Design Considerations for Toxic Chemical and Explosives Facilities

Chapter 7

Interim Design Criteria for Polycarbonate Blast-Resistant Glazing Gerald E. Meyers and James E. Tancreto Naval Civil Engineering Laboratory, Port Hueneme, CA 93043

Glazing is often the weakest element in the protective capability of a structure against blast, fragments, and ballistics. Polycarbonate and glass-clad polycarbonate can overcome these deficiencies. This paper establishes credible and reliable interim design values for blast resistant glazing utilizing polycarbonate as a structured layer. Required design of the frame and edge engagement or bite of the glazing are also included. Glazing i s often the weakest element i n the protective c a p a b i l i t y of a structure against blast, fragments and b a l l i s t i c s . Over the last few years, the U.S. Naval C i v i l Engineering Laboratory has developed and validated design charts and tables f o r thermally tempered glass (Reference 1 and 2) for use where blast overpressure i s the predominate threat. However, t h i s glass does not provide a comparable level of protection against fragments, b a l l i s t i c s , or forced entry. Also, even i f a laminated thermally tempered glass remains intact after fragment or b a l l i s t i c impact, i t w i l l lose both i t s transparency and operational effectiveness. Polycarbonate and glass clad polycarbonate can overcome these deficiencies. As a glazing material, i t has established a long track record against fragments, b a l l i s t i c s and physical assault. However, no design method or practice existed to guide the r e l i a b l e design of polycarbonate to r e s i s t blast. I t i s the intent of t h i s paper to f i l l t h i s immediate and pressing need and to establish credible and r e l i a b l e interim design values for blast resistant glazing u t i l i z i n g polycarbonate as a structured layer. Required design of the frame and edge engagement or b i t e of the glazing are also included as they are r e q u i s i t e for a successful blast resistant design. While conservative engineering assumptions have been employed, a large data base yet needs to be developed to validate the presented design. However, the limited testing i n the engineering l i t e r a t u r e , even at high overpressures, provides i n i t i a l confidence i n the present designs. Also, the dynamic or blast analysis used to generate

This chapter not subject to U.S. copyright Published 1987 American Chemical Society

7.

MEYERS AND TANCRETO

Design Criteria for Polycarbonate Glazing

131

the design charts are independent from those used to create design tables for the physical security s e t t i n g such as i n Reference 3. The close correspondence between the s o l u t i o n methodology employed for t h i s paper (numerical integration of the d i f f e r e n t i a l equations of motion) and that used for the physical security design tables (the response spectra solution of an equivalent l i n e a r e l a s t i c spring-mass model) are mutually confirmatory. Material Characteristics Polycarbonate i s a thermoplastic and i s often marketed under tradenames such as Lexan or Tuffak. I t should not be confused with a c r y l i c p l a s t i c s , marketed under tradenames such as P l e x i g l a s or Lucite, which are flammable and e x h i b i t a b r i t t l e f a i l u r e mode. Polycarbonate i s available m o n o l i t h i c a l l y ( i n a single sheet) i n thicknesses up to 1/2 inch. In t h i s range of thickness, polycarbonate i s twice as expensive as thermally tempered glass. In t h i c k nesses over 1/2 inch where lamination i s required, i t i s roughly three times as expensive as an equivalent thermally tempered laminated l i t e . Other than cost, polycarbonate s main disadvantage i s that i t experiences greater environmental degradation than glass, e s p e c i a l l y due to the effects of u l t r a v i o l e t r a d i a t i o n and abrasion. However, chemical coatings, such as Lexan s MARGARD or Tuffak s CM3, are available to provide some protection from abrasion. U l t r a v i o l e t i n h i b i t o r s are also available for most commercial polycarbonate. Greater protection against both abrasion and u l t r a v i o l e t attack i s afforded by encapsulating the polycarbonate i n glass. I n c i d e n t a l l y , t h i s w i l l enhance both the b a l l i s t i c and chemical resistance of the glass. Unfortunately, t e s t i n g of older glass-clad polycarbonate indicates that even glass-encapsulated polycarbonate with u l t r a v i o l e t i n h i b i t o r s w i l l suffer degradation of load carrying and penetration resistance over time. In recognition of t h i s fact and to be conservative, t h i s paper w i l l assume a reduced maximum stress for polycarbonate and not employ the p o t e n t i a l benefits of d u c t i l e or poste l a s t i c y i e l d design. 1

1

1

Pane Design Theory A maximum f l e x u r a l stress of 9,500 p s i i s assumed for polycarbonate. This conservative stress value should account for degradation i n u l t r a v i o l e t s t a b i l i z e d polycarbonate exposed to long term solar exposure. While more research i s required i n t h i s area, i t i s reasonable to expect at least a ten year useful l i f e for u l t r a v i o l e t s t a b i l i z e d polycarbonate. A Young's modulus of 345,000 p s i and a Poisson s r a t i o of 0.38 are also assumed for polycarbonate. The polycarbonate glazing i s modeled as a simply supported plate subjected to nonlinear center deflections up to 15 times the pane thickness. Using the f i n i t e element s o l u t i o n of Moore (Reference 4), the resistance function i s generated for each pane under considerat i o n . T y p i c a l l y , the resistance i s concave up, as i l l u s t r a t e d for t y p i c a l pane sizes i n Figure 1. This occurs because membrane stresses induced by the stretching of the neutral axis of the pane become more pronounced as the r a t i o of the center pane d e f l e c t i o n to the pane 1

TOXIC CHEMICAL AND EXPLOSIVES FACILITIES

jure 1. Resistance function of polycarbonate.

7.

MEYERS AND

TANCRETO


133

thickness increases. In a few cases of t h i n panes with long spans where the center d e f l e c t i o n associated with a maximum stress of 9,500 p s i i n the plate exceeds 15 pane thicknesses, a smaller design maximum stress associated with a 15-pane thickness i s chosen. This l i m i t a t i o n both r e s t r i c t s the s o l u t i o n to the v a l i d range of the Von Karmen equations used by the f i n i t e element program to develop the resistance function and the p r a c t i c a l edge engagement developed by commercially available frames. A single-degree-of-freedom approach i s used to perform the dynamic or b l a s t analysis. The resistance function i s modeled as f i v e l i n e a r segments and a Wilson-Theta numerical i n t e g r a t i o n of the equation of motion i s performed. A maximum time step of integration smaller than l/25th of the natural period of v i b r a t i o n of the corresponding segment of the resistance function i s used. No damping i s assumed and the e f f e c t i v e mass of the pane i s l i m i t e d by a load mass factor between 0.63 and 0.79 depending upon the aspect r a t i o ( r a t i o of pane length to width). The b l a s t load i s modeled as a triangular-shaped overpressure time curve. The b l a s t overpressure r i s e s instantaneously to the peak overpressure, B, then decays l i n e a r l y with a b l a s t pressure duration, T. The pressure i s uniformly d i s t r i b u t e d over the surface of the plate and i s applied perpendicular to the pane. Monolithic action i s assumed between adjoining polycarbonate layers for the following reasons. F i r s t , recent s t a t i c load t e s t i n g at the Naval C i v i l Engineering Laboratory indicates t h i s to be a good assumption. Second, the large deflections experienced by the r e l a t i v e l y f l e x i b l e polycarbonate means that a r e l a t i v e l y high proportion of load i s being c a r r i e d i n membrane action rather than bending. Interlaminar shear capacity between plates does not a f f e c t t h i s very e f f i c i e n t mode of s t r u c t u r a l capacity. F i n a l l y , i t i s anticipated that the high s t r a i n rates associated with b l a s t loading w i l l further increase the shear capacity of most, i f not a l l , interlaminar plast i c s i n current commercial use. To prevent f a i l u r e due to the disengagement of the pane out of the frame, b i t e or edge engagement depths are required. They are based upon the assumption that the plate w i l l d i s t o r t as a spheroid surface. At the maximum design center d e f l e c t i o n of 15 pane t h i c k nesses, t h i s conservatively approximates the d e f l e c t i o n shape funct i o n . To be conservative, a 0.5-inch safety margin i s added to a l l calculations. Glazing Design Charts Figures 2 through 9 are design charts for u l t r a v i o l e t s t a b i l i z e d polycarbonate under b l a s t load. Charts are provided for pane t h i c k nesses of 1/4, 3/8, 1/2, and 1 inch for pane areas up to 25 f t at pane aspect r a t i o s (pane length to width r a t i o s ) of 1.00, 1.50, 2.00 and 4.00. The charts r e l a t e the peak experienced b l a s t overpressure capacity, B, for convenient pane dimensions across the spectrum of encountered b l a s t durations. Depending on the o r i e n t a t i o n of the window to the charge, the b l a s t overpressure may e i t h e r be incident or r e f l e c t e d . The pane dimensions (measured across the span from the gasket centerline) peak b l a s t capacity at 1000 msec, B, s t a t i c frame design pressure, r ^ , and the required b i t e are printed to the r i g h t

134

TOXIC C H E M I C A L A N D EXPLOSIVES FACILITIES

Figure 2.

Peak b l a s t pressure capacity f o r polycarbonate: a/b = 1.0; t = 1/4 and 3/8 i n .

MEYERS AND TANCRETO


b κ a

Β

r

(in.)

(psi) (psi)

12x12

33.9

86.1

18x18

15.8

43.9

Bite (in.l 0.63

30x30

6 21

18.6

36x36

4 55

13.7

1 0

42x42

3 51

10.6

1 1

48x48 54x54 J 60x60

2 81 2 31 1 94

8.46 6.94 5.82

b χ a (in. )

Β (psi)

(psi)

12x12

105

219

0.55

121

0.63

Γ

0 91

1 2 1 3 1 4

Bite (in. )

18x18

51.8

24x24

34.0

86.1

0.76

30x30

22.3

60.1

0.86

36x36

15.7

43.9

0.96

42x42

11.8

33.8

1.0 1.1

48x48

9.22

27.0

54x54

7.49

22.2

1.2

60x60

6.24

18.6

1.3

D u r a t i o n of B l a s t P r e s s u r e , Τ ( m s e c )

Figure

3.

Peak a/b

blast

=1.0;

pressure t

=

1/2

capacity and

1 in.

for

polycarbonate:

136


Figure 4. Peak blast pressure capacity for polycarbonate: a/b = 1.5; t = 1/4 and 3/8 i n .

MEYERS AND TANCRETO

F i g u r e 5.


Peak b l a s t p r e s s u r e c a p a c i t y f o r p o l y c a r b o n a t e : a/b = 1.5; t = 1/2 and 1 i n .


Figure 6.

Peak blast pressure capacity for polycarbonate: a/b =2.0; t = 1/4 and 3/8 i n .

MEYERS AND TANCRETO

Figure 7.


Peak blast pressure capacity for polycarbonate: a/b =2.0; t = 1/2 and 1 i n .

139

140


Figure 8.

Peak blast pressure capacity for polycarbonate: a/b =4.0; t = 1/4 and 3/8 i n .

MEYERS AND TANCRETO

Figure 9.


Peak b l a s t pressure capacity for polycarbonate: a/b =4.0; t = 1/2 and 1 i n .

142


of each design curve. To r e f l e c t current manufacturing tolerances and to be conservative, design thickness used to calculate blast capacities were limited to 95% of the nominal thickness. It i s worth noting that blast capacity of a polycarbonate pane i s sensitive to the duration of the blast load. Because of t h i s , the t y p i c a l short overpressure duration t e s t i n g of polycarbonate with small c l o s e - i n charges with frame set-ups that permit a rapid pressure clearing time may give an unconservative estimate of blast capacity i n many r e a l world threat scenarios. Engineering judgment i s also required i n assessing the blast capacity of a glass-clad polycarbonate. Because i n most cases the annealed, semi-tempered, or sodium-based chemically tempered glass does not contribute s u b s t a n t i a l l y to the blast load capacity of the cross section, i t i s conservative to base blast capacity upon the polycarbonate layers alone. In many cases, the dynamic amplification factor or the r a t i o of s t a t i c load to dynamic load capacity w i l l exceed two. This i s because of the concave up shape of the resistance function and the mobilization of membrane resistance at large d e f l e c t i o n to thickness ratios. Because of this phenomenon, i t i s unconservative to assume the b l a s t capacity of polycarbonate glazing to be no less than one half of i t s s t a t i c pressure load capacity. At very short blast durations, some small area 1-inch thick panes exhibit s l i g h t l y less blast capacity than panes with larger areas. This occurs because the small panes are acting as linear plates with small deflections under blast loads while the larger panes can mobilize membrane resistance without exceeding the maximum design stress of 9,500 p s i . Frame Requirements To be e f f e c t i v e , the blast load carried by the polycarbonate glazings must be transferred to the frame and ultimately through the structure. I f not properly designed, the pane or pane and frame w i l l d i s engage and become a large and dangerous fragment. Also, care must be taken to properly design the supporting structure for the frame loads. F a i l u r e to do this can increase the p r o b a b i l i t y of structure collapse. This i s especially true i n r e t r o f i t construction. While the design loads for the panes are based upon large def l e c t i o n plate theory, the design loadings for the frame are based on an approximate solution of small d e f l e c t i o n theory for normally loaded plates. Analysis indicates this approach to be considerably simpler and more conservative than using the frame loading based exclusively on large d e f l e c t i o n plate behavior. The effect of the s t a t i c design load, r , applied d i r e c t l y to the exposed frame members of width, w, should also be considered. The design load, r , produces a l i n e shear, V , applied by the long side, a, of tëe pane equal to: V

χ

=

C

r x u

b sin(*x/a) + r w, ' u

lb/in.

(1)

The design load, r , produces a l i n e shear, V , applied by the short side, b, of the pane equal to: ^

7.

MEYERS AND TANCRETO

V

=

y

C

r

y

u


b sin(*y/b) + r

u

w,

lb/in.

143 (2)

The design load, r , also produces a corner concentrated load, R, tending to u p l i f t corners of the window pane equal to: R

=

C

κ D

r

2

u

b,

lb

(3)

D i s t r i b u t i o n of these forces as loads acting on the window frame i s shown i n Figure 10. S t a t i c frame design loads, r , are provided for each pane i n the t h i r d column of the design data to the r i g h t of each design chart. Table I presents the design c o e f f i c i e n t s , C , C , and C for p r a c t i c a l aspect r a t i o s of the pane. Linear i n t e r p o l a t i o n can be used for aspect r a t i o s not presented. Frame deflections should be l i m i t e d to no more than 1/100 the length of the supporting span. This i s a s i g n i f i c a n t benefit compared to the more r i g i d r e s t r i c t i o n s associated with tempered glass. Although frames with mullions are covered i n the design c r i t e r i a , i t i s recommended that single pane frames be used. Experience indicates that mullions complicate the design and reduce r e l i a b l e f a b r i c a t i o n of b l a s t - r e s i s t a n t frames. R

Table

a/b 1.00 1.50 2.00 4.00

C

R

0.065 0.085 0.092 0.094

I.

C

C

y

X

0.495 0.581 0.623 0.687

0 0 0 0

495 574 614 685

Frame B i t e Minimum frame b i t e s or frame edge engagements are required f o r poly carbonate to provide enough edge support to carry the b l a s t load and prevent pane disengagement. The fourth column to the r i g h t of each design chart presents the required b i t e for each pane. Rebound Response to the dynamic b l a s t load w i l l cause the window to rebound with a negative (outward) d e f l e c t i o n . The outward pane displacement and the stresses produced by the negative d e f l e c t i o n must be safely r e s i s t e d by both the pane and frame. I f operational requirements d i c t a t e an operational window a f t e r the b l a s t , the frame, connec t i o n s , and w a l l should be designed to also r e s i s t the s t a t i c frame design load, r , i n the outward d i r e c t i o n . I f the window can be permitted to a f t e r the p o s i t i v e b l a s t pressure has decayed, more economical frames can be used, as the negative s t a t i c design load can be reduced to 0.67 of r . For b l a s t durations greater than 250 msec, s i g n i f i c a n t rebound does not occur during the p o s i t i v e pressure phase.


corner

10.

load. R = C_, r b * R u

/

Frame design loading to be applied by the pane to the frame.

7.

MEYERS AND TANCRETO


Literature Cited 1. Meyers, G.E. "Design Criteria and Acceptance Test Specification for Blast Resistant Thermally Tempered Glass Glazing," Department of Defense Explosive Safety Seminar, Anaheim, CA, Aug 1986. 2. Physical Security: Shore Station Engineering Design Criteria, (MIL-HDBK-1013/1), (to be published). 3. Physical Security - Planning and Requirement Guide. U.S. Army Corps of Engineers, Omaha, Neb, Mar 1987. 4. Moore, J.M. FSA Task Report No. 5101-291: Thickness Sizing of Glass Plates Subjected to Pressure Loads. Jet Propulsion Laboratory, Pasadena, CA, Aug 1982. RECEIVED April 21, 1987

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Design Considerations for Toxic Chemical and Explosives Facilities

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