strongly suggest you design to mining standards..... and then some...... before worrying about the seismic aspects. The owner will not thankyou if the structure has to be replaced within say 3 years of normal heavy duty abuse.

 

[Agent666]

Miningman, nothing at all to do with mining, its primarily a carpark structure.

I thought that was a no-no in your earthquake zones!

Well that depends on what 'that' means and what you've interpreted 'that' as .

I did neglect to mention, the transfer structure is not the lateral load resisting system. It's just required to be designed for deformation compatibility... Going along for the ride if you like with beam and columns detailed appropriately for the design actions from this or for the overstrength capacities if columns are required to undergo any inelastic action. I certainly wouldn't be designing a transfer beam as the primary lateral load resisting system as a moment resisting frame. We'll leave that for others that don't know better...

The 34MPa logic does not work for transfer beams in our opinion. It was meant more for members like columns affected by blast loading

Yeah the more I've read on it I came to the same conclusion. Intended to be some sort of lateral load to ensure elements that cannot be removed have sufficient robustness. Not really the additional vertical or lateral load factor I was hoping to find to ensure robustness in the gravity and lateral load path with respect to a transfer structure.

We certainly won't have the opportunity to deal with column removal or horizontal/vertical tying or catenary action given the spans involved.

Currently only exterior columns and any elements of the lateral load resisting system are direct to ground. Internal columns are intended to be supported from multiple bay long transfer beams at 1/4 points of transfer beam as originally noted.

 

[KootK]

From a philosophical perspective, I'm not big on amplified loads as a means of supplying robustness and, rather, favor:

1) Strong connections.

2) Redundancy.

3) Ductile failure modes.

Thinking along such lines:

4) For the beams, I'd be generous with the shear reinforcing and supply top and bottom mild steel capable of resisting a substantial proportion of the governing midspan moment. I suspect that this would not be too difficult as your beams are probably sized for serviceability concerns.

5) For the columns, I'd like to proportion them such that demand fell below the P-M balanced points and column axial capacities would be relatively insensitive to moment demand.

6) For the all important beam to column connections, I might apply that 34MPa to the columns in each direction, non-concurrently and track through the implications of that in the design of the joint between the beams and columns. I think that, combined with your seismic drift compatibility design, would lead to fairly robust connections.

 

[rapt]

Kootk,

Agreed, except for the exceptional case, like a cantilever or simply supported transfer beam with no redundancy, or a very long span transfer beam where vertical ties and redistribution through the structure just does not work, e.g. if there are 2 or 3 columns transferring on each transfer span.

Even long ago when we did not have earthquakes in Australia (until suddenly we did), I always over-designed transfer beams, both because of peace of mind at nominal extra overall building cost and for construction sequence freedoms and stage stressing.

Now that we do have earthquakes, I am even more glad I did it.