New for DoD Designers: Structural Design of Mass Timber Exposed to Blast Loads

Transcription

1 Karagozian & Case, Inc. 700 N. Brand Blvd., Suite 700 Glendale, CA (818) New for DoD Designers: Structural Design of Mass Timber Exposed to Blast Loads By: Mark K. Weaver, S.E. Leonardo M. Torres, S.E. Presented at: Mass Timber Structural Design Quarterly Webinar Series March 14, 2018 Disclaimer: This presentation was developed by a third party and is not funded by WoodWorks or the Softwood Lumber Board 2018 Karagozian & Case, Inc.

2 pg 2 The Wood Products Council is a Registered Provider with The American Institute of Architects Continuing Education Systems (AIA/CES), Provider #G516. Credit(s) earned on completion of this course will be reported to AIA CES for AIA members. Certificates of Completion for both AIA members and non-aia members are available upon request. This course is registered with AIA CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.

3 Course Description pg 3 Facilities constructed for the U.S. Department of Defense (DoD) must often be designed for blast loads in accordance with antiterrorism requirements stipulated in UFC As cross-laminated timber (CLT) and other mass timber solutions continue making inroads in federal construction projects, demand for a design methodology complete with response limits for CLT construction exposed to blast loads has emerged. This presentation will provide a primer on general blast design requirements for DoD facilities and introduce a blast design methodology for CLT construction based on two years of testing research.

4 Learning Objectives pg 4 1. Provide an overview of DoD antiterrorism design criteria. 2. Review essential concepts used to analyze structural components for blast loads. 3. Discuss analytical response limits appropriate for CLT construction exposed to blast loads based on a suite of test data. 4. Introduce a design methodology for CLT construction exposed to blast loads that considers both panels and connections.

5 Background pg 5 Inhabited DoD buildings must comply w/ UFC Conventional construction concept. Cross-laminated timber (CLT) is not **currently** conventional construction.

6 Overview pg 6 Topics CLT introduction Airblast load analysis basic concepts Resistance function suitable for SDOF dynamic analysis Response limits based on test results Analysis guidance assumes: Far-field explosion Airblast load idealized as transient uniformly-applied load

7 CLT Introduction Overview pg 7 Developed in Austria & Germany Engineered wood panel Bonded with structural adhesives and pressed CLT Handbook, US Edition Panel variation Ply number Wood species Grade classification distinction

8 CLT Introduction Relevant Standards & References pg 8 ANSI/APA PRG 320 Standard for Performance-Rated Cross-Laminated Timber (2017) National Design Specification (NDS) for Wood Construction (2018) CLT Handbook, US Edition (2013) Section of Unified Facilities Guide Specifications (UFGS) Manufacturer Product Data Protective Design Center-Technical Report (PDC-TR) (Under Development)

9 Brief Introduction/Review of Blast Loading and Dynamic Analysis for Blast Effects pg 9 The objective of this introduction is to briefly describe the concepts and methods employed in the blast-resistant design of facilities. Important definitions and acronyms (ASCE 59-11) Standoff, R Charge Weight, W Scaled Distance, Z = R/W 1/3 Far Range: ~ Z > 3.0 Near Range: ~ Z < 3.0 Rate Effects Dynamic Increase Factor, DIF R 1, W 1 R 2, W 2

10 Blast Loads pg 10 Define an explosion. Define characteristics of a blast load and the parameters used to define it. Introduce tools used to compute blast loads. Goals: Know how blast loads are computed on structures for simple scenarios. Understand limitations of simple models.

11 What is an Explosion? pg 11 Baker et al. (1983) definition: an explosion is said to have occurred in the atmosphere if energy is released over a sufficiently small time and in a sufficiently small volume so as to generate a pressure wave of finite amplitude traveling away from the source. However, the release is not considered to be explosive unless it is rapid enough and concentrated enough to produce a pressure wave that one can hear. National Fire Protection Association (NFPA) definition: The sudden conversion of potential energy (chemical, mechanical, or nuclear) into kinetic energy that produces and violently releases gas. Key points: Sudden release of energy Produces, as a minimum, an audible sound Not tied to actual or potential damage

12 Combustion Basics pg 12 Basic chemistry: y y C x H y x O2 xco2 H2O energy / heat 4 2 High pressures and temperatures leads to an expansion of the gas Deflagration vs. Detonation Deflagration: subsonic combustion (< Mach 1) Detonation: supersonic combustion (> Mach 1)

13 Important Characteristics of a Blast Load pg 13 Point in space (incident pressure) Point on reflecting surface (Reflected pressure)

14 Example scenarios illustrating the effects of reflecting surfaces pg 14 Explosive Pressure Gauge Pressure Gauge o Free Air Burst Surface Burst Pressure Gauge Pressure Gauge Surface Burst w/ Reflecting Structure Barrier Surface Burst w/ Reflecting Structure and Barrier

15 Roof and rear blast loadings Incident pressures Dynamic pressures Axis of Symmetry Explosive 4 o o 3 o 2 o 1 5 o pg 15 Pressure Gauges 6 o o 7 o 8 o 9 Ground

16 How to calculate blast loads pg 16 Definitions: R = Standoff or range [ft, m] W = Charge weight/mass [lb-f, kg] Z=R/W 1/3 = Scaled range Type of reflecting surfaces and confinement Four general approaches exist: Hand calculations or look-up charts. Software implementations of look-up charts. Engineering-based shock reflection codes. Computational fluid dynamic (CFD) codes.

17 Unified Facilities Criteria UFC pg 17 UFC Also known as Army TM , NAVFAC P-397, AFR Provides the best compendium of material that is publicly available on the subject of blast effects on structures. Material is basic in nature and represents the traditional approaches to blast engineering. A lot of useful charts for blast load calculation covering many scenarios. Detailed discussion on design approaches and procedures for hardened structures.

18 Look-up charts for simple blast loads, i.e., pressure and impulse pg 18 Free Air Burst Simple Blast Loading Categories Charge Confinement Category Pressure Loads Unconfined Explosions 1. Free air burst 2. Air burst 3. Surface burst 4. Fully vented a. Incident b. Reflected c. Internal shock d. Leakage Surface Burst Confined Explosions 5. Partially confined c. Internal shock e. Internal gas d. Leakage Confined Explosions 6. Fully confined c. Internal shock e. Internal gas

19 Type of Explosive pg 19 TNT equivalency Explosive Pressure Factor Impulse Factor TNT ANFO C PENT

20 Hemispherical Surface Burst pg 20 Can be used for assessment and design. Note scale terms.

21 Influence of Explosive Shape pg 21 Charge shape may make a significant difference at a small standoff distance.

22 Load Magnifiers, Site Layout pg 22 Some site layouts substantially magnify airblast pressures Due to reflections Lack of venting Blast reflections under overhang Blast reflections inside setback Blast reflections off other nearby buildings

23 Tools for computing blast loads for HE pg 23 Hand calculations or look-up charts. TM (U.S. Government, open-access) TM (U.S. Government, Controlled Distribution) Software implementations of look-up charts. CONWEP (U.S. Government, Controlled Distribution) SHOCK (U.S. Government, Controlled Distribution) FRANG (U.S. Government, Controlled Distribution) Engineering-based shock reflection codes. BLAST-X (U.S. Government, Controlled Distribution) SHOCK (U.S. Government, Controlled Distribution) Computational fluid dynamic (CFD) codes. FEFLO (Private/U.S. Government, Controlled Distribution) CTH (U.S. Government, Controlled Distribution) GEMINI (U.S. Government, Controlled Distribution) AutoDYN (Commercial)

24 Structural Response pg 24 Introduce the concept of a resistance function. Describe the models that can be used to estimate their response. Range-to-effect and Pressure-Impulse (PI) models Single-Degree-of-Freedom (SDOF) models Finite element (FE) or Computational Solid Dynamics (CSD) models Goals: Understand the basic structural behaviors seen under blast loads. Know how the effects of blast loads are determined for structures under simple blast scenarios. Understand the limitations (and complexities) associated with each approach.

25 The Concept of a Resistance Function (a.k.a., Resistance-Deflection curve) pg 25 Analogous to a load-deflection response, or in structural engineering, a push-over curve. Particular to a component under a known (or assumed) response mode and a prescribed load. Resistance [Units: Pressure, force/area ] R u Plastic Elastic-Plastic Elastic Perfectly plastic behavior Softening behavior Membrane behavior Deflection [Units: Length ]

26 How to determine a resistance function pg 26 (1) Compute it using structural mechanics (2) Experiments (3) Computational finite element models In reality, all three are needed to develop reliable resistance functions. Much research has gone into this over the last decade. Software programs available for different components

27 SDOF models for blast engineering pg 27 Same equation of motion Inertial Force (f I ) Damping Force (f D ) Resisting Force (f s ) Loading Function (p) c u(t) m p(t) Linear system Nonlinear system Conversion of structure to equivalent SDOF using transformation factors 2 M e m x x dx e F p x x dx e s R f x x dx 2 Ce c x x dx Transformed SDOF equation K M K L K R = K L K C = K M 1 k (Linear) k t (Nonlinear) General SDOF model. p(x,t) m(x), c(x), EI(x) x Concept of generalized SDOF using transformation factors (modal participation factors).

28 Equivalent SDOF Properties: Load/Mass Factors for Beams pg 28 Define: K Load/Mass Factor Typical Load/Mass Factors LM K K M L Equation for SDOF solution Resisting Force (f s ) Loading Function (p) Illustration of plastic mechanisms in a beam with fixed-fixed boundary constraints.

29 Blast vs. seismic design from an SDOF perspective pg 29 Earthquake Loading is proportional to the mass. Blast Loading is proportional to the exposed area. More mass is bad. More mass helps. Ductility is desirable. Ductility is desirable. Damping helps. Damping helps, although is typically not accounted for.

30 Example: Steel beam subjected to 234 psi, 2808 psi-msec blast load pg 30 W18x50 strong-axis properties (12 tributary) I = 800 in 4 Z = 101 in 3 E = 29,000,000 psi F y = 50,000 psi W = 150 psf (including beam) Spring constant, k 4 384EI ,000,000psi 800in k 716psi / in 4 4 5L b 5 ( 120in) 12in Ultimate bending moment, M p M p Maximum resistance, R u Period, T ZF 101in 3 50,000,000psi 5, 050k in y 8 Mp 8 5, 050k in R m 234 psi 2 2 L b (120in) 12in T 2 M k 2 150psf in / s 144in 716psi / in sec Resistance T k 716psi / in sec R m 234psi Deflection

31 Solution using the SBEDS package or TM charts pg 31 Solution: Dmax 0.92in Load p =234 psi i = 2,808 psi-msec t d = 24 msec t d /T = / = 2.0 R m /F t = R m /p = 1.0 D yield = R m / k = 0.33 in From Chart m = 3.2 D max = D yield *m = 0.33 in * 3.2 = 1.06 in

32 Tools for computing the blast effects response of structures pg 32 P-I Curve Packages CEDAW (U.S. Army Corp. Protective Design Center) SDOF Packages TM Charts SBEDS (U.S. Army Corp. PDC Various and general form) WinGARD/WinLAC (GSA - Windows) CBARD (K&C/DTRA - Columns and retrofits) FE Codes with Explicit Time Integration LS-DYNA (Livermore Software Technology, Commercial) ABAQUS Explicit (SIMULIA, Commercial) DYNA3D (Lawrence Livermore National Laboratories, Controlled Distribution)

33 Resistance Function Overview pg 33 Idealized resistance vs. out-ofplane displacement relation r u Outermost CLT ply ruptures Process Investigate post-peak response through testing Quantify initial stiffness and ultimate resistance Average static strength Strain rate effects r r k D e Innermost CLT ply ruptures D u Schematic Resistance Function for 3-Ply CLT Panel w/ Simple Boundary Conditions

34 Resistance Function UMaine Panel Tests No Axial Load (Video) pg 34

35 Resistance Function 3-Ply Grade E1 Panel pg 35

36 Resistance Function 3-Ply Grade V1 Panel pg 36

37 Resistance Function 5-Ply Grade V1 Panel pg 37

38 Source: CLT Handbook, US Edition Resistance Function Stiffness Computation pg 38 Two-step process: Compute apparent bending stiffness, EI app : EI app = EI eff 1 + K sei eff GA eff L 2 k EI eff = Effective bending stiffness from manufacturer data GA eff = Effective shear stiffness from manufacturer data L = Span K s = Shear deformation influence constant (see below)

39 Resistance Function Stiffness Computation pg 39 Compute stiffness, k: k = C EI appb w k b bl 4 k C = Relevant adjustment factors from NDS excluding those associated with load duration (i.e., C m, C t ) b w = Section width b = Loaded tributary width k b = Bending influence constant (e.g., 5/384 for simple boundary conditions)

40 Resistance Function Out-of-Plane Strength Computation pg 40 Smaller of: Bending strength, F b S eff : r u F b S eff = SIF b DIF b C F b S eff SIF b = Static increase factor (see next slides) DIF b = Dynamic increase factor (see next slides) C = Relevant adjustment factors from NDS excluding those associated with load duration (i.e., C m, C t ) F b S eff = Allowable bending strength from manufacturer data Flatwise shear strength, V s : V s = SIF s DIF s C V s SIF s = Static increase factor for flatwise shear (see next slides) DIF s = Dynamic increase factor for flatwise shear (see next slides) V s = Allowable flatwise shear strength from manufacturer data

41 Resistance Function Static Increase Factor pg 41 Transforms allowable strength to average strength Ten minute duration of load assumed (C D = 1.6) Two step process Allowable => Characteristic (5% exclusion) PRG 320 testing safety factors Source: PRG , Table 1 footnote d Characteristic (5% exclusion) => Average Coefficient of Variation (COV) associated with wood species / stress type COV = σ μ = Standard Deviation Mean

42 Resistance Function Static Increase Factor pg 42 Grade E1 Grade V1 Grade SL-V4

43 Resistance Function Dynamic Increase Factor pg 43 Investigators have recommended DIF between 1.20 and 1.35 for Grade E1 CLT Lowak (2015, 2016) Doudak (2018) DIF of 1.25 (i.e., 2.0 / 1.6) good estimate for airblast load analysis Source: 2015 NDS

44 Resistance Function Ultimate Resistance Computation pg 44 r u Source: UFC , Table 3-1

45 Validation Tests Overview pg 45 7 arena tests on full-scale CLT structures Tests 1-3 Unloaded structures 3-ply panels Grades E1, V1, & SL-V4 Tests 4-5 Loaded structures 3-ply panels Grades E1, V1, & SL-V4 Tests 6-7 Unloaded structures Alternative front panel configurations 5-ply Grade V1 CLT Alternative connection configurations 2x4 NLT

46 Validation Tests Full-Scale CLT Structures (Test 3 Video) pg 46

47 Validation Tests CLT Panel Response (Test 3 Video) pg 47

48 Validation Tests Tests 1 3 Results: 3-Ply Grade V1 pg 48 Displacement Gage Location

49 Validation Tests Tests 1 3 Results: 3-Ply Grade E1 pg 49 Displacement Gage Location

50 Validation Tests Tests 1 3 Results: 3-Ply Grade SL-V4 pg 50 Displacement Gage Location

51 Validation Tests Tests 6 7 Results: 5-Ply Grade V1 pg 51 Displacement Gage Location

52 Validation Tests AFCEC Panel Tests With Axial Load (Video) pg 52

53 Validation Tests 3-Ply Grade V1 Different %F c pg 53

54 Validation Tests 5-Ply Grade V1 Different %F c pg 54 Flatwise shear limit computed using (Ib/Q) eff as defined in CLT Handbook Flatwise shear limit computed using V s value as defined in PRG 320

55 Validation Tests Tests 4 5 Structure Loading Rotate Roof Panels 90 pg TYP Remove & Replace Front Wall Panels Structure Grade No. of Blocks / Structure Roof Floor V E1 4 8 SL-V4 4 4

56 Validation Tests Tests 4 5 Results pg 56 W i t h A x i a l L o a d Grade V1 Grade E1 Grade SL-V4 W i t h o u t A x i a l L o a d

57 Validation Tests Tests 4 5 Results pg 57 Grade V1 Grade E1 Grade SL-V4

58 Response Limits Overview pg 58 Response limits provide means to evaluate analysis results SDOF dynamic analysis response limits defined in defined in PDC-TR No CLT response limits, currently Source: PDC-TR 06-08

59 Response Limits Recommendations pg 59 r u Outermost CLT ply ruptures Innermost CLT ply ruptures k r 1 D e D u 2D e Schematic Resistance Function for 3- Ply CLT w/ Simple Boundary Conditions r u r 3 k Outermost CLT ply ruptures Middle CLT ply ruptures Innermost CLT ply ruptures Source: PDC-TR r 1 D e 2D e D u Schematic Resistance Function for 5-Ply CLT w/ Simple Boundary Conditions Controlling Limit State B1 B2 B3 B4 m m m m Flexure Shear

60 Response Limits Test Results Review pg 60 HAZARDOUS FAILURE HEAVY DAMAGE MODERATE DAMAGE Assumptions Coefficient of Variation Grade V1: 0.40 Grade E1: 0.10 Grade SL-V4: 0.40 Panel Density Grade V1: 35 pcf Grade E1: 32.5 pcf Grade SL-V4: 30 pcf Supported Weight: 0 psf SUPERFICIAL DAMAGE SL-V4

61 Response Limits Load Bearing Wall CCSD Comparison pg 61 Wall Type Sections Span Min. Static Material Strength EWI Standoff Distance EWII Standoff Distance Reinforced Concrete ,000 psi Reinforced Masonry ,500 psi CLT EIFS 3-ply Grades E1, V1, and SL-V Steel Studs EIFS 600S162-43; 600S162-54; 600S ,000 psi Table shows proposed conventional construction standoff distances (CCSDs) for CLT assuming a LLOP based on a response limit of m < 1.5. This table has not been reviewed or approved by USACE. 2 Table does not consider openings; localized reinforcement may be required around openings for the CCSDs shown. 3 Assumed COV: 0.40 for Grades V1 and SL-V4; 0.10 for Grade E1 4 Assumed panel density: 35 pcf for Grade V1; 32.5 pcf for Grade E1; 30 pcf for Grade SL-V4 5 Assumed supported weight: 10 psf

62 Summary pg 62 Blast load generation overview Dynamic analysis overview, specifically single-degree-of-freedom methods Proposed resistance function for CLT panels exposed to out-of-plane airblast loads Tests to validate resistance function Quasi-static laboratory Full-scale structure arena Recommended response limits for CLT construction exposed to airblast loads PDC-TR to formalize guidance

63 Questions? This concludes The American Institute of Architects Continuing Education Systems Course pg 63 Please Contact: Mark Weaver, S.E. Karagozian & Case, Inc. (818) or Leo Torres, S.E. Karagozian & Case, Inc. (818)