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

B-18-10

© 2018Karagozian & Case, Inc.

Karagozian & Case, Inc.700 N. Brand Blvd., Suite 700Glendale, CA 91203(818) 240-1919www.kcse.com

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

B-18-10pg 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 ofhandling, 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.

B-18-10pg 3Course Description

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 4-010-01. 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.

B-18-10pg 4Learning Objectives

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.

B-18-10pg 5Background

Inhabited DoD buildings must comply w/ UFC 4-010-01.

“Conventional construction” concept.

Cross-laminated timber (CLT) is not **currently** “conventional construction”.

B-18-10pg 6Overview

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

B-18-10pg 7

CLT IntroductionOverview

Developed in Austria & Germany

Engineered wood panel

Bonded with structural adhesives and pressed

Panel variation Ply number Wood species

Grade classification distinction

© CLT Handbook, US Edition

B-18-10pg 8

CLT IntroductionRelevant Standards & References

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 06 17 19 of Unified Facilities Guide Specifications (UFGS)

Manufacturer Product Data

Protective Design Center-Technical Report (PDC-TR) (Under Development)

B-18-10pg 9

Brief Introduction/Review of Blast Loading and Dynamic Analysis for Blast Effects

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, DIFR1, W1

R2, W2

B-18-10pg 10Blast Loads

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.

B-18-10pg 11What is an Explosion?

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

B-18-10pg 12Combustion Basics

Basic chemistry:

High pressures and temperatures leads to an expansion of the gas

Deflagration vs. Detonation Deflagration: subsonic combustion

(< Mach 1) Detonation: supersonic combustion

(> Mach 1)

heatenergyOHyxCOOyxHC yx /24 222

B-18-10pg 13

Important Characteristics of a Blast Load

Point on reflecting surface(Reflected pressure)

Point in space(incident pressure)

B-18-10pg 14

Example scenarios illustrating the effects of reflecting surfaces

Free Air Burst Surface Burst

Explosive

oPressure “Gauge”

Pressure “Gauge”

Pressure “Gauge”

Pressure “Gauge”

Barrier

Surface Burst w/ Reflecting Structure

Surface Burst w/ Reflecting Structure and Barrier

B-18-10pg 15Roof and rear blast loadings

Incident pressures Dynamic pressures

Ground

Explosive

Axis of Symmetry

o 1

Pressure “Gauges”

o 2

o 3

4o

5o

6o

o 9

o 8

o 7

B-18-10pg 16How to calculate blast loads

Definitions: R = Standoff or range [ft, m] W = Charge weight/mass [lb-f, kg] Z=R/W1/3 = 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.

B-18-10pg 17

Unified Facilities CriteriaUFC 3-340-02

UFC 3-340-02 Also known as Army TM 5-1300,

NAVFAC P-397, AFR 88-22

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.

B-18-10pg 18

Look-up charts for simple blast loads, i.e., pressure and impulse

Simple Blast Loading CategoriesCharge

Confinement Category Pressure Loads

Unconfined Explosions

1. Free air burst2. Air burst3. Surface burst

a. Incidentb. Reflected

Confined Explosions

4. Fully ventedc. Internal shockd. Leakage

5. Partially confined

c. Internal shocke. Internal gasd. Leakage

6. Fully confinedc. Internal shocke. Internal gas

Free Air Burst

Surface Burst

Confined Explosions

B-18-10pg 19Type of Explosive

TNT equivalencyExplosive Pressure

FactorImpulse Factor

TNT 1.0 1.0

ANFO 0.87 0.87

C-4 1.20 1.19

PENT 1.27 1.27

B-18-10pg 20

Hemispherical Surface Burst

Can be used for assessment and design.

Note scale terms.

B-18-10pg 21Influence of Explosive Shape

Charge shape may make a significant difference at a small standoff distance.

B-18-10pg 22Load Magnifiers, Site Layout

Blast reflections under

overhang

Blast reflections off other nearby buildings

Blast reflections inside setback

Some site layouts substantially magnify airblast pressures Due to reflections Lack of venting

B-18-10pg 23Tools for computing blast loads for HE

Hand calculations or look-up charts. TM5-1300 (U.S. Government, open-access) TM5-855-1 (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)

B-18-10pg 24Structural Response

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.

B-18-10pg 25

The Concept of a Resistance Function(a.k.a., Resistance-Deflection curve)

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.

Deflection[Units: Length ]

Softeningbehavior

RuR

esis

tanc

e[U

nits

: Pre

ssur

e, fo

rce/

area

] Perfectly plasticbehavior

Membranebehavior

Elastic

Elastic-Plastic

Plastic

B-18-10pg 26How to determine a resistance function

(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

B-18-10pg 27SDOF models for blast engineering

Same equation of motion

Conversion of structure to “equivalent” SDOF using “transformation factors”

Transformed SDOF equation

Linear system

Nonlinear system

m

u(t)

p(t)

k (Linear)

kt (Nonlinear)

c

General SDOF model.

Resisting Force (fs)

Loading Function (p)

Damping Force (fD)

Inertial Force (fI)

2eM m x x dx

eF p x x dx e sR f x x dx

KM

KL

KR = KL

Concept of generalized SDOF using transformation factors (modal participation

factors).

2eC c x x dx KC = KM

p(x,t)

m(x), c(x), EI(x)

x1

B-18-10pg 28

Equivalent SDOF Properties:Load/Mass Factors for Beams

Define: Load/Mass Factor

Typical Load/Mass Factors

Illustration of plastic mechanisms in a beam with

fixed-fixed boundary constraints.

MLM

L

KKK

Resisting Force (fs)

Loading Function (p)Equation for SDOF solution

B-18-10pg 29

Blast vs. seismic design from an SDOF perspective

Earthquake

Loading is proportional to the mass.

More mass is bad.

Ductility is desirable.

Damping helps.

Blast

Loading is proportional to the exposed area.

More mass helps.

Ductility is desirable.

Damping helps, although is typically not accounted for.

B-18-10pg 30

Example: Steel beam subjected to 234 psi, 2808 psi-msec blast load

W18x50 strong-axis properties (12” tributary) I = 800 in4

Z = 101 in3

E = 29,000,000 psi Fy = 50,000 psi W = 150 psf (including beam)

Spring constant, k

Ultimate bending moment, Mp

Maximum resistance, Ru

Period, T

inpsik /716

sec012.0T

DeflectionR

esis

tanc

e

inpsiinin

inpsibL

EIk /71612)120(5800000,000,29384

5384

4

4

4

inkpsiinZFM yp 050,5000,000,50101 3

psiinin

inkbL

MpRm 23412)120(

050,58822

sec012.0/716

144/4.386150

2222

inpsiinsin

psf

kMT

psiRm 234

B-18-10pg 31

Solution using the SBEDS package or TM5-1300 charts

max 0.92D in

Solution:

Load p =234 psi i = 2,808 psi-msec td = 24 msec

td/T = 0.024 / 0.012 = 2.0 Rm/Ft = Rm/p = 1.0 Dyield = Rm / k = 0.33 in

From Chart m = 3.2 Dmax = Dyield*m = 0.33 in * 3.2 = 1.06 in

B-18-10pg 32

Tools for computing the blast effects response of structures

P-I Curve Packages CEDAW (U.S. Army Corp. Protective Design

Center)

SDOF Packages TM5-1300 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)

B-18-10pg 33

Resistance FunctionOverview

Idealized resistance vs. out-of-plane displacement relation

Process Investigate post-peak response

through testing Quantify initial stiffness and ultimate

resistance Average static strength Strain rate effects

k

ru

rr

De Du

Outermost CLT ply ruptures

Innermost CLT ply ruptures

Schematic Resistance Function for 3-Ply CLT

Panel w/ Simple Boundary Conditions

B-18-10pg 34

Resistance FunctionUMaine Panel Tests – No Axial Load (Video)

B-18-10pg 35

Resistance Function3-Ply Grade E1 Panel

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Resistance Function3-Ply Grade V1 Panel

B-18-10pg 37

Resistance Function5-Ply Grade V1 Panel

B-18-10pg 38

Resistance FunctionStiffness Computation

Two-step process: Compute apparent bending stiffness, EIapp:

EIeff = Effective bending stiffness from manufacturer data GAeff = Effective shear stiffness from manufacturer data L = Span Ks = Shear deformation influence constant (see below)

𝐸𝐼𝑎𝑝𝑝 =𝐸𝐼𝑒𝑓𝑓

1 +𝐾𝑠𝐸𝐼𝑒𝑓𝑓𝐺𝐴𝑒𝑓𝑓𝐿2

Source: CLT Handbook, US Edition

k

B-18-10pg 39

Resistance FunctionStiffness Computation

Compute stiffness, k:

C = Relevant adjustment factors from NDS excluding those associated with load duration (i.e., Cm , Ct)

bw = Section width b = Loaded tributary width kb = Bending influence constant (e.g., 5/384 for simple boundary

conditions)

𝑘 = 𝐶 ∗𝐸𝐼𝑎𝑝𝑝𝑏𝑤𝑘𝑏𝑏𝐿4

k

B-18-10pg 40

Resistance FunctionOut-of-Plane Strength Computation

Smaller of: Bending strength, FbSeff’:

SIFb = Static increase factor (see next slides) DIFb = Dynamic increase factor (see next slides) C = Relevant adjustment factors from NDS excluding those

associated with load duration (i.e., Cm , Ct) FbSeff = Allowable bending strength from manufacturer data

Flatwise shear strength, Vs’:

SIFs = Static increase factor for flatwise shear (see next slides) DIFs = Dynamic increase factor for flatwise shear (see next slides) Vs = Allowable flatwise shear strength from manufacturer data

𝐹𝑏𝑆𝑒𝑓𝑓′ = 𝑆𝐼𝐹𝑏 ∗ 𝐷𝐼𝐹𝑏 ∗ 𝐶 ∗ 𝐹𝑏𝑆𝑒𝑓𝑓

𝑉𝑠′ = 𝑆𝐼𝐹𝑠 ∗ 𝐷𝐼𝐹𝑠 ∗ 𝐶 ∗ 𝑉𝑠

ru

B-18-10pg 41

Resistance FunctionStatic Increase Factor

Transforms allowable strength to average strength Ten minute duration of load assumed (CD = 1.6)

Two step process Allowable => Characteristic (5% exclusion)

PRG 320 testing safety factors

Characteristic (5% exclusion) => Average Coefficient of Variation (COV) associated with wood species / stress type

Source: PRG 320-2017, Table 1 footnote d

𝐶𝑂𝑉 =𝜎𝜇=𝑆𝑡𝑎𝑛𝑑𝑎𝑟𝑑 𝐷𝑒𝑣𝑖𝑎𝑡𝑖𝑜𝑛

𝑀𝑒𝑎𝑛

B-18-10pg 42

Resistance FunctionStatic Increase Factor

Grade E1Grade V1Grade SL-V4

B-18-10pg 43

Resistance FunctionDynamic Increase Factor

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

B-18-10pg 44

Resistance FunctionUltimate Resistance Computation

ru

Source: UFC 3-340-02, Table 3-1

B-18-10pg 45

Validation TestsOverview

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

B-18-10pg 46

Validation TestsFull-Scale CLT Structures (Test 3 Video)

B-18-10pg 47

Validation TestsCLT Panel Response (Test 3 Video)

B-18-10pg 48

Validation TestsTests 1 – 3 Results: 3-Ply Grade V1

Displacement Gage Location

B-18-10pg 49

Validation TestsTests 1 – 3 Results: 3-Ply Grade E1

Displacement Gage Location

B-18-10pg 50

Validation TestsTests 1 – 3 Results: 3-Ply Grade SL-V4

Displacement Gage Location

B-18-10pg 51

Validation TestsTests 6 – 7 Results: 5-Ply Grade V1

Displacement Gage Location

B-18-10pg 52

Validation TestsAFCEC Panel Tests – With Axial Load (Video)

B-18-10pg 53

Validation Tests3-Ply Grade V1 Panel @ Different %Fc

B-18-10pg 54

Validation Tests5-Ply Grade V1 Panel @ Different %Fc

Flatwise shear limit computed using (Ib/Q)eff as defined in CLT Handbook

Flatwise shear limit computed using Vs value as defined in PRG 320

B-18-10pg 55

Structure Grade Roof Floor

V1 4 12

E1 4 8

SL-V4 4 4

No. of Blocks / Structure

Remove & Replace Front Wall Panels

1’-0” TYP

Rotate Roof Panels 90°

Validation TestsTests 4 – 5 Structure Loading

B-18-10pg 56

Validation TestsTests 4 – 5 Results

W i t h A x i a l L o a d

W i t h o u t A x i a l L o a d

Grade V1 Grade E1 Grade SL-V4

B-18-10pg 57

Validation TestsTests 4 – 5 Results

Grade V1

Grade E1 Grade SL-V4

B-18-10pg 58

Response LimitsOverview

Response limits provide means to evaluate analysis results

SDOF dynamic analysis response limits defined in defined in PDC-TR 06-08 No CLT response limits, currently

Source: PDC-TR 06-08

B-18-10pg 59

Response LimitsRecommendations

k

ru

r1

De Du

Outermost CLT ply ruptures

Innermost CLT ply ruptures

Schematic Resistance Function for 3-Ply CLT w/ Simple Boundary Conditions

k

ru

r3

De Du

Outermost CLT ply ruptures

Innermost CLT ply ruptures

Schematic Resistance Function for 5-Ply CLT w/ Simple Boundary Conditions

2De

r1

Middle CLT ply ruptures

2De

Source: PDC-TR 06-08

Controlling Limit State

B1 B2 B3 B4 m m m m

Flexure 0.9 - 1.5 - 1.75 - 2 - Shear 0.9 - 1.5 - 1.75 - 2 -

B-18-10pg 60

Response LimitsTest Results Review

SUPERFICIAL DAMAGE

MODERATE DAMAGE

HEAVY DAMAGE

HAZARDOUSFAILURE

AssumptionsCoefficient 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

SL-V4

B-18-10pg 61

Response LimitsLoad Bearing Wall CCSD Comparison

Wall Type Sections Span

Min. Static

Material Strength

EWIStandoff Distance

EWII StandoffDistance

Reinforced Concrete ≥ 6” 12’ – 20’ 3,000 psi 66 16

Reinforced Masonry 8” – 12” 10’ – 14’ 1,500 psi 86 30

CLT – EIFS 3-ply 10’ – 12’Grades E1, V1,

and SL-V490 35

Steel Studs – EIFS 600S162-43; 600S162-54;600S162-68 8’ – 12’ 50,000 psi 361 151

1 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 E14 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

B-18-10pg 62Summary

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

B-18-10pg 63Questions?

Please Contact:Mark Weaver, S.E.

Karagozian & Case, Inc.(818) 240-1919

weaver@kcse.com

or

Leo Torres, S.E.Karagozian & Case, Inc.

(818) 240-1919torres@kcse.com

This concludes The American Institute of Architects Continuing Education Systems Course