Dormant Curtain Wall Anchors: Relative Stiffness Oversights

1
Dormant Curtain Wall Anchors Relative Stiffness
Oversights

• Mark Schmidt
• Wiss, Janney, Elstner Associates
• Dave Dunkman
• The University of Texas at Austin

SEI Structures Congress Buildings I Building
Facade 25 April 2008, 130 300 pm
Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
2
Agenda

• Topic Overview
• 3 Case Studies
• Material Location Scope of Service
• EIFS Pacific Island Expert Witness
• Stone Midwest Facade Inspection
• Aluminum Mountain West Design Review
• Various materials, locations, and scopes of
  service but all (eventually) involved remedial
  design
• General Conclusions

Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
3
Deciphering Presentation Topic

• Dormant Curtain Wall Anchors Relative
  Stiffness Oversights
• Uneven load distribution among anchors caused by
  anchorage components with different relative
  stiffness
• Common design approach for load distribution
• Assume connecting elements to be rigid, anchors
  to be deformable
• Often results in even load distribution

Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
4
Deformable Anchors? Yes, but
Photo Bendigo, Hansen, and Rumpf (1963)

• Experimental studies show anchors deformable, but
  load unevenly distributed in certain situations
• Design with reduced capacity rather than tested
  capacity

Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
5
Topic Relevancy to Design Practice

• Other design considerations of importance to the
  satisfactory performance of the connected
  material, such as block shear rupture, shear lag,
  prying action, and connection stiffness and its
  effect on the performance of the structure, are
  beyond the scope of this Specification and
  Commentary.
• Specification for Structural Joints 2004
• Research Council on Structural Connections

Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
6
Case I Underdesigned EIFS

• Relatively new hotel on US territorial island
  built by design-build contractor
• Damage from typhoon winds
• Broken windows
• Loss of EIFS panels (1.5 by area) near building
  corners
• Typhoon wind loads only 60-75 of design wind
  pressure
• Case proceeded to arbitration

Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
7
Typhoon Damage to EIFS
Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
8
Cladding System

• Key components
• Expanded polystyrene boards
• Gypsum sheathing
• Light-gage metal framing

adhesive screws
Panels detached when screws pulled through gypsum
sheathing
Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
9
Inadequate Remedial Design

• Install new anchors installed through EPS board
• Use proprietary plastic washers to engage EPS

Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
10
Inadequate Remedial Design

• Install new anchors installed through EPS board
• Use proprietary plastic washers to engage EPS
• Install sufficient new anchorages so that Pnew
  Pexist gt Wind Design Load with an acceptable
  factor of safety

Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
11
Flaws in Remedial Design Scheme
No consideration of difference in relative
stiffness
Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
12
Robust Remedial Design

• Core 1 in. holes through EPS board
• Install supplemental screw anchors similar to
  existing screw anchors
• High level of workmanship
• Recladding judged more economical
• Arbitrators awarded compensation for recladding
  cost to owner

Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
13
Case II Deficient Stone Anchorage

• 15-year-old high-rise office building in Midwest
• Routine facade inspection
• Loose stone panel discovered
• Inspection opening made at interior of panel
• 2 epoxied anchors at loose panel one failed,
  other loose
• Top-to-bottom facade review requested by owner

similar building
Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
14
Failed Epoxied Anchor
Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
15
Stone Facade System

• Panels
• 1-¼ in. granite
• Supported by kerf anchors (gravity loads) and
  epoxied anchors (wind loads)
• Epoxied Anchors
• Bent threaded rods embedded ¾ in.
• Fastened to aluminum strongbacks

5 ft x 7 ft panel relevant to case study
Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
16
Aluminum Strongback

• Aluminum channel
• 5 anchors at 12 spacing
• Ends attached (hooked) to curtain-wall framing

Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
17
In-Depth Facade Review Scope

• Design review of anchorage details
• In-place load testing
• Repairs as justified

Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
18
Design Review Anchorages
Original Design Pmax 174

WIND
Design Review Pmax 614 underdesigned
zzz
Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
19
Topic Relevancy to Design Practice

• Other design considerations of importance to the
  satisfactory performance of the connected
  material, such as block shear rupture, shear lag,
  prying action, and connection stiffness and its
  effect on the performance of the structure, are
  beyond the scope of this Specification and
  Commentary.
• Specification for Structural Joints 2004
• Research Council on Structural Connections

Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
20
Load Testing

• Proof load selected to limit tests
• Several failures, but all confined to 5th 7th
  floors
• Atypical epoxy color noted on these floors
• Failures attributed to construction defect
  (improper epoxy installation)

Pink (5th 7th)
Grey (typ.)
Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improved Overcladding
Conclusion
21
Repairs to 5th 7th Floors
Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
22
Case III Improper Overcladding

• 50-year-old high-rise office building in Mountain
  West
• Glass-and-aluminum curtain wall scheduled for
  rehabilitation
• Spandrel panel overcladding
• Design review for schematic design phase

similar building
Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
23
Curtain Wall System

• Aluminum spandrel panels
• Corrugated aluminum
• 2 ft x 5ft or 6 ft x 5ft
• 1/16 thick
• Wet-glazed within perimeter framing
• Glass
• Stone and brick

Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
24
Panel in Need of Rehabilitation
Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
25
Proposed Overcladding
Fasten new spandrel panel directly to existing
spandrel panel
Existing Curtain Wall Framing
Panel Perimeter Aluminum Framing
Existing Spandrel Panel (1/16 thick)
New Spandrel Panel
Aluminum Receiver
Stainless Steel Rivet (12 spacing)
Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
26
Issues with Proposed Overcladding

• Air and water infiltration
• Accommodation of thermal movements
• Overstressed rivets ?
• Wind load assumed to be resisted evenly by all
  rivets
• Aluminum receiver and extrusion significantly
  stiffer than existing spandrel panel

end rivets overstressed
Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
27
Mockup Evaluation
Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
28
Mockup Evaluation

• Pull-out testing revealed end rivet connection
  grossly underdesigned
• Overcladding details revised to include more
  substantial anchorage

Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
29
Conclusions

• Performing simplified analyses that neglect the
  relative stiffness of connected curtain wall
  components can result in overstressed anchorages
• Structural calcs dont have an easy button
• Ask the structure Jack Janney
• Consideration of relative stiffness in addition
  to strength especially important for remedial
  design

Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
30
Conclusions

• Why no catastrophic curtain-wall collapses ?
• Structural redundancy
• Conservative safety factors
• Low frequency and duration of near-design loads

Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion
31
Questions ?
Thanks for your attention!
Introduction
Underdesigned EIFS
Deficient Stone Anchorage
Improper Overcladding
Conclusion