Reinforced Concrete Wall vs. Block Wall

[cb1331]

Our firm has been in the design stage of a three story office building for the last several months and is now in construction. We have specified a 10" reinforced concrete wall internal to the buidling envelope to resist the lateral loads on the buidling. The Owner has asked if we can revise this to a 10" block wall but I am hesitant to due so, mainly due to the structural inadequacy of a reinforced concrete block wall vs a reinforced concrete wall (height ~= 40' so h/t <=30, so block would have to be atleast 16" and this doesnt even include eccentricity). I am preparing a response but I believe his main concern is cost and speed of construction. Could any of you, in your experinces, please shed light on this issue of whether a reinforced concrete vs. reinforced masonry block wall is better in regards to cost and/or speed of construction?

Your comments are appreciated.

Thank you.

 

[JedClampett]

That's a huge change, not to be taken lightly. Different codes, seismic response, construction details, etc. If the wall unbraced height is 40', it's not going to work. But I think you're confusing the wall height with the effective height.
This should of been finalized during design. Everything is going to have to stop while you redesign the building. How is this going to save any time?

 

[JAE]

I'd agree with Jed on this too. The cost-speed issue is also one that would have to be referred back to the contractor as well - they would have a much better view of cost-speed that you.

 

[azcats]

About a decade ago when I was fresh out of school working at a large firm, there was a project being constructed that utilized CIP concrete elevator cores as the lateral resisting system. On that job the contractor unwittingly used CMU under the supervision of the firm's inspector. The project engineer was horrified when he visited the site one day.

I'm not sure how it all worked out, but I know it cost the inspector his job and the firm quite a bit of time/effort to 'make' the CMU cores work...along with the embarrassment.

 

[steellion]

As Jed said, you can't just swap one out for the other. It's a total redesign.

See how the contractor feels about the situation when you hit him with a six-figure design change order.

 

[cb1331]

Thank you. Unfortunately the General Contractor on this project is the Owner. Needless to say it has been a very painful few months. It is a difficult situation, but I have responded to him that we will not do a block wall as this will require re-design and subsequent re-design of architectural as well if we had to increas the thickness of the wall.

Thanks for your comments.

 

[BAretired]

1331,

Was there no dialog between you and your client during design? If he is the General Contractor and the Owner, he must have known what you intended from the beginning. Why did he not speak up earlier?

BA

 

[frv]

For the good of the profession, you should stand your ground. Do not redesign this unless you charge him for the FULL design of the new system. Scope creep seems to be an ever increasing problem and this goes way beyond scope creep. This is an entirely (or nearly) new design.

 

[dik]

Generally office building construction does not lend itself to masonry wall construction. If the location is in a high seismic area then it may be difficult to use masonry, else it may not be that difficult to change. Can you provide added info about the framing and is the project under construction?

10" concrete walls seem fairly thick for RC for a 3 storey building; I've done six storey buildings with 6" walls and 18 storey with 8" reinf conc block walls. Is there other structural support other than bearing walls?

Dik