New Load on Existing Members

[GalileoG]

I am trying to gauge what is normal practice when it comes to reinforcing existing members for new load (I practice in a Canadian jurisdiction if that helps).

Say I am adding 10% new load to an existing member. Can I design the reinforcing elements to carry the 10% load increase only? The problem here is that I am assuming that the existing member was adequately designed for the original loads.

Can I make that assumption and meet the standard of care?

What if the building is 30+ years old with a satisfactory past performance and no visual signs of distress in the present? Does this not demonstrate that the building has satisfactory capacity to resist loads (other than earthquake)? Does that change your answer?

What if I have the original design structural drawings that indicate the original design loads? Would that change your answer as well? And if I do not have the original design drawings, can I deduce the original design loads based on the age of the building and the building code that would have governed at the time?

Thanks

 

[tony1851]

If you design the 'reinforcing elements' only to take the 10% increase in loading (say on a beam) how would you ensure that the connection between existing and new elements can transmit the shear?

If the building is >30 years old and shows no signs of distress, how can you be sure it will be adequate to take the additional loads plus the loads imposed by your 'reinforcing elements'?

I shouldn't necessarily rely on od drawings to identify what the structure is about.

 

[CELinOttawa]

I practice in Ontario, and when I upgrade a member I take full responsibility for complete design of the existing member. You must.

In reinforced concrete that is even harder than it sounds... I must ensure that the beam does not become OVER reinforced (ie: in danger of sudden failure, under-reinforcing being a fundamental precept of good RC design), and this means ensuring or correcting for the function of old bars and new together.

There are many ways of acheiving this in all materials, but frankly I see you as being wholly responsible for all loads impossed as well as the proper behaviour of the final product; This is the only way to safeguard the public - Your first and most important duty.

 

[Once20036]

I think it largely depends on the original beam and how you intend to reinforce it.
If you have a steel beam and want it to carry additional load - I would strengthen it such that the new section can carry the proposed load. That is to say, in the past I haven't worried about existing stresses within the steel and have assumed that 100% of the load would be applied to the reinforced section.

I don't feel that satisfactory performance to date comes into play if you're changing the use of the space and adding load. I think that satisfactory performance to date can justify to focusing on a beam if there's no change in use, however, as CEL says if you're changing the load, you take full responsibility.

If I have existing structural drawings, I`ll still field measure ~50% of the existing members, to ensure that the as built construction conforms to the drawings. If I start finding discrepancies I`ll measure more and more until I`m confident that I know how the structure was built.

Also, don't forget about the connections.

 

[GalileoG]

Thank you for the responses.

CELinOttawa, I am of the same opinion but was completely stomped when I read Sentence 18 of Commentary L "Evaluation Based on Satisfactory Past Performance" in the NBCC 2010 Commentaries. Can you read this Sentence and let me know what your interpretation is?

 

[Triangled]

What is the material?

 

[CELinOttawa]

Hi GalileoG,

The NBCC code commentary section you attached is a reflection of CBD-230, the Canadian Building Digest which used to be published by the Institute for Research in Construction. I know it well, and have relied upon this many times in the past.

This sentence relates to situations where there has been no change of use. While they do not state it in that section, this is about field testing of an existing structure. It is not applicable to a case where you are modifing the structure in any way.

You should have a read of CBD-230; It and nearly all the other CBDs are available online at

http://archive.nrc-cnrc.gc.ca/eng/ibp/irc/cbd/digest-index.html

They are of great use to practicing engineers, and the fact that they have stopped publishing them is a great loss.

 

[Parrapit]

Hi All

Let me put it this way I have done what you propose to do in the past. My situation was were we had to move a wall to the end of a cantilever floor or edge but the supporting wall underneath was 2m away from the edge of the cantilever.

So we casted a beam as an up-stand that can carry the load of the wall above but left a 30mm gap (with jointex). 25mm Gap was found from the long term deflection and we added another 5mm. So the idea is the beam does not actually lie on the slab and therefore does not distribute the load to the existing structure. We also arched the beam to minimize the deflection.

So actually might not be the same as to your situation.

While we are on this topic to increase an existing RC-beam depth to carry more load than what it was designed for, would you support it underneath chip say 40% of the beam depth away so the existing steel is exposed, re box the beam adding steel at the top (compression area) and and fix shear steel to existing stirrups and re-cast the beam. Also we did do the original beam design so know exactly what the beam was designed for and what steel is present in it.