• Field Notes 08.11.2012

    I hope by the time you read this, you will have voted.  And, all the political ads will have been pulled; the Facebook endorsements and hate emails will have stopped.  And that it’ll be at least 3.5 years before we see another political ad. Can I at least hope?

    Regardless of who wins, can we all just agree that in a democracy, the people choose who our next president is.  And that the choice has been made. Just don’t blame me, I probably voted for the looser.  If you didn’t pick the winner, join the club, there were up to 49.9 percent others who didn’t pick the winner either.  The next four years, the winner shall be President of these United States.  Can we all have just a little respect for the office regardless of who occupies it?  And, agree that this democracy process generally works?  If your guy didn’t win – wait four years.  We get to do it all over again.  Love it or leave it, but ain’t it great?

    In the same way choosing a president impacts the economy and daily life long after the election, selecting curtain wall anchors and embeds usually sets the precedent for how the job will go.

    Someone once explained that if on any window or curtain wall project you could save 25 percent on an anchor design or the typical way in which anchoring the curtain wall was accomplished, you’d surely win the next job.  It was cheaper to add more material to the anchor design if it could be offset by faster installation, since the field labor savings helped offset the costs.

    Besides structural silicone glazing, unitized walls have driven much of the innovation regarding how walls are connected to buildings.

    The first jobs I worked on were stick curtain walls.  Two steel plates were bolted to the aluminum mullions and headed / threaded bolts were embedded in the concrete.  Steel clip angles were attached to those embed bolts (usually to the face of the concrete spandrel beam) and then welded to the plates on the side of the mullions.

    On the first unitized wall I ever worked on, we put steel plates into the face of the concrete spandrel beams, and welded two shelf angles for every unit to those plates, then thru-bolted the unitized wall panels to the clips through the sill intermediate horizontals.  The reasoning for this approach was that the depth of the spandrel beam was almost as tall as the spandrel panel, thus denying any access to the sides of the mullions.

    And then the world got fancy.  Aluminum extrusions, by utilizing higher strength alloys such as 6005 and 6061, could be fabricated to allow for all manner of adjustments, in/out, usually by slotting the base anchor plate, and jacking screws allowing for up/down final adjustments.  There are all sorts of variation on this theme.  The anchor plates could be mounted on the face of a slab, or recessed into block-outs in the top of the slab, never being seen in the final installation.

    One of the largest contributors to the overall design variability in anchors and embeds has been the advancement in embeds, most notably the development of hot rolled steel sections used in concrete construction.  Such sections allow adjustment in the direction of their length; the bolts come in a variety of diameters and lengths, and are generally more forgiving in terms of placement tolerances.

    While most of this subject deals with concrete embeds, steel anchors welded or otherwise attached to a steel structural member carry equally weighted concerns.  But a misplaced steel anchor on a steel frame can be more easily moved or replaced than their concrete counterparts, by means of grinding off welds, removing bolts, etc, and relocating the anchor to the correct location.  Not so with concrete embeds.

    Most concrete frames will allow the use of steel anchors embedded in the concrete or in precast concrete panels.  In steel structural frames, the use of embeds is usually limited to the composite concrete deck slabs.  If the embed can’t go in the composite deck slab, embeds usually won’t be required, but connection directly to the steel frame is likely to change the whole anchor design.

    When dealing with concrete, embeds are usually the first materials delivered to the project.  They are required when the slabs or structures are poured, usually months in advance of the curtain wall or window so they get some immediate attention in terms of shop drawings, structural calculations, and material purchasing.  Because of this lead time, there’s not a lot of leeway to play with in terms of their design and execution.  It has to be done right and completed rather quickly.

    So as prominent as they are to good anchor design and execution, there’s a lot that goes into embeds. The load imposed on the embed, the strength of the concrete the embed is placed in to resist that loading, the design of the embed itself, the associated material and labor costs all go toward determining if a proposed design is correct.  Although the labor cost for placing embeds in formwork prior to the concrete pour is usually borne by the general contractor, it too plays a role if the GC doesn’t buy into the “means and methods” of doing so.

    There’s nothing worse than embed problems when it’s time to actually install a wall.  I once worked on a composite deck project where the embeds pulled out of the concrete at about 1/8 of their design strength.  Turns out the concrete in the corrugated decking was left exposed after it was poured, it rained, and the concrete basically turned to sand.  Every embed was tested, a lot of concrete was torn out and replaced to fix this problem.  A lot of cost and delay to the schedule were incurred.

    The same contractor told my employer at the time that they could locate the edge of slab and finished floor datum to within 1/8” of nominal.  As the industry standard is generally +/-1” (and is often specified) in or out, up or down, left or right, holding 1/8” was music to our ears.  When some of the slab and embed locations came in out of tolerance by as much as 4”, we had to scramble with redesigned anchors, new materials, and delays while the new anchors were fabricated.  Needless to say, there was a lot of discussion about who was responsible to pay for all of this added effort in time, material, and resources.  This job ended up at the courthouse – never fun for anyone.

    And there’s always one or two embeds that somehow don’t make it into the forms prior to the concrete being poured.  Field fixes are not easily dealt with, and can delay erection two to three days, again waiting for redesigned anchors and fabrication.

    In determining an anchoring scheme, involve field personnel and get their buy-in since they’re the ones who will actually work with it.  Be prepared for missing embeds, or embeds that are placed outside of the +/-1” location tolerance.  And if given the luxury, impress upon the general contractor and the subcontractor putting the embeds in the concrete how much accuracy and thoroughness matter in getting ALL embeds in, and in the proper location.

    The bottom line is the crunch is on when it comes to embeds, or any anchor scheme for that matter, because their delivery is so far in advance of when the wall itself is delivered to the site.  And, you only have one chance in a very small schedule-driven window to get it right.  But when it is done correctly, the whole job will benefit from it.  If you can’t set anchors efficiently, then the rest of the job will be staggered to follow that.  No way around it:  set off on the right course with good solid embed and anchor design and execution, and the entire job will go smoother.

    Posted by Bryan @ 10:21 am

  • One Response

    • Well done Chuck. Interesting how you moved from politics to construction also. Always enjoy reading your blogs. Keep up the good work. Lyle Hill

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