Large missile vs. small missile impact (LMI vs SMI). What is the difference?

Two ratings, two kinds of debris.

Every impact window and door sold in South Florida is tested against flying debris. The tests come in two sizes. Large missile impact (LMI) simulates a piece of lumber hitting the glass, the kind of debris that flies near the ground. Small missile impact (SMI) simulates roof gravel and small fragments, which travel higher and faster. The rating tells you which test the product passed, and that decides where on a building it can be installed. A product can be rated LMI, SMI or both. You will see these letters on Miami-Dade Notices of Acceptance (NOAs) and Florida Product Approvals, often right in the product title.

The large missile: a 9-pound 2x4.

In the High-Velocity Hurricane Zone (HVHZ), which covers Miami-Dade and Broward counties, the test is set by the Florida Building Code and Testing Application Standard (TAS) 201. The large missile is a piece of timber with nominal dimensions of 2 in. by 4 in., weighing 9 pounds. An air cannon fires it at 50 feet per second. Three complete test units are built exactly as they will be installed, with the same frame, anchors and fastener spacing. Each unit takes two hits on the glass: one near the center and one near a corner. Essential facilities such as hospitals are tested at 80 feet per second; homes use the 50 feet per second test.

• Missile: 2x4 timber, 9 pounds.
• Speed: 50 feet per second.
• Specimens: three complete units.
• Hits: two per unit, center and corner.

The small missile: 30 steel balls.

The small-missile test uses solid steel balls, each with a mass of 2 grams and a 5/16 in. nominal diameter. Each ball strikes the glass at 130 feet per second. Every test unit takes 30 hits, delivered in three groups of 10: one group spread over a 2 square foot area at the center, one at the middle of the long edge and one at a corner. Like the large-missile test, it is run on three complete units. The steel balls carry far less energy than a 2x4, but they arrive faster and in clusters, which tests how the outer glass and interlayer resist many small, sharp blows.

The cyclic pressure test that follows.

Surviving the hits is only half the test. Each impacted unit then goes through cyclic wind pressure loading under TAS 203 in the HVHZ, or ASTM E1886 elsewhere. The cracked unit is pushed and pulled by air pressure to mimic the gusts of a long storm: 4,500 cycles pushing inward, then 4,500 cycles pulling outward, 9,000 cycles in all. Each cycle lasts 1 to 3 seconds. To pass, the unit must reject every missile without penetration and finish the cycles with no crack longer than 5 in. and 1/16 in. wide through which air can pass. Separately, TAS 202 checks air, water and structural performance under uniform static pressure.

• Impact first, then 9,000 pressure cycles.
• No penetration by any missile.
• No through-crack longer than 5 in. and 1/16 in. wide.

Where ASTM E1996 and E1886 fit in.

Outside the HVHZ, Florida approvals rely on two national standards. ASTM E1886 is the test method: missile impact followed by cyclic pressure differentials. ASTM E1996 is the performance specification that sets which missile to use and what counts as a pass. It groups missiles into lettered levels. Missile Level A is the 2-gram steel ball at 130 feet per second, the small-missile test. Missile Level D is the 9-pound 2x4 at 50 feet per second, the common large-missile test for homes. Missile Level E fires the same 2x4 at 80 feet per second for essential facilities. The TAS and ASTM tests are close cousins, but an approval names the exact standard used.

The 30 ft rule.

Height decides which rating you need. In the HVHZ, the large-missile test applies to products used up to and including 30 feet in height, and the small-missile test applies to products used above 30 feet. Elsewhere in Florida's wind-borne debris regions, Section 1609.1.2 of the Florida Building Code says the same thing in its own words: glazed openings within 30 feet of grade must meet the ASTM E1996 large-missile test, and openings more than 30 feet above grade must meet the small-missile test. For a typical single-family home, every window and door is below 30 feet, so it needs LMI. SMI-only products belong on upper floors of taller buildings, such as condo towers.

Why some products hold both.

A large-missile rating does not automatically include small-missile approval; each test has to be run and listed. Many products are tested both ways so one window can serve the whole building, from the lobby to the top floor, with the same look. The two ratings often carry different design pressures. The ES-H340 single-hung has separate Miami-Dade NOAs: 25-0519.03 LMI at +90/-160 psf and 26-0304.06 SMI at +80/-90 psf. Some products come as a pair: the ES-5500 awning is LMI, and the ES-5600 is its small-missile version. Small-missile versions usually cost a little less than their large-missile counterparts, which is why they make sense high on a tower.

• ES-SW340 slider: Miami-Dade NOA 26-0506.05 LMI +80/-90 psf; 26-0506.03 SMI +80/-110 psf.
• ES-P300 casement: Florida approval FL40521-R6, LMI and SMI, +95/-105 psf.
• ES-46T outswing door: Florida approval FL21835-R11, 21835.2 LMI and 21835.1 SMI, +100/-120 psf for both.

When an SMI product needs shutters.

An SMI-only product is not a substitute for large-missile protection near the ground. If it is used below 30 feet, the opening still needs protection that meets the large-missile test, usually impact shutters. The approval spells this out. Series 2100, an ES horizontal sliding window, is SMI only, and its NOA requires impact shutters up to 30 ft above grade. That is why an SMI-only window is a poor fit for a house: you would pay for impact glass and still need shutters on every opening.

How it shows up on NOAs and Florida approvals.

Miami-Dade Product Control issues NOAs, and the product title usually states the rating, for example "Large Missile Impact Resistant" or "Large and Small Missile Impact Resistant." Florida Product Approvals mark a product as impact resistant and list the standards it was tested to, and the installation drawings name the missile level. Read the rating together with the details: the approved sizes, glass makeups, design pressures and anchors are tied to the rating that was tested. An LMI table and an SMI table for the same frame can differ.

• Check the rating: LMI, SMI or both.
• Check the design pressure listed for that rating.
• Check the glass type and size in the drawing for your opening.

The bottom line.

For most homes, every opening needs a large-missile rating. Small-missile products are built for the upper floors of taller buildings. Either way, the approval for the exact product, size and glass governs, not a brochure or a headline rating. True Impact confirms the missile rating, design pressure and approval for each opening before we order, using the current version of each approval.