web shear buckling bearing strength · • box beams tested at the civil engineering structures lab...
TRANSCRIPT
Casey Briscoe, Susan Mantell, Jane Davidson Department of Mechanical Engineering, University of Minnesota
Prototype Testing
• Box beams tested at the Civil Engineering Structures Lab
• Experimental validation of the interaction between structure and foam
• Web shear buckling • Bearing failure
Prototypes
• Four-point bending test • Examined shear buckling and postbuckling
behavior of the webs • Good agreement between predicted and
observed buckling strength
Shear Buckling Test • Shear buckling failure mode:
• Core shear failure mode:
• Bearing failure mode:
Bearing Failure Test
• Three-point bending test (end one-flange loading condition)
• Good agreement between model and results • AISI demonstrates superposition of web
and foam strength • Analytical model works with
• Up to 3 day erection time requiring skilled labor
• Loose fill insulation has gaps and thermal bridges
• On-site construction waste
• Off-site manufacture and ½ day field installation
• Open attic space • Reduced construction waste • Possible energy savings up to 35%
A Better Performing Roof The objective of this study is to develop a one-piece modular roof panel system that is manufactured in a continuous process and provides a more energy efficient building envelope.
Panel Concepts
Truss Core Panel Web Core Panel
Two panel concepts were investigated:
• Metal structural component • Insulating component fully
separated from structural • Insulation is attached to
interior or exterior surface
• Structural and thermal components are integrated
• Can partially separate the insulating component to use less foam
Sponsored By: U.S. Department of Energy
Face Sheet Buckling
Panel Deflection
Web Shear Buckling
Web Core Failure Modes
Core Shear Failure
Bearing Failure • Balance between structural and thermal requirements
• Foam core material used in novel way to strengthen sheet metal components
• New structural models developed
• Snow/wind (live) loads • Self weight (dead) loads • Sustained/cyclic loading
• Insulating R-Value • Thermal bridging due to webs • Temperatures up to 80°C at
exterior surface
Roof Panel Requirements Structural Thermal
Integration between Structural and Thermal Functions
Web Core Panels
Bearing Strength
• Plastic collapse mechanism (Roberts and Newark, 1997):
Analytical Model Semi-Empirical Model (AISI)
• Bearing failure involves deformation/crushing of foam
• Foam strength superimposed with web strength
• Factor FC accounts for variability in bearing test data
• Based on unified web crippling model used in prescriptive steel design codes (AISI)
• Factors CR, Cc, and Ch are functions of web geometry and construction
• Effect of foam crushing strength accounted for using superposition
• Matches current design practice
Panel Designs • Failure mode map (feasible
designs shaded):
• Most designs limited by thermal requirement and shear buckling
• Can separate design process into two steps: • Design web geometry
(thermal/shear buckling) • Design face sheets
(deflection/face buckling)
• Minimum weight designs developed • Four panel types compared:
• Truss core panels • Web core panels with carbon steel
webs • Web core panels with stainless
steel webs • Web core panels with exterior foam
layer (separated) • Southern US: low loads and R-value • Northern US: high loads and R-value
Southern US: q = 1576 N/m2, R = 5.3 m2-K/W Depth (mm) Weight (N/m2)
Truss Core 272 265 (Carbon) Web Core 284 205 (SS) Web Core 275 206 Separated Web Core 280 203
Northern US: q = 3537 N/m2, R = 6.8 m2-K/W Depth (mm) Weight (N/m2)
Truss Core 359 354 (Carbon) Web Core ---- ---- (SS) Web Core 400 283 Separated Web Core 400 407
Web Shear Buckling Plate on Pasternak Elastic Foundation
Buckling Solutions
Foundation Modeling
• Relate the foundation constants KW and KP to foundation material properties
• Model validated using finite element analysis
• Pasternak model applicable to deep foundations with high shear stiffness
Application to Panels
• Buckling mode shapes:
• Buckling coefficient vs. web spacing:
• Wider web spacing increases the effect of the face sheets
• Plate buckling model • Foam modeled as a Pasternak foundation
• Buckling coefficient χ determined analytically using energy methods
• Finite element model:
• Buckling coefficient vs. web slenderness:
• Analytical model under-predicts buckling strength by 11–21%
• Face sheets provide added rotational resistance to webs
• Buckling coefficient: • Buckling mode shapes: