Storefront Into an Out-of-Level Masonry Opening
A glazing contractor is installing a new aluminum storefront into an existing cast-in-place opening in a 1960s masonry building. The head measures 12 ft 0 1/4 in, the sill 11 ft 11 3/4 in, and the head is out of level by 1/2 in across the width; the jambs are plumb within 1/8 in. The storefront anchors through 1/2 in × 2 in slotted holes into cast-in anchors already set in the concrete that cannot be relocated. Glazing is 1 in insulating glass; the system provides 1/2 in edge bite with a nominal 1/4 in glazing clearance. Five lites have been fabricated and shipped at 46 in wide × 84 in tall. Installation day temperature is 38°F and the design summer temperature is 96°F. The lead installer proposes to set the frame square and "spring" the head with the anchor bolts, then shim the sill to hide the resulting gap.
1. Evaluate the proposal to spring the head into place. What does it do to the frame, the anchors, and the glass?
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Springing the head is the wrong answer, and it is the kind of shortcut that only shows up months later.
The frame. Storefront aluminum is extruded to tight tolerance and is not designed to be bent into a crown. Pulling a 1/2 in bow out of the head with the anchor bolts works against the extrusion's own stiffness, so the frame stays in a preloaded, distorted state. That distortion runs down the jambs - the glazing pockets are no longer parallel, the fixed lights are no longer square, and the door opening (if any) is racked.
The anchors. The 1/2 in slots exist to let the frame move thermally and to absorb construction tolerance. Springing the frame consumes that slot travel at the head, so when the frame expands or contracts there is nowhere left to go - the load goes into the cast-in anchors or back into the glass. Slots are not adjustment for forcing geometry.
The glass. A distorted frame with non-parallel pockets will not hold the specified 1/4 in glazing clearance. Some lites lose clearance at one corner and gain it at another, so glass bears on metal - the classic point-load break. The frame must be self-supporting and square before any glass goes in. If the frame is carrying the glass, the glass is carrying the frame's mistakes, and glass is a terrible structural member.
2. Calculate the thermal movement the frame and glass will see, and explain what that does to the head and sill detailing.
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Movement is small but not zero, and on a 12 ft span it directly competes with the 1/4 in clearance.
Aluminum: use about 1.3 × 10⁻⁵ in/in/°F. ΔT = 96 − 38 = 58°F. Length = 144 in.
144 × 1.3 × 10⁻⁵ × 58 ≈ 0.109 in, call it 1/8 in.
Glass: coefficient is roughly 4.7 × 10⁻⁶ in/in/°F.
144 × 4.7 × 10⁻⁶ × 58 ≈ 0.039 in, call it 1/32 in.
Differential: about 0.07 in across the width - the aluminum grows and shrinks about 2.8 times as much as the glass.
What it means:
- The frame needs at least 1/8 in of movement capability at each end of the head and the sill, and the anchor slots must have room in both directions - checked cold, since the frame is installed at 38°F and will run to 96°F.
- The head-to-structure sealant joint must be sized for that 1/8 in plus gasket and construction tolerance: typically a minimum 1/4 in width, never less, and a depth-to-width ratio of about 2:1 so the sealant can flex rather than tear.
- The sill closure must absorb the movement without loading the glass or the setting blocks. Shimmed gaps that simply hide the out-of-level head with a lump of sealant will crack and leak.
- A 1/2 in out-of-level head is a measurement problem first. If the 1/2 in is taken up as visible shim and closure at the head, the glass sizes must be field-verified against that condition rather than trusting the original shop drawing.
3. Give the correct sequence to resolve the out-of-level head and get the five lites in.
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Sequence it so no measurement is taken twice and no glass touches metal.
- Field verify before fabrication, not after. Measure the opening at head and sill, plumb the jambs, and check both diagonals. The head/sill difference (12 ft 0 1/4 in vs. 11 ft 11 3/4 in) is 1/2 in - diagonal check confirms whether the opening is parallelogram or whether the head itself is out of level. If the lites were shipped before this check, verify them against as-built and stop if they don't fit without forcing.
- Build the frame square. Set jambs plumb, set the head level, and let the 1/2 in discrepancy live between the head and the concrete - not inside the frame. Use a continuous shim or a fabricated closure plate so the frame still bears evenly and the anchors fall in the middle third of their slots.
- Anchor in the middle of the slots, snugged so the frame can still move, using the specified fasteners, then verify no anchor has bottomed out in the slot.
- Set glass on setting blocks at the quarter points of each lite, with the specified clearance all around, checked with a feeler gauge before gaskets or sealant.
- Seal the head joint at the correct width and depth with backer rod and a sealant rated for the movement, and detail the sill with positive slope to the exterior and a functioning weep path.
If any lite would require forced clearance, the lite is recut - not wedged.