OVERVIEW OF UFC 3-340-02, STRUCTURES TO RESIST THE EFFECTS OF ACCIDENTAL EXPLOSIONS

OVERVIEW OF UFC 3-340-02, STRUCTURES TO RESIST THE EFFECTS OF ACCIDENTAL EXPLOSIONS

Author: Patrick F. Acosta, PE, US Army Engineering and Support Center, Huntsville; ATTN: CEHNC-ED-CS-S (Acosta); PO Box 1600; Huntsville, AL 35807-4301; phone 256-895-1661; fax 256-895-1602; e-mail: [email protected].

Abstract

UFC 3-340-02, Structures to Resist the Effects of Accidental Explosions, was recently approved by the Services. Publication of UFC 3-340-02 represents the culmination of a 5-year, Department of Defense Explosives Safety Board (DDESB) effort to update DoDs mandatory blast design requirements for explosives safety applications, as provided in Army TM 5-1300/NAVFAC P-397/AFR 88-22 (TM 5-1300), revision 1, November 1990. As an unlimited distribution document approved for public release, TM 5-1300 has long provided both government and private sector engineers with an invaluable source of blast effects and loading data and with step-by-step procedures for blast analysis and design. UFC 3-340-02 continues this tradition, using straightforward guidance and examples to illustrate and explain protective construction design requirements. While based upon TM 5-1300, UFC 3-340-02 incorporates several beneficial changes to the manuals reinforced concrete design requirements. This presentation provides an overview of the more significant of these changes. Anticipated, future revisions and additions to other chapters also are presented and discussed. UFC 3-340-02 may be downloaded from the Whole Building Design Guide website. For users who have the DPLOT computer program, readable versions of some manual figures also are available through this website. In 2003, the Department of Defense Explosives Safety Board (DDESB) established a Technical Working Group to revise the tri-service blast design manual, Structures to Resist the Effects of Accidental Explosions, Army Technical Manual 5-1300/NAVFAC P-397/AFR 88-22. As an unlimited distribution document approved for public release, TM 5-1300 provides both government and private sector engineers with an invaluable source of blast effects and loading data and with step-by-step procedures for blast analysis and design. This paper provides a status update of TWG activities and plans for future work. Emphasis will be given to revisions to chapter 4, Reinforced Concrete Design. Anticipated revisions and additions to other manual chapters also will be presented and discussed.

Introduction When US Army Technical Manual 5-1300/NAVFAC P-397/AFR 88-22, Structures to Resist the Effects of Accidental Explosions (TM 5-1300) was first published in 1969, it provided

1454Structures Congress 2011 ASCE 2011

engineers with groundbreaking, quantitative procedures for analyzing and designing structures to withstand non-nuclear blast effects (Department of the Army, 1969) [1]. Based upon the results of numerous explosive test programs and accident investigations, the manual included detailed design procedures and step-by-step design examples to illustrate their application. Since reinforced concrete performs particularly well under intense, short duration blast pressures, the manual emphasized the design of reinforced concrete structural elements. DDESB in 2001 solicited input from a broad range of TM 5-1300 users on recommended changes, corrections, and additions to the manual. APT Research, Inc., Huntsville, AL, consolidated the resulting comments in a summary report (APT Research, 2002) [3]. In October 2002, Naval Facilities Engineering Service Center (NFESC) submitted supplementary comments (NFESC, 2002) [4]. A wide range of input was received. The blast design community strongly recommended the update of the manual, both to remove outdated criteria and to incorporate research conducted since publication of revision 1. In addition, TM 5-1300 users requested the addition of new guidance on various topics such as the retrofit of existing structures, constructability, and the applicability of computer codes in blast resistant design. Based upon these comments, DDESB established the Technical Working Group to Update Army Technical Manual 5-1300/NAVFAC P-397/AFR 88-22 (TWG) in March 2003. The TWG provides direction to the revision effort and makes technical decisions related to the manuals content. The TWG charter specifically limits the scope to incorporation of existing procedures and products; no new research will be funded. Mr. Bill Zehrt, DDESB, serves as TWG chair. At the June 2003 organizational meeting, TWG members reviewed recent advances in blast resistant design meriting inclusion in Revision 2. A wide range of areas were considered including blast load prediction and modeling, analytical methods, design procedures, retrofit of existing structures, and innovative blast resistant materials. Per the TWG charter, discussions centered on items that were sufficiently developed to allow their incorporation without additional research. In addition, the TWG-funded studies to evaluate reinforced concrete research and test data published since the development of revision 1. When warranted, draft revisions to chapter 4 requirements were prepared for TWG consideration. At the June 2006 TWG meeting, USAESCH was tasked with developing a draft final interim UFC that incorporates both the foregoing changes and other updates approved at the meeting, including the revision of outdated references in chapter 4 to satisfy current code requirements (ACI, 1983) [6] and (ACI, 2008) [7]. Draft final revisions to the manual then were developed and disseminated, first through 2006 DoD Explosives Safety Seminar papers (Woodson and Zehrt, 2006) [8] and (Zehrt, Woodson and Beck, 2006) [9], next through draft final mark-ups to TWG members. Since that time, TWG comments have been resolved and incorporated in chapter 4. Chapter 4 Revision Highlights


Due to the extent of chapter 4 revisions, it is not feasible to discuss each change in this paper. Instead, we will illustrate the scope of the changes by providing portions of the pending revision 2 guidance in four manual sections: Section 4-13.2. Dynamic Increase Factor; Section 4-22. Design of Non-Laced Reinforced Slabs Introduction; Section 4-55. Prediction of Concrete Spalling; and Section 4-66.3.1. Single Leg Stirrups. 4-13.2. Dynamic Increase Factor

The dynamic increase factor, DIF, is equal to the ratio of the dynamic stress to the static stress, e.g., fdy/fy, fdu/fu and f'dc/f'c. The DIF depends upon the rate of strain of the element, increasing as the strain rate increases. The design curves for the DIF for the unconfined compressive strength of concrete are given in Figure 4-9 for 2,500 < fc < 5,000 psi and in Figures 4-9a and 4-9b for fc = 6,000 psi. Test values for the DIF in tension (before cracking) also are given on Figures 4-9a and 4-9b. The DIF design curves for the yield and ultimate stresses of ASTM A 615 Grade 40, Grade 60 and Grade 75 reinforcing steel are given in Figure 4-10. Grade 40 steel is not permitted in new protective construction. Thus, Grade 40 data are provided for comparative purposes and for use in evaluating existing construction. The curves were derived from test data having a maximum strain rate of 300 in./in./sec. for concrete and 100 in./in./sec. for steel. Values taken from these design curves are conservative estimates of DIF and safe for design purposes.

Figure 4-9 Design curve for DIF for ultimate compressive strength of concrete (2,500 psi < fc < 5,000 psi)


Section 4-22. Design of Non-Laced Reinforced Slabs - Introduction

Conventional reinforced concrete elements are for the purpose of this manual, members without lacing. These non-laced elements make up the bulk of protective concrete construction. They are generally used to withstand the blast and fragment effects associated with the far design range but may also be designed to resist the effects associated with the close-in design range. Non-laced elements may be designed to attain small or large deflections depending upon the protection requirements of the acceptor system.

A non-laced element designed for far range effects may attain deflections corresponding to support rotations up to 2 degrees under flexural action. Single leg stirrups are not required to attain this deflection. However, shear reinforcement is required if the shear capacity of the concrete is not sufficient to develop the ultimate flexural strength.

Type A, Type B, or Type C single leg stirrups, as defined in section 4-66.3, must be provided when a non-laced element is designed to resist close-in effects. The shear reinforcement must be provided to prevent local punching shear failure. When the explosive charge is located at scaled distances less than 1.0, Type C single leg stirrups or lacing must be employed. For scaled


distances greater than 1.0 but less than 3.0, single leg stirrups must be provided, while for scaled distances greater than 3.0, shear reinforcement should be used only if required by analysis.

Type A stirrups may be used only if concrete spalling is prevented and the scaled distance is greater than 1.0. If these requirements are satisfied, a slab with Type A stirrups may attain deflections corresponding to support rotations up to 2 degrees under flexural action.

A slab with Type B or Type C stirrups may attain deflections corresponding to support rotations of up to 12 degrees. While Type B stirrups may be used only if the scaled distance is greater than 1.0, Type C stirrups are allowed as long as the minimum separation distance requirements of section 2-14.2.1 are satisfied. It should be emphasized that the Section 2-14.2.1 separation distances are the minimum clear distance from the surface of the charge to the surface of the element. The normal scaled distances RA (center of charge to surface of barrier) corresponding to these minimum clear separation distances are equal to approximately 0.25 ft/lb1/3.

A type I and II cross-section provides the ultimate moment capacity and mass to resist motion for elements designed for 2 and 6 degrees support rotation, respectively. If spalling occurs then a type III cross-section would be available. In addition, a non-laced element designed for small deflections in the close-in design range is not reusable and, therefore, cannot sustain multiple incidents.

A non-laced reinforced element may be designed to attain large deflections, that is, deflections corresponding to incipient failure. These increased deflections are possible only if the element has sufficient lateral restraint to develop in-plane forces. The element may be designed for both the close-in end far design range. A type III cross-section provides the ultimate moment capacity and mass to resist motion.

4-55. Prediction of Concrete Spalling

As previously explained, direct spalling is due to a compression wave traveling through a concrete element, reaching the back face and being reflected as a tension wave. Spalling occurs when the tension is greater than the tensile strength of the concrete.

Many spall tests have been conducted on the configuration shown in Figure 4-65, where a cylindrical charge, cased or bare, is oriented side-on at a stand-off distance from a wall slab and oriented end-on in contact with the ground. Tested variations to this configuration include non-cylindrical charge shapes, charges off the ground, and charges in contact with the slab. Test data for all of these cases have been compiled and analyzed and are plotted in Figure 4-65a. The test data in this figure are reported in terms of the observed severity of spall, i.e., as either no spall, spall (no breach), or breach. Threshold spall and breach curves are plotted as approximate upper bounds to the spall and breach data points, respectively and may be used in design. The spall threshold curve is given by Equation 4-178:

5.05.2

1++= cbaR

h 4-178


where: h = concrete thickness (ft.) R = Range from slab face to charge center of gravity (ft.)

a = -0.02511 b = 0.01004 c = 0.13613 = spall parameter (equations 4-180a and 4-180b)

The breach threshold curve is given by Equation 4-179:

21

++= cbaRh 4-179

where: h = concrete thickness (ft.)

R = range from slab face to charge center of gravity (ft.) a = 0.028205 b = 0.144308 c = 0.049265 = spall parameter (Equations 4-180a and 4-180b)

The spall parameter for noncontact charges is given by Equation 4-180a:

333.0

353.0266.0926.0 '

+=

cadj

adjadjc WW

WWfR 4-180a

Equation 4-180b gives the spall parameter for contact charges: 341.0308.0972.0 '527.0 = adjc WfR 4-180b where: = spall parameter

R = range from slab face to center of charge (ft.) fc = concrete compressive strength (psi) Wadj = adjusted charge weight (lb) Wc = steel casing weight (lb) The spall parameter equations have limits of 0.5 14. The adjusted charge weight, Wadj, is the weight of a hemispherical surface charge that applies an equal explosive impulse at the target to that of the actual charge (see Figure 4-65) and is given by Equation 4-181a: Wadj = BfCfW 4-181a where: Wadj = adjusted charge weight (lb) Bf = burst configuration factor = 1.0 for surface bursts, 0.5 for

free air bursts W = equivalent TNT charge weight (lb) Cf = cylindrical charge factor given by equations 4-181b and 4-181c:


;2

11)16/3(

21 333.0667.02

+=

WR

LDLDC f L > D and R/W

0.333 < 2.0 4-181b

Cf = 1.0; all other cases 4-181c where: L = charge length [ft] D = charge diameter [ft] R = range from slab face to charge center of gravity [ft] W = equivalent TNT charge mass [lb] The burst configuration factor, Bf, is used to correct to a surface burst condition, such as the ground in Figure 4-65. The charge shape factor, Cf, is used to correct to a hemispherical charge geometry in the case of close-in cylindrical charges oriented side-on to the slab. These corrections are applicable to both standoff and contact charges. It should be considered in design that when the munition position is variable, a contact burst may not be worst-case. The spall effect for cased charges can be greatest at a small standoff, particularly for heavy casings. The test data range listed in Table 4-15a for each parameter shows that the data spans a wide range of subscale and full-scale tests. Although subscale tests predominate in the data base, the applicability of Figure 4-65a to large full-scale weapons is enhanced by the fact that concrete strain rate effects are accounted for in the term.

Figure 4-65 Typical geometry for spall predictions


Figure 4-65a Threshold spall and breach curves for slabs subject to high-explosive bursts in air (standoff and contact charges, cased and bare)

Table 4-15a Parametric ranges for spall prediction Parameter Max. Min. Avg. R, in. 360 0.1 21.0 Charge Weight, W, lb. 2299 0.03 24.4 Case length, in. 60.0 0.80 8.8 Case diameter, in. 18.0 0.80 4.0 Case thickness, in. 0.62 0.00 0.05 1R/W1/3, in/lb1/3 12.1 0.008 0.70 Concrete thickness, T, in. 84.0 2.00 9.23 fc, psi 13815 1535 5067 Rebar spacing, S, in. 11.8 1.25 7.16 Reinf. Ratio, 0.025 0.0005 0.0054

1 Per section 4-32, the minimum allowable design value for R/W1/3 is approximately 0.25 ft/lb1/3.


4-66.3. Elements Reinforced with Single Leg Stirrups

4-66.3.1. Single Leg Stirrups

A single leg stirrup consists of a straight bar with a hook at each end. Minimum bar bend requirements for single leg stirrups depend upon both the design support rotation and the scaled distance of the charge from the element, as follows:

1. Type A Single leg stirrup with a 90-degree hook on one end and a 135-degree hook on the other end. Type A stirrups may be used only if the scaled distance from the center of the charge to the element is greater than 1.0 ft/lb1/3, the design support rotation is 2-degrees or less, and concrete spalling is prevented in accordance with section 4-55. Placement requirements for Type A stirrups are summarized in Figure 4-101. For elements designed for blast loading on one-face only, the 90-degree leg shall be placed on the blast face. For elements designed for blast loading on either face, the 90-degree leg shall be alternated between each face.

2. Type B Single leg stirrup with 135-degree hooks on both ends. Type B stirrups may be used only if the scaled distance from the center of the charge to the element is greater than 1.0 ft/lb1/3. Type B stirrups are acceptable for all protection categories and thus, may be used for design support rotations up to 12-degrees.

3. Type C Single leg stirrup with 180-degree hooks on each end. Type C stirrups may be used for all charge separation distances allowed by this manual. Type C stirrups also are acceptable for all protection categories and thus, may be used for design support rotations up to 12-degrees.

Hooks shall conform to the ACI 318 Building Code. At any particular section of an element, the longitudinal flexural reinforcement is placed to the interior of the transverse reinforcement and the stirrups are bent around the transverse reinforcement (Fig. 4-101).

The required quantity of single leg stirrups is calculated in the same manner as lacing. It is a function of the element's flexural capacity while the size of rebar used is a function of the required area and spacing of the stirrups. The maximum and minimum size of stirrup bars are No. 8 and No. 3, respectively, while the spacing between stirrups is limited to a maximum of d/2 or dc/2 for type I and type II or III cross-sections, respectively.

The preferable placement of single-leg stirrups is at every flexural bar intersection. However, the transverse flexural reinforcement does not have to be tied at every intersection with a longitudinal bar. A grid system may be established whereby alternate bar intersections in one or both directions are tied within a distance not greater than 2 feet. The choice of the three possible schemes depends upon the quantity of flexural reinforcement, the spacing of the flexural bars and the thickness of the concrete element. For thick, lightly reinforced elements, stirrups may be furnished at alternate bar intersections, whereas for thin and/or heavily reinforced elements, stirrups will be required at every bar intersection. For those sites where large stirrups are required at every flexural bar intersection, the bar size used may be reduced by furnishing two stirrups at each flexural bar intersection. In this situation, a stirrup is provided at each side of longitudinal bar.


Single leg stirrups must be distributed throughout an element. Unlike shear reinforcement in conventionally loaded elements, the stirrups cannot be reduced in regions of low shear stress. The size of the stirrups is determined for the high stress areas and, because of the non-uniformity of the blast loads associated with close-in detonations, this size stirrup is placed across the span length to distribute the loads. For two-way elements, diagonal tension stresses must he resisted in two directions. The size of stirrup determined for each direction is placed to the same extent as the lacing shown in Figure 4-92. However, the distribution does not apply for cantilever elements since they are one-way elements requiring only one stirrup size which is uniformly distributed throughout.


Figure 4-92 Typical locations of continuous and discontinuous lacing


Figure 4-101 Placement requirements for Type A single-leg stirrups.


Future Work Future TM 5-1300 TWG tasks will focus on two areas. First, we will continue to update and expand the content of TM 5-1300. We anticipate that this work will include the revision of the gas pressure calculation procedure for partial containment cells (Bogosian & Zehrt, 1998) [10], (Tancreto & Zehrt, 1998) [11] and (Hager, Doolittle and Needham, 2006) [12]; the update and expansion of structural steel and masonry design guidance; and the addition of new guidance on innovative materials and retrofit of existing structures. Second, we will support both JUM development and the corresponding revision of TM 5-1300 to provide guidance specific to explosives safety applications. Conclusions and Recommendations Since its initial publication in 1969, Army TM 5-1300/NAVFAC P-397/AFR 88-22 (TM 5-1300, 1969) [1] has provided uniquely practical and intuitively straightforward procedures for analyzing and designing blast resistant structures. With its unlimited distribution, TM 5-1300 is the blast design manual of choice of both government explosives safety experts and private A-E firms throughout the world. The September 11, 2001 terrorist attacks on the United States underscore the need for up-to-date, blast design guidance. To obtain maximum benefit from recent research advances, pertinent data must be disseminated quickly to the blast design community in an open distribution document. Whenever possible, guidance should be written so it can be understood and applied by a veteran structural designer with little or no blast experience. Although the interim TM 5-1300 UFC chapter 4 revision will provide updated guidance in several key areas, additional revisions and supplementary coverage of new, innovative systems and materials are sorely needed. Future TWG work will concentrate on developing and disseminating this guidance, either in a future TM 5-1300 revision or in the proposed Joint Use Manual for the explosives safety, hardened structures, and AT/FP communities.


References 1. Departments of the Army, the Navy, and the Air Force (1969), Structures to Resist the Effects of Accidental Explosions, Army Technical Manual No. 5-1300, Navy Publication NAVFAC P-397, and Air Force Manual AFM 88-22, Washington, DC. 2. Departments of the Army, the Navy, and the Air Force (1990), Structures to Resist the Effects of Accidental Explosions, Army Technical Manual No. 5-1300, Navy Publication NAVFAC P-397, and Air Force Manual AFM 88-22, Revision 1, Washington, DC. 3. APT Research, Inc. (2002), Final Report TM 5-1300 Comments and Suggested Revisions, Huntsville, AL. 4. Naval Facilities Engineering Service Center (NFESC) (2002), Proposed Revisions Army TM 5-1300 Navy NAVFAC P-397, Air Force AFR 88-22 Structures to Resist the Effects of Accidental Explosions, Port Hueneme, CA. 5. The Departments of the Army, Air Force, and Navy and the Defense Special Weapons Agency (1998), Design and Analysis of Hardened Structures to Conventional Weapon Effects, UFC 3-340-01 (formerly Army TM 5-855-1, Air Force AFPAM 32-1147(I), Navy NAVFAC P-1080, DSWA DAHSCEWMAN-97), Washington, DC. 6. American Concrete Institute (1983), Building Code Requirements for Reinforced Concrete (ACI 318-83), Detroit, MI. 7. American Concrete Institute (2008), Building Code Requirements for Structural Concrete (ACI 318-08) and Commentary (ACI 318R-08), Detroit, MI. 8. Woodson, S. C. and Zehrt, W. H., Jr., Investigation of Army TM 5-1300/NAVFAC P-397/AFR 88-22 Diagonal Tension Requirements at Low Scaled Distances, Thirty-Second DOD Explosives Safety Seminar Proceedings, Philadelphia, PA, August 2006. 9. Zehrt, W. H., Jr., Woodson, S. C., and Beck, D. C., Investigation of Army TM 5-1300/NAVFAC P-397/AFR 88-22 Bar Bend Requirements for Single Leg Stirrups used as Diagonal Tension Reinforcement, Thirty-Second DOD Explosives Safety Seminar Proceedings, Philadelphia, PA, August 2006. 10. Bogosian, D. D. and Zehrt, W. H., Jr. (1998), Assessment of Analytical Methods Used to Predict the Structural Response of 12-inch Concrete Substantial Dividing Walls to Blast Loading, presented at 28th DoD Explosives Safety Seminar, Orlando, FL. 11. Tancreto, J. E. and Zehrt, W. H., Jr. (1998), Design for Internal Quasi-Static Pressures from Partially Contained Explosions, presented at 28th DoD Explosives Safety Seminar, Orlando, FL.


12. Hager, K., Doolittle, C. and Needham, C. (2006), Proposed Gas Pressure Rise-Time Model for TM 5-1300 , Technical Report TR-2273-SHR, Naval Facilities Engineering Service Center, Port Hueneme, CA.


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OVERVIEW OF UFC 3-340-02, STRUCTURES TO RESIST THE EFFECTS OF ACCIDENTAL EXPLOSIONS