Curtain wall engineers can assist in heading off problems with post tension slabs.
If using concrete embeds glazing contractor should make sure the installation of embeds is coordinated with the post tension strand installation. Paying attention to this issue can save glazing contractors time and money.
Ed D’Ambrosio, Senior Project Manager at Jangho Curtain Wall Americas shares.”We recently experienced this very problem on a post tensioned concrete structure in Los Angeles. The general contractor allowed the concrete sub-contractor to submit shop drawings too late (sometimes a few days before a pour) for any coordination of embeds with tendons or stud rails. Despite our numerous requests. Coordination was impossible as there were no documents for anyone to review. The result was much finger pointing, and weeks of frantic re-engineering. Seems to happen every job lately. Why?”
Chuck Knickerbocker, TGP Curtain Wall Manager, Technical Glass Products commented. “Seems like someone should have raised the flag about coordinating this at the very start of the job. So if not in the estimate and / or proposal, maybe even using the pre-bid RFI process if available. An ounce of prevention being the operable mode in this regard.
Hopefully that puts this issue on everyone’s radar,
and puts the GC on notice that there’s a lot of coordination that’s REQUIRED.
One favor to ask the structural engineering community. The curtain wall is going to have to be hung off the edge/top/bottom of the slab edge. Why not include the embeds in the structural drawings.
The weight of the wall and the imposed loads can be estimated.
Why not put the embeds into the structural drawings of the floor slab.
Do up front what’s going to be required.
(that being the coordination of embeds and PT cables)
So that the coordination issue is negated?
Wouldn’t that be a simpler approach?
Even if the embeds are over-designed, and they fall out of the typical scope of the building envelope contractor, isn’t that a simpler approach?”
Ed D’Ambrosio replied.“Chuck – there was a lot of discussion, RFI’s, and fist pounding on tables among our group concerning embeds. When it came to asking for coordination. Also there was no way to coordinate as the concrete sub had not submitted anything, and the GC allowed that to happen. (which appears to be standard operating procedure on every concrete project lately). So requests to the EOR for conflicts with the tendons. Cones and other elements with the channel embeds came back that they are not responsible for attachment of any element to the slab. They are concerned solely with the structural aspects and the movement of the concrete structure.
I totally agree that the EOR needs to be responsible for all characteristics of the structure. Including the forces imposed by the exterior envelope, and taking the lead in determining the attachment for the exterior wall so that there are no conflicts requiring weeks of redesign. Also i have been subject to all sorts of excuses by EOR’s over the years. As if they design a building as a shell and are not even aware that there are other elements that factor into the capacity of the slab. So if they design a post tensioned slab with no ability to hang a curtain wall with current attachment technology and technique. Also what is a glazing contractor, with no background in structural post tensioned slabs, to do?
This attitude is part and parcel of where construction is heading. General contractors take little to no responsibility for design issues; it is now the sub-contractors responsibility to “coordinate” with the other subs (which begs the question “What do we need GC’s for?”). Owners don’t want to pay architects to furnish complete drawings. So much is left out and becomes the responsibility of the sub. As the architect does not specify properly (or at all). Engineers of record do not want to address issues brought up by subs as they also are not “responsible”. In the end, the subs have to work out issues. Chewing up what little margin is left in the project, and no one wins.
Never used to be that way. ”
Tom Jackson, Vice President and Architectural Division Manager, Steel Encounters, Inc pipes in. “I think this relates to a bigger problem. The general mentality of the low bid process, how cheap can we build this building.
Good contractors know that is takes staff to check and coordinate these critical drawings and qualified staff costs money.
which much be built into a good bid. It’s true that you get what you pay for!
A few lessons I have learned the hard way:
A good checking process carries from the office into the field, embeds should always be surveyed for proper location in all three axis before concrete is poured. Also be careful to not assume responsibility for catching the errors of others if pushed into field surveying of embeds. So expect to be paid to check embeds if forced into it by contract, it takes labor, which costs money..
Final layout on finished post tensioned concrete slabs for curtain wall placement should always include checking embeds, especially if placed on the face of slab. So get your layout done early to gain as much time as possible for correction of work.
Most subcontracts require notification within 24 to 48 hours after discovery of any issue impacting your work. My foreman have use field observation reports they must fill out and turn into the contractor when they discover an issue. If the contractor is non-responsive. We create a log with an aging report which my PMs will email it to the GC every week (if needed). My last lesson, be careful to be friendly and remind the customer you are only doing this because they require written notification by contract. When given the opportunity, be a team player!
I hope sharing some of my hard lessons helps you!”
Carrie Jeske, project development at JEI Structural Engineering says. “We had a long talk about this post in our Tuesday morning meeting. Also i do believe it’s public conversations like this that build awareness for the issues and create lasting change.
Thanks to Ed, Chuck and Tom for your collaboration in helping others.”
Stewart Jeske, P.E., Principle of JEI Structural Engineering added,
“We try to address embeds far in advance, but sometimes we get stung, like the rest of you.
Good curtain wall engineers will address the issue early if they are responsible for designing embeds in post tension slabs. The way we do this at JEI is by raising the issue in an RFI. Embed anchors of our trade, embeds of other trades and all post tension anchors should be coordinated by either the Structural Engineer of Record (EOR) or by the Post Tension Slab Engineer (if this is a delegated design). Curtain wall engineers are not qualified to make decisions about how close embeds may be to post tension equipment. In addition, the curtain wall engineer may not know about other trades that require post installed anchors or imbeds and these need to be taken into account in an overall coordination effort.”