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Protective Design

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Blast & Forced Entry/Ballistic Protection

Buildings that house critical assets often have to meet blast as well as forced entry/ballistic resistance (FE/BR) design requirements to mitigate the hazardous effects associated with terrorism. Historically, these buildings have been constructed using concrete and steel. A significant amount of testing has been performed to demonstrate the ability of these building materials to resist blast and FE/BR threats. 针对类似的威胁，对木结构进行的测试较少. At least part of this stems from the relative difficulty of designing light-frame wood construction to resist these threats efficiently and economically.

However, 大量木结构建筑的出现, 尤其是交叉层压木材(CLT), both in the U.S. and internationally, presents an opportunity to provide a sustainable building material alternative to owners and architects developing high-security buildings. The solid, panelized nature of CLT allows for both inherent strength and ease of construction.  Further, as connections for CLT panels typically consist of steel and timber elements, 延性吸能板附件相对容易设计. 

Past U.S. Forest Service-sponsored efforts have been used to generate design guidance for CLT construction exposed to blast loads—i.e., Protective Design Center Technical Report (PDC-TR) 18-02: Analysis Guidance for Cross-Laminated Timber Construction Exposed to Airblast Loading (PDC-TR 18-02), published by the U.S. Army Corps of Engineers. Additionally, testing funded by a 2019 Wood Innovation Grant demonstrated the feasibility (in terms of performance, cost, and weight) of embedding thin steel plates within a CLT panel to defeat Department of State (DoS) blast, ballistic, and forced entry threats. Below is a summary of the testing performed to date, along with pertinent reports and references.

Blast Protection

Blast design requirements are specific to a facility’s governing jurisdiction and defined in various standards. 与爆破设计相关的几个众所周知的标准包括:

  • UFC 4-010-01: U.S. 国防部(DoD)的防爆设计要求用于反恐
  • UFC 3-340-02: U.S. DoD blast design requirements for accidental explosions of ammunition or explosives
  • ISC风险管理流程:非军事美国.S. 美国联邦政府爆炸设计要求.S. facilities
  • OBO Building Code: U.S. DoS blast design requirements
  • ASCE 59-11:非政府设施的爆破设计要求

到目前为止，CLT板的爆炸测试主要集中在美国.S. DoD antiterrorism requirements included in UFC 4-010-01 (2013); however, 满足其他爆破设计要求的爆破试验正在进行中. Highlights from several blast tests performed to date are included in the following videos: 

Video of Blast Test 3:

In this test, the timing of the negative phase blast loading aligned with the wall panel’s rebound response, 是什么导致了回弹时的位移响应大于入流时的.  This test underscores the importance of designing CLT wall panel connections potentially exposed to blast loading for both inbound and rebound response, 特别是用于承重CLT墙板.

Video of Blast Test 5:

This test was identical to Test 3, except an axial preload of roughly ten percent of F’cAparallel 被放置在面对爆炸的CLT墙板上. 而试验3墙板破裂, the Test 5 wall panels did not, illustrating the positive effect of axial load on the displacement response of CLT panels dynamically stressed near their elastic limit.

Video of Blast Test 7:

Test 7 was designed to rupture 5-ply CLT wall panels installed on one of the three CLT test structures. As the other two structures had much weaker wall panels facing the detonation (i.e., 3层CLT和2×4钉层压木材(NLT), this test illustrated the response of mass timber panels that were intentionally overloaded by the imposed blast loading.

Video of Blast 7-ply Test:

7-ply Test was designed to displace 7-ply CLT wall panels well beyond their elastic limit. The test indicated that, even at a displacement ductility of 3, 与7层CLT板相关的危险碎片可以忽略不计.

How do I design CLT for blast loads?

PDC-TR 18-02: Analysis Guidance for Cross-Laminated Timber Construction Exposed to Airblast Loading gives engineers trained in structural dynamics information they need to analyze and design CLT structures for blast loads. WoodWorks参与了导致本文档的测试, 并且还创建了资源来帮助工程师们使用它.

迄今为止进行的爆炸试验的结果

Mass timber structural systems can effectively resist blast loads in the elastic range with little noticeable damage. 由于大质量木板的强度相对较高，刚度较低, significant blast loads can be resisted by mass timber panels in the elastic response range.

大质量木板的峰后响应比较脆. However, for CLT systems, the presence of multiple plies allows for measurable residual strength following initial panel rupture. Additionally, the two-way action inherent in CLT provides a means for load distribution across the panel, 从而限制了在峰值加载位置的损伤. NLT systems do not have this advantage of cross lamination and thus do not exhibit these post-peak response benefits.

前提是紧固件穿透深度足够, significant blast loads can be resisted and transferred through CLT connections that are both simple and quick to install. An added benefit is that dowel-type connection limit states associated with CLT construction are often ductile in nature due to the propensity for wood to crush and/or steel to yield when loaded in shear beyond their respective elastic limits.

The results of the blast demonstration testing indicate that single degree-of-freedom (SDOF) dynamic analysis can be used to approximate the peak out-of-plane displacement of CLT panels without openings. The static and dynamic increase factors used to approximate the expected ultimate resistance defined in PDC-TR 18-02 show good correlation with arena blast test results.

Load-bearing CLT panels that exhibit localized panel rupture due to blast loading are capable of resisting their service axial loads provided the panel’s displacement ductility is limited to two.

CLT面板的回弹响应通常控制其入站响应, thus highlighting the importance of considering the negative phase of blast loading when designing CLT components and systems for blast loading.

Visually-graded CLT panels demonstrate significantly greater out-of-plane bending strength than that associated with the characteristic values defined in ANSI/APA PRG 320 Standard for Performance-Rated Cross-Laminated Timber.

While the testing conducted to this point provides a solid foundation upon which to base blast design guidance for CLT structures, the following areas of additional research would serve to curtail conservatism in the analysis and design approaches for such structures:

  • The minor strength direction bending strength values for CLT panels in Annex A of PRG 320 appears to be too conservative from an ultimate response perspective. Further testing to justify more representative peak bending strengths in the minor strength direction may allow for openings in blast-loaded structures to be designed more economically. Additionally, better quantification of the minor strength direction strength and stiffness will enable more explicit consideration of the two-way action inherent in CLT construction.
  • Additional blast testing on 5-ply, 7-ply, and 9-ply panels is necessary to assess the robustness of the SDOF resistance function methodology defined in PDC-TR 18-02.
  • Dynamic characterization of different timber species and dowel-type connections at blast-relevant strain rates will assist in refining the dynamic increase factors (DIFs) used in design.

Download blast test reports below.

Ballistic Protection

Ballistic design requirements are specific to a facility’s governing jurisdiction and defined in various standards. 与弹道设计相关的几个众所周知的标准包括:

  • STD-SD-01.01, Revision H: U.S. DoS ballistic design requirements
  • UFC 4-023-07: U.S. 国防部直接火力武器设计指南
  • ASTM F1233:弹道冲击试验程序

Much of the ballistic testing on CLT panels performed to date has focused on U.S. 军事远征设施和DoS设施.

迄今为止进行的弹道试验结果

Through a series of ballistic tests performed on softwood CLT targets, Sanborn et al. (2019) discovered that the equations in UFC 4-023-07 consistently overpredict the penetration of spherical steel projectiles by a wide margin. (K. Sanborn, T. R. Gentry, Z. Koch, A. Valkenburg, C. Conley and L. K. Stewart, 交叉层压木材(CLT)的弹道性能,《皇冠99hg现金网》, vol. 128, pp. 11-23, 2019.)

CLT panels can be modified to defeat the ballistic threat defined in SD-STD-01.以一种相对划算的方式. 在考虑的候选面板设计中, the optimal means of defeating these projectiles is by introducing steel plates into the layup. 而钢板的增加增加了成本, the cost markup associated with integrating plates is smaller than introducing wire mesh or species of hardwood that could defeat the DoS ballistic threat projectiles.

下载下面的弹道测试报告.

大型木材CLT块准备进行弹道测试

Forced Entry Protection

Forced entry design requirements are specific to a facility’s governing jurisdiction and defined in various standards. 与强制进入设计相关的几个众所周知的标准包括:

  • STD-SD-01.01, Revision H: U.S. DoS forced entry design requirements
  • UFC 4-026-01: U.S. DoD forced entry design requirements
  • ASTM F1233:钝/锋利工具冲击的试验程序

Only one known forced entry test has been performed on CLT to date, which is described below.

迄今为止进行的强行进入试验的结果

一个标准的7层CLT面板组成的No. 2 SPF-S lumber in both the major and minor strength directions resisted a FE attack performed in accordance with SD-STD-01.01 for 44 minutes. As most elements in U.S. DoS设施只需要15分钟或更少的FE评级, this test result indicates the typical 7-ply CLT panel is more than adequate for most hardened envelope situations in DoS facilities. 适用于DoS设施内的高度保安地点(例如.g.(安全港)，需要60分钟的FE抵抗. To achieve 60-minute FE protection, thicker and/or more plies, a harder and/or denser wood, or some kind of steel element can be incorporated into the CLT panel’s layup to provide this protection.

下载下面的强制进入测试报告.

New Tests Underway

几项涉及CLT和爆破的测试工作, ballistic, 强行进入的威胁正在进行中:

  • Quasi-static testing of full-scale CLT panels with embedded ductile elements. 这些加固的CLT板是为了抵抗爆炸而设计的, ballistic, 以及DoS设施的强制进入设计要求. 如果测试结果显示出希望, the intent is to extend this research effort to include blast testing on reinforced CLT panels.
  • 5层及更厚的CLT板的爆炸试验. These component-level tests aim to investigate the potential of increasing the response limits included in PDC-TR 18-02 for non-load bearing CLT panels and develop an analytical methodology for CLT panels with openings exposed to large blast loads.
  • FE/BR testing of typical detailing conditions encountered in DoS facilities. These tests aim to certify typical detailing conditions according to SD-STD-01.01, Revision H.

爆炸，弹道和强行进入测试报告


Does all this mean CLT can be used in a project with protective response requirements?

CLT可用于许多有防护设计要求的工程. 在PDC-TR 18-02中记录了设计用于爆炸载荷的CLT板的方法, and it is anticipated that a revised version of PDC-TR 18-02 will be generated in the near future based on results of the testing described above. 典型的FE/BR细节和铺层指导.S. DoS facilities are also expected to be included in DoS facility design guidance documentation in the coming years. 

适用于PDC-TR 18-02范围以外的应用, 关于这些FE/BR细节的询问, 或者其他与木材工程防护设计相关的问题, 联系你当地的木厂区域总监.