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Frequently Asked Questions - Specialty Doors

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SDI’s online AIA course on Specialty Steel Doors provides more detailed information on acoustic, blast, bullet, tornado, and more. It provides one AIA HSW hour.


What are the reasons for not over-specifying the STC rating?

Typically, doors with higher STC ratings have increased cost, weight and lead time.

Are STC rated doors provided with a label, similar to fire rated doors?

STC rated doors are generally supplied with a label that is either Mylar or metal, but the products are not listed like fire doors by agencies such as Underwriters Laboratories or Warnock Hersey. Labels are for manufacturer and rating identification purposes only.

What is the maximum rating tested for STC so far?

STC ratings would be specific to each manufacturer and design, typical higher range ratings are between 50 and 55.

What is the difference between STC and decibels?

TC is a single number rating that indicates the sound transmission loss over a defined range of frequencies of a door assembly between adjacent closed rooms. Higher values equate to better sound reduction performance.

Decibels dB, are used to express the intensity of a sound wave. Examples of decibels levels are:

  • 100-120 Deafening industrial machinery, jet engines
  • 80-100 Very loud cocktail party, boom box
  • 60-80 Quiet speech
  • 20-40 Faint soft whisper, recording studio
  • 0-20 Very faint acoustical laboratories, deep caves

Is it a requirement that STC metal frames be filled with grout or a sound retarding material?

Most manufacturers conduct acoustic tests with grout filled frames to optimize the lab performance. Acoustic performance will be reduced on frames over STC 40 when grout or sound retarding material is eliminated.

What is STC?

STC stands for "Sound Transmission Class", a measure of the extent to which sound is prevented from being transferred from one area to another. The higher the STC value, the less sound transferred from one area to another.

The STC scale is a logarithmic progression, meaning that a jump of 3 points in an STC rating equates to a doubling in a door's ability to prevent sound transmission. Acoustical doors are tested as an assembly comprised of the door, frame, hinges, and sound sealing system. See SDI 128 for more information.

The following chart illustrates the sound retardant performance associated with a range of STC values.

I have a specification that references an OITC value, is this the same as STC?

OITC stands for Outdoor-Indoor Transmission Class and is a measure of the ability of a building material or product’s ability to retard sound transmission. It is similar to STC, but it is not the same. As the name indicates, OITC is a measure of sound transmission from the external environment into the building envelope. Testing to establish an OITC value utilizes ASTM E-1332 Standard Classification for the Determination of Outdoor–Indoor Transmission Class. This standard uses a range of frequencies lower than the STC testing, to more closely replicate the sounds of rail and vehicular traffic.

Do you recommend a specific type of core for acoustical doors?

No, we recommend that the door selection be made on the basis of the STC value that has been specified. Multiple door core types may be capable of achieving the specified value, and can be documented by Certificates available from the manufacturer.

What level STC door do you suggest for an office building near an airport?

There are many variables that would determine the proper STC value, including the physical proximity of the office building to the airport. To ensure that the specification is properly developed, an acoustic consultant should be engaged to determine the Outdoor / Indoor Transmission Class (OITC) values.

STC Value Rating Description
50-60 Excellent Loud sounds heard faintly or not at all
40-50 Very Good Loud speech heard faintly, but not understood
35-40 Good Loud speech heard but hardly intelligible
30-35 Fair Loud speech understood fairly well
25-30 Poor Normal speech understood easily and distinctly
20-25 Very Poor Loud speech audible



I saw an image of a tornado resistant door with a vertical, top and bottom rods, but the bottom rod is no longer acceptable per ADA. Do tornado resistant doors have to have bottom rods?

Openings, specifically hardware, have been tested to comply with FEMA 320 and 361 that have the bottom bolt engaging into the hardware rabbet of the frame. This eliminates the bottom rod engaging into the floor and can be single acting function that would comply with ADA requirements.

Do SDI members offer products that meet FEMA 361 “Design and Construction Guidance for Community Safe Rooms”? What is approved?

Yes, SDI members offer door assemblies certified to FEMA 361. These openings must be tested and installed as a system including the hinge, door, frame, anchors and latching hardware. The architect or specifier should investigate the doors available that would satisfy the requirements for those openings.

We are building a safe room and the only tornado doors I find have a solid metal appearance. Are there tornado doors that are more aesthetically pleasing?

Some manufacturers have options for aesthetic finishes on these doors, but due to the severity of the testing requirements, almost all tornado shelter doors are very heavy-duty steel designs.

How do hurricane resistance ratings work? Are they pass/fail or are their levels like with fire ratings?

Hurricane resistance ratings are expressed in pounds-per-square-foot (PSF) values, and unlike fire ratings, there are no standard increments. In addition to the PSF values, they are either listed with or without “missile” impact ratings. In this instance, a missile refers to a 2 x 4 piece of lumber weighing 9 lbs. that is fired at the door at 35 miles-per-hour. A door assembly must withstand 3 impacts in order to have a missile impact listing. A typical door listing would be expressed as;

  • Design Pressure Rating +75/-65 PSF
  • Large Missile Impact - Yes
This listing indicates that the door assembly is rated for 75 PSF when loaded against the stops of the frame, and 65 PSF when loaded away from the stops of the frame

Where are higher hurricane design pressures and missile impact ratings required?

As a general rule, the closer a building is located to the coastline, the higher wind speeds it is expected to experience. The wind speed and debris impact zones for Florida are defined by a map that is found within the Florida Building Code.

Can oversized hollow metal doors meet Miami Dade or Florida Building Code requirements or do they need to be standard sized doors?

Many SDI manufacturers have single swing doors that are 4’0” x 8'0", and pairs that are 8'0" x 8'0", listed with the Florida Building Commission. Special sizes are also permitted by the code to be analyzed by a Registered Design Professional if they fall outside of existing approvals.

I have a specification that references ICC 500 and FEMA 361. Are these the same?

No, they are not the same, but they are both related to the construction of tornado shelters. FEMA 361“Design and Construction Guidance for Community Safe Rooms” is a guidance document, but FEMA does not regulate building construction. ICC 500 “Standard for the Design and Construction of Storm Shelters” is written by the International Code Council® and is referenced in the International Building Code in all Editions since 2009.


Blast Resistant

Can STC and blast resistant doors be fire rated? If so, can they be rated up to 3 hours?

The ability to fire rate STC and blast doors would be specific to each manufacture and it is recommended that you contact a quality manufacturer for their specific listing. Follow this link to a listing of SDI manufacturers to ensure you are supplied a top quality door.

What criteria should I look at when specifying a blast door?

To properly specify the desired blast resistance for a door assembly, the following information needs to be provided:

  • Peak blast pressure — such as 5 psi.
  • Either the blast duration (e.g., 60 milliseconds) or blast impulse (e.g., 150 psi-msec).
  • Direction of blast pressure loading (i.e., either seated or unseated).
  • Acceptable level of post blast event damage: typically limited to Response Categories I, II or III. In general terms, these ratings correspond to (I) no permanent damage, (II) permanent damage but the door remains operable, and (III) more severe damage with the door being inoperable after the blast.

Can blast resistant doors have veneers?

There is nothing in the standards that prohibit veneers, but they would have to be documented within the manufacturer’s certification to ensure that they pose no hazard to the building’s occupants.

Can blast resistant doors have glasslights?

Yes, the primary focus of the Department of Defense’s Unified Facilities Criteria is to protect against injury from architectural glazing, so many SDI member companies have certifications for doors with glasslights.

I have a specification that references “UFC 4-010-01 Minimum Antiterrorism Standards for Buildings”. What type of doors are required to meet this spec?

The Department of Defense created the referenced document to provide a minimum level of protection from blast forces from acts of terrorism. The door assemblies offered by SDI members are specifically engineered and certified for these applications, with a wide variety of blast resistant capabilities. In order to supply the correct door assemblies, you will need to know the PSI/MSEC value (pounds per square inch / millisecond). This is a value that has been calculated by a blast consultant, and would be expressed as a numerical value, i.e. 50 PSI/MSEC. The higher the value the more blast force the opening is designed to withstand. Matching that value to a manufacturer’s certification will allow you to select the correct door, frame, glazing and hardware configuration.

Note: There are many other blast resistant specifications in addition to UFC 4-010-01. Please also keep in mind that the version of the specification is critical, particularly with the UFC 4-010-01 specification.



How should I store steel doors and frames onsite?

All frames shall be stored under cover. Assembled frames shall be stored vertically. The units shall be placed on at least 4" (102 mm) high wood sills or in a manner that will prevent rust or damage, even if they are galvanized or primed. The use of non-vented plastic or canvas shelters that can create a humidity chamber shall be avoided.

Are knock down frames standard for all door openings?

The type of frame is up to the specifier’s discretion. To answer the question directly, knock down (K-D) frames are not standard for all openings, as many are specified as welded frames.

What is the definition to A, B, C, D, E and F cores?

Per SDI-100-1991 the cores were listed as follows:
A - Kraft Honeycomb
B - Polyurethane
C - Polystyrene
D - Unitized Steel Grid
E - Mineral Fiberboard
F - Vertical Steel Stiffeners

These are now archaic references and should be treated as such.

Our firm designs healthcare facilities, and uses AIA MasterSpec as a basis for our specifications. In the Steel Door Frame section, it calls for a "bituminous coating; cold applied asphalt mastic SSPC-Paint 12 compound for 15 mil thickness per coat." We have a subcontractor questioning the need for the bituminous coating. I seem to recall that the coating is primarily required at exterior door frames where moisture getting in behind the frame may be a problem. Can you shed any light on the reason for bituminous coating?

Some Architectural Specifications require steel frames to be back-coated with a "bituminous" product for corrosion protection and sound control. The term “bituminous” is defined as an asphalt or tar material obtained as a residue from heat refined petroleum. For years it was not recommended by the Steel Door Institute for frames to be factory back-coated due to issues with packaging, shipping and handling. This procedure was more effectively accomplished at the jobsite by the contractor or appropriate trade immediately prior to installation of the frame.

Modern materials now offer manufacturers the opportunity to back-coat frames with a more user and environmentally friendly product. Some of these coating s are VOC (Volatile Organic Compounds) free and do not present the handling issues common to bituminous products. Today, multiple SDI manufacturers offer the option of factory back-coated frames which utilize these newer materials.

In Appendix B of SDI-100, "G" type galvanizing is not recommended. Is it correct to infer that the "A" Types are recommended? I understand that A60 is the thickest option. In a marine climate, would you say painted A60 is our best bet?

SDI recommends type A, both A40 and A60, over type G due to the superior primer adhesion qualities. A60 is superior to A40 for rust inhibiting.

SDI Technical Data Series 100 has been republished as an ANSI Standard, A250.8.In addition, please see our Technical Data Series 112 "Zinc-Coated Standard Steel Doors and Frames." The difference in the A and G designations are explained. Table 1 of that document gives coding designations for A40, A60, G60 and G40.

What modifications am I eligible to make to the opening protective?

Modifications are only eligible when referenced and described in both the third-party certification and the manufacturer’s published instructions.


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