ICC IS-STM 500, Chapter 8

Draft of Proposed Commentary Statements 1 st Draft by Ernst W. Kiesling 08/09/07 Chapter 8 – Test Methods for Impact and Pressure Testing

Commentary Committee: Waller (Chair), Kiesling; Roeper Review: Levitan, Tezak, Graber, Messersmith;Wills

Chapter 8 -- Testing

C 804.9.7 Various configurations are employed in alcove or baffled entry systems to allow access/egress without doors or to shield, in whole or in part, doors from the impacts of windborne debris. Some examples are shown. Insufficient research or testing has been done on the impact of windborne missiles on baffled entry systems to permit a purely analytical approach to predicting debris impact performance of partially shielded doors. Debris impact testing may be required to establish the performance of a baffled entry system.

C 804.9.7 Doors on alcove or baffled entry systems may be added for privacy, security, environmental control, or for debris impact protection. Such doors shall meet pressure test requirements of 806.3 to assure that doors will not suddenly open when affected by an extreme wind event. Doors that meet wind load requirements and the debris impact criteria of Section 804.9.7 are deemed adequate to withstand rebound impacts from a design missile or fragments from the first impact.

C 804.9.7 Debris Impact Mechanics – Some insight into the mechanics of debris impacts may be gained by treating impacts as elastic, i.e., (θin = θout). The variables that affect the debris response to the angle of impact include the coefficient of restitution and dynamic friction between the debris and the impacted surface. The elastic response angle is decreased as the coefficient of restitution and dynamic friction is added to the impact mechanics. Hence, the response angle (θout) of the impact will be a conservative value.

C 804.9.7.3 Testing by Method 2 of Section 804.8.7.3 is known to be conservative: the missile traveling at design missile speed and striking at an angle has less perforation potential than the same missile striking perpendicular to the surface traveling at the speed of the velocity component perpendicular to the surface.

C 804.10.2

The Standard states that “Dislodgement that occurs in a test shall be demonstrated to be harmless by failing to perforate a # 70 unbleached kraft paper witness screen …”

The 70-lb basis weight kraft was suggested because for many years, this paper was one of the most widely produced products for use in grocery bags. It can still be found in many places in bulk (e.g. Delta Paper Corporation) and on the internet. 60# weight is more readily available, probably it is now being more widely used for grocery bags.

The basis weight number (60-lb, 60#, 70-lb, 70#) is actually the weight of a ream of Kraft paper (500 sheets, 24-in x 36-in in size). The #60 basis weight paper is 15% lighter than #70 and has about a 10% drop in burst strength. These are relatively minor differences in regards to how it is being used as a witness screen. Virgin, unbleached, 'No. 2', or 'brown' paper should be used rather than the bleached white version, which may be weakened by the bleaching process.

C 806 Multipliers on Test Pressures

ASTM E 1886 – 05 defines the air pressure differential for cyclic testing as follows: the maximum air pressure differential (P) is specified or is equal to the design pressure. The design pressure is defined as the uniform static air pressure difference, inward or outward, for which the test specimen would be designed under service load conditions using conventional structural engineering specifications and concepts. This pressure is determined by either analytical or wind tunnel procedures (such as are specified in ANSI/ASCE 7).

To provide a high level of protection for life safety, we adjust the test pressures upward by a factor of 1.2 times the shelter design wind pressure to be very conservative.

Optional Section Prepared by: Navin K. Galani C 804.9.7.1

Debris impact tests on baffled entry system Tests were conducted at Texas Tech University Wind Science and Engineering Research Center, in order to investigate the impact of debris on a baffled entry system of tornado shelter. The experiments were carried out to find the angle of attack of missile, at which it perforates a debris impact plywood wall (two layers of ¾” plywood framed with 2x4s; held together with 3” # 8 wood deck screws 6” on centers). It was assumed that the critical angle of attack should be such that the normal component of the angled impact is equal to the threshold value for the barrier (wall). It was found that the calculated values of angle of impact (based on above assumption) were on an average 17 degrees smaller than the values collected from the tests. Thus it can be concluded that, using threshold value for the system as the horizontal component of the angled impact in the above equation appears to result in conservative values for angle of impact. Another experiment was carried out to find out the path followed by the missile after impacting an impact resistant wall. The impact resistant wall used was reinforced brick cavity wall with concrete infill. A 15lb-“2 x 4” missile was used to impact the system at a specified angle (α) with a speed of 100 mph. The setup had the impact resistant wall placed at right angles to another wall. It was observed that the impact with primary wall caused the front end of missile to deform and bend a bit. Further the missile slid for a while along the wall and did not bounce instantly as anticipated based on momentum analysis of particles. Sliding of missile can be attributed to the deformation of its front edge and the length of missile, which averaged over 13.8 ft. The back end of missile broke off due to whipping action against the primary wall edge. The missile eventually took off from the primary wall, after sliding for a while to hit the witness wall. Due to deformation the missile lost a major part of its energy and took off from the primary wall at a very small response angle. This test provides a qualitative picture of the motion of debris after impact. Question: Should we pull some things from the NSSA Standard on lightning? There is nothing to refer to in the ICC/NSSA Standard but we get occasional questions.