Chapter #1 – General Layout of Construction Drawings
Reading a set of construction drawings requires that you learn basic drafting guidelines and the symbols that are used to convey meaning, much like a traffic sign tells you what to expect up ahead. The larger and more complex the project, the more drawings will be contained within the set. All the drawings together will provide for a fully constructed project, covering everything from the structure to the mechanical systems.
Reading Construction Drawings
In this first chapter we will cover how to interpret the meaning of the various lines, references, sections and the general layout of construction drawings.
Drawing List could include the following Trade Drawings;
Cover Sheet
Index
“G” General
“C” Civil
Roads
Parking
Site Utilities
Grading Plan
“L” Landscape
“A” – Architectural
“S” – Structural
“M” – Mechanical
HVAC Sheet Metal
HVAC Piping
“E” – Electrical
Power
Lighting
“P” – Plumbing
“FP” – Fire Protection
General Drawing Layout
Title Block
The main purpose of the title block is to identify the project and engineers involved in the design. Also included is a place for the individual responsible for the design and a date; a place to record the progression and date of any milestones such as a plan check set, addendums and their date. Title blocks can run horizontally along the bottom of the page or vertically along the right edge of the drawing. The title block should be visible when the drawings are rolled up, that way you can tell without having to unroll them which project it is related to.
Drawing Cover Layout
Revision Block will show any changes made to the drawings since they were issued. You might also find the original issue date and comments such as “Issued for Construction”. A revision number will be given that corresponds to clouded areas on the drawing where changes were made under this revision number.
Grading plan – shows the new and existing grading, the contour of the land.
Location map shows where the property is located on a Google map or other source.
Key Plan – this shows you where in the building the current drawing is located in reference to other sections of the building.
Site Plan (Plot Plan) – this shows how the building structure is situated on the plot of land it’s built upon. The site plan shows the contours, boundaries, roads, utilities, trees, structures, and other significant physical features on or near the construction site. It shows the locations of proposed structures in outline.
The site planshows the survey marks, including the bench mark (BM), with the elevations and the grading requirements. Surveyors use the plot plan shown below to set up the corners and perimeter of the building using batter boards and the line stakes. The plot plan furnishes the essential data for laying out the building.
The yellow lines highlighted in the image below shows the property line, while the blue highlighted area shows the building area as it’s located on the property.
There will be a north arrow somewhere on the drawing providing the orientation of the building to North, this is especially important when doing heating and cooling loads. Depending on which hemisphere you’re in, this will determine which exposure get shade and how the sun hits various aspects of the structure, effecting its heat gain or loss. The north arrow is shown in the upper right hand corner of the below drawing.
The site plan is drawn at a small scale so that everything fits on one page including the property boundaries. As you can see in the example site plan below, the driveway is shown entering off of the roadway. A dimension is shown indicating how far each corner of the building is in relationship to the property line or boundary.
Site / Plot Plan
Another item of interest on the site plan is the building finished floor elevations for the garage and floor plan. The length of the properties boundaries are also shown.
Electrical Drawings
You should be able to find all the HVAC equipment that requires electrical power on the electrical drawings. Shown below are two VAV’s (VAV 1-5 & VAV 1-6) that are provided with electrical power. You can read the description as circled in red as the electrical panel number and the circuit in that panel, such as; P5-16 (Panel 5, Circuit 16).This indicates that VAV 1-5 has electrical conduit and wire coming from panel 5 and circuit #16 in that panel.
Electrical Drawings
Structural Drawings
The structural engineer will design the support framing for the building in addition to miscellaneous equipment supports and concrete pads. They will do all of the calculations to ensure that the structure can support the various weights, wind loads, seismic activity and other stresses bearing on the building and its components. For the HVAC contractor this could include the design of special roof duct supports and equipment pads.
Below is a concrete pad designed by the structural engineer and shown as a detail on the structural drawings.
Equipment Pad
Column Lines
“Column lines help you find common areas on different floors.”
If you locate the intersection of column line “4” & “N” on the first floor, then you can find the same location directly above on the 2nd floor by finding the same column lines. This is helpful when trying to trace sheet metal, piping or utility risers that penetrate the floor.
Architectural, structural and all mechanical drawings should have the same column line references.
Column Lines General Drawing Layout
Dimension lines
Dimension lines are used to indicate the distance between two points.
Dimension Lines
Dimension lines can be shown many different ways, such as shown here with arrows as end points or hash marks.
Dimension Lines Arch Drawings
Hidden Lines
Hidden lines represent lines traveling under or on the inside of an object. These invisible portions of an item are often represented by dashed lines. Hidden Lines give an indication of items that are behind or below another item, as shown here highlighted in yellow. If you see dashed lines, those are part of a hidden item. The hidden items in this drawing show other sheet metal air ducts traveling under the duct above.
Hidden Lines – General Drawing layout
Elevation View
This is the view that shows the outside or inside of the building from a standing position, eyes forward.
Elevation views are used to determine sheet metal riser lengths.
Elevation View
Watch this Video for a quick overview of a complete set of Construction Drawings.
Chapter #9 – Sheet Metal Shop Fabrication Productivity
You must be able to determine the productivity rate of your shop as measured in pounds per hour. How many pounds per hour of galvanized rectangular fittings can your shop produce? How many pounds per hour of coil line ductwork can your shop produce? If you fabricate round ductwork and fittings the same question would apply.
You must have some measure of productivity in order to measure whether or not your shop is becoming more or less productive over time. This measure of productivity will allow you to focus on those areas of your shop that need improvement to be more productive.
Also, does your shop ship its duct KD (Knocked Down, Unassembled)? The main reason ductwork is shipped to the jobsite knocked down is to be able to nest (stack) more ducts onto the trucks, or because access into the building is limited
The fabrication shop is the best place to assemble the duct and fittings because it’s done under a controlled environment, although this may increase your trucking cost.
Coil Line Productivity
Sheet Metal Coil Lines usually feed other pieces of equipment in a fully automated shop such as a Beader, Shear, Snap Lock or Pittsburgh Seamer, TDC/TFC Flange Roll Former, Duct Liner with Glue and Pins, and lastly a Break that bends the finished duct into the size required.
The sheet metal gets cut from the coil and then goes through all the processes of beading and forming seams and joints before being finished with liner if required, then bent into shape (Either as one piece or as two halves that get assembled together).
Productivity will vary according to how much of the work is being done by the equipment and how much has to be done by hand. How many pounds per hour can your coil line produce lined and unlined ductwork? For an example maybe the shop did 20,000 lbs. this week and incurred 160 hours of labor. This would equate to 125 Lbs./Hr. (20,000 Lbs. / 160 hrs. = 125 Lbs./Hr.)
Rectangular Fittings
If your sheet metal fabrication shop has been fortunate enough to have a plasma cutter you can avoid the time consuming method of hand layouts and cutting. If you don’t own some form of automation cutter, your productivity will probably be at least half of that of those that do. Either way you should know how many pounds of fittings your shop can produce per hour.
Also, any short straight pieces that are not run on the coil line should be figured as a fitting. As an example you might have 10,000 Lbs of metal run through the plasma cutter and have spent 250 hours of labor. (10,000 Lbs. / 250 Hrs. = 40 lbs/hour).
Round Ductwork
If your sheet metal fabrication shop owns a spiral machine, then you will want to know the productivity of this machine. This can also be figured in pounds per hour, or feet per hour per size depending on how the shop prefers to inventory materials. As an example if you produced 20,000 Lbs in 100 hours, you would get a productivity of (20,000 Lbs. / 100 Hrs. = 200 Lbs./Hr.)
Round Fittings
If you cut and assemble your own round fittings it is imperative that you keep track of the cost to fabricate. How would you know if it is cheaper to fabricate then to purchase them? The only way to know for sure the true cost of fabricating is to measure the cost.
Also it can be very difficult to compete against a shop that uses Blue Label Workers; these are production, assembly line type of workers that have limited work task and a lower pay scale.
If a sheet metal fabricator is only in the business of fabrication and not contracting, they might have an advantage of being able to combine multiple orders from many contractors into to one fabrication run that helps optimize efficiency and increase productivity, making their prices highly competitive.
Remember that the examples here are just that, examples! Do not use these values for your adjustments. Each fabrication shop produces at various levels of productivity based on the machinery they own and the talent and experience of their shop labor. You must know your shops productivity factors in order to make the appropriate adjustments based on your unique circumstances.
Above is a rectangular duct & fitting report from a popular computer estimating program. This report shows the total full-length pieces which can be considered from your coil line, plus the total quantity of short pieces and fittings (#1). From this report we can analysis the fabrication shops productivity. If your computer program summarizes these numbers in the format you need, you’re a step ahead.
The first thing to do is to determine what type of productivity the program is coming up with. If you rely 100% on the database numbers that printout of your estimating program then you can skip this step, but you should still verify the numbers from time to time.
If you notice from the report above there is no way of using the numbers without doing some type of calculation to get it into the format that is comparable to your shop reports of Lbs./Hour. Your Estimating program or if by manual takeoff you should obtain the following information.
#1(Total Full Length Pieces, Total Short Pieces and Total Fittings)
#2(Quantity of Pieces). This will help you determine how many hours it takes to fabricate or install each piece.
#3(Total Linear Feet). Again we use these values to measure productivity.
#4(Total Pounds). You will use the total pounds for each category to determine how many pounds of metal can produced within an hour.
#5(% of Total Pounds). This is important to analyze the fitting ratio when thinking about field labor. Projects with more fittings will take longer and thereby have a slower productivity value.
#6(Total Material Cost). The sheet metal material cost that is in your computer program or that you calculate by hand.
#7(Total Fabrication Hours). These should be your unadjusted values from your computer estimating program or if you have already put the net values into your database for shop labor, then those values are represented here.
#8 (Total Field Hours). Just like the shop hours, these are based on your labor database values unadjusted or net. If they are net labor units, then you should still review them to ensure they still make sense.
Sheet Metal Fabrication Shop Analysis
Put the numbers from the report above into the Shop Fabrication Analysis portion of the Sheet Metal Labor Review form shown below and calculate the productivity of the report without adjustment. The spreadsheet will automatically calculate LBS/HOUR (#1) and HRS/PIECE (#2) after you enter the information from the estimating program. You’re fortunate if your program provides these metrics as this will help avoid these extra steps.
Sheet Metal Shop Productivity
After you complete filling out the form and doing the calculations compare it to the form shown below. You can see that for each item (Straight Duct, Short Pieces, Fittings) there is now a calculated productivity metric shown by item #1 in the chart below. Straight Duct has a productivity of 150 LBS/Hr, while Short Pieces 75 LBS/HR. and Fittings 60 LBS/Hr.
This is the value that has come from the computer estimating program, now its time to adjust these values (#2) to that which is derived from your companies historical data or performance metrics. This will provide you with an Additive or Deductive Hours (#3) which you will enter into your estimating spreadsheet.
Sheet Metal Shop Productivity Factors
As shown above, the coil line or straight duct pieces are reporting out at 150 Lbs./Hour or 0.28 Hrs./Piece. However you analyze your productivity, now is the time to adjust the factor to your shops productivity. The Computer Estimating program may be setup with SMACNA labor units and your fabrication shop may be doing better or worse than these values, so an adjustment will be required.
After you have entered the numbers from your computer estimating report, it’s time to decide what factors to apply for the shop adjustment. Let’s use the numbers in the examples shown above at the beginning of this chapter of 125 Lbs./Hour for straight pieces and 40 Lbs. / Hour for short pieces and fittings as shown below by item #1. After these new adjustment factors (#1) you will either have additional hours to add to your estimate or hours to deduct (#2).
Sheet Metal Shop Productivity Adjustments
As you can see we have adjusted the straight pieces to 125 Lbs. / Hour, and the short pieces and fittings to 40 Lbs. / Hour. This gives us a shop add of 95.22 hours, which equates to a 47% add. (95.22 / 202.99 = 47%)
Other things to consider when adjusting the shop are how many pounds are on this one project compared to your average job. Is this considered a smaller job than usual? Will it be penalized at fabrication time because the shop is slow? Will the foreman submit cut sheets one at a time or in complete batches to improve productivity? Some of these factors are out of your control, but it is good to be aware of those things that have an impact on productivity.
If your productivity numbers are as those shown in our example at the beginning of this chapter, your adjustment might look as shown below.
Shipping
How do you figure the cost of transporting the ductwork from the fabrication shop to the job site? Does your company use an independent trucking company, or do they own their own trucks, or do you purchase your duct FOB job site?
Shipping
If you fabricate your own ductwork and fittings you will have to determine the cost to ship it from the shop to the jobsite. A common way of figuring shipping is by determining the amount of pounds that can be shipped per truck load. If you know that you have 20,000 lbs of ductwork and that your truck can deliver 4,000 lbs per trip, the formula would look like this; (20,000 Lbs. / 4,000 Lbs. = 5 Truckloads.
The next thing to consider is what the cost to the company is for operating and maintaining the truck. This will include items such as fuel, lease payments, insurance, maintenance, all other burdens in addition to the labor for the truck driver, warehouse loading and delivery time.
The charge can be handled in many different ways, such as a burden rate added on top of the shop labor hourly rate, or by the hour for the labor and a fixed rate of recovery for the truck for each trip or miles driven.
For example you might figure using 12 hours per truckload at a rate of $50/hr. Based on our previous example of 5 truckloads, this would equal (5 Truckloads x 12 hours/Truckload = 60 hours) = (60 Hours x $50/Hr = $3,000). Or, you could use a combination of the two, where you separate the labor cost from the truck cost. Whichever method you use, be sure to consider all cost associated with shipping.
Remember to consider whether you are shipping your ductwork KD (Knocked Down) or if it is pre-assembled by the shop. You should be able to get a little more than twice the pounds per truckload in K.D. form.
Watch the below video to get a sense of the magnitude of the work required in a sheet metal shop. This shop by McCorvey Sheet Metal Works will give you an idea of what a large sheet metal fabrication shop entails. Most fabrication shops will not be this large, but this will give you an idea of what is capable on a larger scale and how productivity is also related to how well a shop is organized and professionally operated.
There are projects that will require large plenums or casings, such as in built up system where components of the air conditioning system are bought in separate pieces and then assembled. In order to build these larger sheet metal plenums you will need to use additional methods of construction than those for traditional duct fabrication. These larger sheet metal plenums are often built the size of a small or large room and house fans, coils, filter banks and dampers. These large plenums will have doors to provide access to the various sections.
Gas Station Panel – Built-up Plenum
The purpose is to provide an area for supply, return or exhaust air to accumulate before or after the fan section, so they will need to be built to the appropriate pressure class. In some cases walls and floors of the building will make up one or more of the sides of the plenum, requiring that the plenum be sealed tight at this connection. Any penetration of the casing or plenum will require proper sealing to prevent leakage or infiltration.
SMACNA states that all ductwork fabricated on the suction side of the fan are built to 2 in. wg (500 Pa) pressure classification, while the discharge matches the design pressure classification of the system. If the casing or plenum exceeds negative 3 in wg (750 Pa) then you may build according to SMACNA’s Rectangular Industrial Duct Construction Standards.
As the height or width (#1) of the Casing/Plenum increases, the thickness (#2) of the material, Steel Angles (#3) and Standing Seams (#4) will increase as shown in SMACNA Figure 9-1 below.
Built Up Casings
Construction methods include the use of Standing Seams, Gas Station Panels or TDC/TDF Flanges. As can be seen from the chart above, as the height increases so does the requirement for a thicker gage material, larger angle and standing seam. Openings in the plenum or casing will be framed with angle to provide reinforcement.
Gas Station Panel Roll Forming Machine
There are not many fabrication shops that own their own Gas Station Panel Roll Forming Machine because the cost of these machine are high compared to the volume of this type of sheet metal required by the average commercial construction company. Unless your company builds a lot of large built up systems, then chances are they won’t own one of these, but will use other methods that are less productive but yet provide a similar product.
The below chart shows what determines the requirements for building a large Plenum or Casing. The columns are as follows: Span (#1) defines the Panel Height of the casing, Panel Gage (#2) defines the thickness of the metal, Depth (#3) is the Panel Thickness (2”, 3” or 4”), Load Class (#4) is determined by the pressure class (Static Pressure wg.) of the system.
Maximum Gas Station Panel Size
For example in the chart above, if you had a Panel Span (#5) of 8 feet and a system Static Pressure (#6) of 4” wg., and you wanted to use a Panel Depth (#7) of 3” with a Panel Width (#8) of 16”, this will inform you to use a Panel Gage (#9) of 20 Ga. The above chart includes additional Panel Span Heights: 10’, 12’, 14’, 16’ and 18’, (See SMACNA Manual for Additional Chart Data)
Sheet Metal Fabrication shops that have a roll forming machine will want to make efficient use of the coil sizes in inventory without waste. The below image shows a panel that was made from a standard 24” coil without any waste. If you add up all the dimensions of the below panel you will come up with 24”. Although the panel width in the below example is only 15-1/2”, you would use the 16” panel width and 3” wide column for all your panel requirements.
Gas Station Panel Section
Many of these roll forming machines are used to fabricate metal roofing panels on large industrial projects.
The casing can be lined with insulation and a solid (Double Wall) or perforated sheet metal interior skin. There are vendors, who sell prefabricated insulated plenum wall panels, but they are usually more expansive then those that your shop can fabricate or that the shop you purchase your duct from can fabricate for you.
Estimating Built Up Casings / Plenums
In order to determine how much sheet metal is included in the design you will need to measure the linear feet of the Panel walls and ceilings across its Face Area. As shown below a typical Panel Width of 15-1/2” when stretched out or flatten, the actual width is 24”, which includes all the sides and parts of the metal that was rolled formed. The stretch-out of the panel above would look like the panel below. This would allow the fabrication shop to use a standard 24 inch wide panel or coil.
Gas Station Panel Stretch-out
Example:
20 linear feet of a 16” (15-1/2”) Panel Width would be equivalent to much more because an additional 8” or 50% of the metal is consumed in the roll forming process using a 24” sheet metal coil.
#1 Panel Height 8 Feet
#2 Casing Wall Width 25.83 Feet (310 Inches)
#3 Casing Wall Width 5 Feet
Gas Station Panel Estimate
A rough way to estimate how much panel material there is in the above casing walls is as follows;
Step #1 (Calculate Quantity of Panels in Casing Wall #2)
The first step is to determine how many Full Height Panels (#1) are required in Casing Wall Width #2 by turning the total footage into Inches and then dividing by the Panels Face Width.
Step #3 (Calculate the Total Square Feet of Material)
Sum the total quantity of Casing Panels.
20 Panels + 5 Panels = 25 Casing Panels
Figure the Total Stretch-out Square Footage based on Panel Face Width and Material Width used to create the panel. The width of the material is determined by the wall width (2”, 3” or 4”). In the example above we were using a 24” coil width with a 3” Casing Panel Width, so we will use 24” for a stretch-out material width. Remember that the visible Face Panel Width is only a portion of the total material that you see or use in creating a panel, the rest is used in making the roll formed panel width (2”, 3” or 4”).
Using Panel Height (8 Feet) we can now calculate the total Square Feet of the material required to make this plenum or casing.
25 Panels x 24” width/Panel = 600 Inches / (12 Inches/Foot) = 50 Feet total width of material stretched out.
50 Feet (Width) x 8 Feet (Panel Height) = 400 Ft2
Answer 400 Ft2 of Material
Step #4 (Determine Total Poundage of Material)
Using the Maximum Allowable Panel Width chart above, we can see that with a Panel Height of 8 feet (#5) and a width of 3” (#7), and a static pressure of 4” wg, we find that using a panel width of 16 inches (#8) we get the required sheet metal gage of 20 ga. Note that we are using a panel width of 15-1/2”, and not 16”. The chart allows any size smaller then that listed, but not one greater than the listed panel width. The reason we are using 15-1/2 inches is that allows for you to use a perfect 24” sheet metal coil are flat stock without any waste.
Maximum Gas Station Panel Gauge
Now that we know the Gage and the square footage of the material required to make the panels we can determine the total poundage.
400 Ft2 x 20 ga (1.656 Lbs/Ft2) = 400 x 1.656 = 662.4 Lbs
Galvanized Material Thickness
Whenever you have a bunch of calculations to perform its best to use a computer program or application for this like Microsoft’s Excel program.
There are additional materials when figuring out the cost of a Casing or Large Plenum, such as Angles, anchors, gaskets, sheet metal screws, Access Doors, etc . There are also other methods of fabricating casings, but these are the ones illustrated in the SMACNA manual.
Access Doors
With large built up HVAC systems there will be a need for access doors in the plenum walls to provide a means for maintenance staff to replace the filters, clean the coils or to replace fan motors. SMACNA prefers that you limit the amount of doors so as to maintain the integrity of the plenum.
This traverse duct connector is fabricated from the same piece of metal as the duct (#3), and has a gasket (#10) inserted between the two joints of duct or fitting and is locked in place by a cleat (#9). Each corner will be bolted together. Watch the videos below to get a better sense of how this joint is assembled.
TDC Joint
Ductmate Duct Connectors (Slip on Flange)
The Ductmate duct connector is a proprietary connector manufactured by Ductmate Industries. One of the differences between this joint and the TDC joint above is that this is a slip on flange (#11) that is not fabricated from the piece of ductwork (#3) but is attached (slipped on) and secured to the raw end of the duct or fitting. This joint also requires a gasket (#10) between the flanges and cleats (#9) to fasten the two joints together. Watch the video below to get a better understanding of this proprietary joint.
Slip On Flanges like Ductmate
Watch the following video for a better understanding of what Ductmate offers.
Duct Flange Rollformer Machine
This machine will make a slip-on flange to make duct joints. A fabrication shop that has this machine will be able to make their own flange instead of having to buy a proprietary flange like that manufactured by Ductmate.
Rollformer
The following video shows a demonstration of how the attached duct connector is assembled onto the end of a piece of ductwork.
Four Bolted Corner Flange
The pieces that make up the TDC or Ductmate joint is comprised of the Flange (#4) on the end of the duct (#5) or fitting, the Gasket (#1) that provides an air tight seal, the Clip or Cleat (#2) that locks the two pieces together and the corner pieces (#3) that bolt together at each corner.
Duct Joint Assembly
To ensure a proper sealing of flanged joints the use of gaskets with bolted corner pieces will make a tight fit that reduces the chances of leakage. The gasket is a continuous length of approved material with a minimum of 16 gauge corner pieces and 3/8” bolts. The clips are used to lock together the two adjoining pieces of duct and are a minimum of 6 inches in length.
In lieu of clips, screws can be installed at 6” maximum intervals starting at a maximum of 1” inch from the corners. Clips are to be spaced at a maximum of 15” inches on center for 3” static pressure and less, and at 12” centers for 4” static pressure and above.
Watch this video on how to install Cleats/Clips.
Cleat Tool
The sheet metal fabrication shop has several options when making a rectangular flanged connection. One is to use have the flange an integral piece make from the duct, or to add a proprietary flange (#1) on to the raw end of the duct such as the below from Hardcast, a Carlisle Company.
The flange (#1) has a piece for each side of the duct or fitting, and four corner pieces (#2). Cleats (#3) locks two adjoining sections of ductwork with each other or a fitting by using the Cleat Installing Tool (#4). Watch the video above to see how the cleat is attached.
Hardcast Joints
TDC Corner Pieces
The corner of each section of duct will require a corner piece that allows separate sections of duct to be bolted together at their corners. The machine in the video below can do this automatically, otherwise they will be installed manually.
Vertical Duct Seam Closing Machine
S & Drive Joint
Here is another method of attaching ductwork and fittings together using an Slip & Drive connector. This requires two (2) Slips and two (2) Drives for each connection.
S and Drive
#1 in the below image is the Drive portion that gets driven at the joint.
This joint can be used at any length when exposed to 2 inch static pressure and less, and up to 36 inches in length for 3 inch static pressure and less, or 30 inch maximum length for ducts exposed to 4 inches of static pressure. Not approved for any duct static pressure over 4 inches. In the image below you have the Drive Slip (#1) that gets driven down at the joint ends (#2) of the two pieces being attached together.
Drive Slip Joint
And the S-Slip (#3) shown below holds the two pieces of duct (#2) in place on the other ends of the duct, so that you end up with two Drive-Slips and two S-Slips.
S-Slip
There is another version of the S-Slip called a Hemmed S-Slip because as you can see circled in red below the Hemmed S-Slip (#4) has a hem at each end.
Hemmed S Slip Joint
Watch the video below to see how the Slip and Drive is attached to the end of a piece of ductwork.
Slip and Drive JointSlip and DriveSlip PlacementInstalling Drive
This next video shows how easy this joint really is. As you will see, the sheet metal fabrication shop worker attaches two “S” slips to the end of two sections of ducts before pushing them together, then hammering the drives on each side to lock the two pieces together.
Watch the video below to see how a manual Drive Cleat/Slip is made in the shop. There are automated drive cleat machines that would produce these faster.
Below is an automatic cleat bender that is powered by a pneumatic operator.
Standing Drive Slip
Standing Drive Slip is similar to S and Drive except for the additional material and strength gained by a perpendicular extension of the Drive joint. This joint can also be reinforced with a bar for additional strength. This joint can be used at any length when exposed to 2 inch static pressure and less, and up to 36 inches in length for 3 inch static pressure and less, or 30 inch maximum length for ducts exposed to 4 inches of static pressure. Not approved for any duct static pressure over 4 inches.
Standing Drive Slip
Reinforced S-Slip
This joint uses a reinforced drive slip (#1) by using a 16 gauge angle (#2) that stands 1 inch in height. The angle slips within the drive section and is fasten in place with screws. This reinforced S-Slip is good on ductwork fabricated up to a maximum of 2” static pressure. The two sections of adjoining ductwork is shown by reference in the image below as (#3).
S Slip Reinforced
Standing S
The Standing S joint is similar to the reinforced S-Slip, except that the standing portion (#4) is fabricated from the same metal piece as the S connector. Below you can see that the two adjoining duct sections (#3) shown in red are inserted into the “S” section.
Standing S Joint
Watch this video to see how a standing “S” lock is made.
Standing “S”
Companion Angles
This is another way to reinforce the strength of the joint by adding companion angles (#6) on each end of a duct or fitting with a gasket (#7) between them. The companion angles can be attached to the duct or fitting by rivets (#8), screws or by tack welding, while the companion angles are secured together with bolts.
Companion Flange
Welded Joints
Welded joints (#5) are attached by methods of welding various material types together. Most commonly welded in the HVAC industry would be Galvanized, Stainless Steel or Black Iron. This will necessitate having heavier gauge fittings and ductwork, as lighter gauges can’t handle the temperature of the welding process.
Black iron grease ductwork is a typically used for grease ducts. Two sections of ducts or a duct and a fitting are brought together and welded where their flanges meet. There are other methods of joining in welding, but the flanged joint is commonly used.
Welded Joints
Bell Type Joint
The Bell type joint is also approved for use by the IMC (International Mechanical Code). The choice to use a particular joint is based on your sheet metal shop fabrication standards within the code approved methods. (See chapter on Grease Exhaust Systems for additional information)
Grease Exhaust Duct – Bell Type Joint
Joint Length
The length of the duct will vary depending on which joint you use to fabricate your ductwork. As shown in the image below, the use of Ductmate (#1) a proprietary duct joint doesn’t require roll-forming as opposed to the other two joint types.
Remember Ductmate (#1) gets slipped onto the raw end of the duct, while the TDC/TDF (#3) flange is roll-formed from the same piece of sheet metal that the duct is made from. Since Slip & Drive (#2) and the TDC/TDF Flange (#3) are made from the same material as the duct, this cause the overall length to be shorter than one that made for the use of Ductmate.
Ductmate Joint Duct Length = 60”
Slip & Drive Joint Duct Length = 59”
TDC/TDF Joint Duct Length = 56”
Joint Length Differences
What this means is that for every 100 feet for each type of joint there will be varying amounts of duct sections as shown below. There will be a quantity of 20 Ductmate pieces, 21 sections of Slip & Drive, and 22 sections of TDC/TDF joints. This is a great difference, but we want to make sure you understand that the roll-formers use some of the end of the duct to make the joints for TDC/TDF and the Slip and Drive joints.
duct sections
Round Duct Joints
Flanged Round Duct Joints
There are various methods of attaching round duct and fittings together. Here is a proprietary system by Ductmate® called “Spiralmate”. The duct can come with the flanges already attached from the fabrication shop or can be field installed. Care must be taken not to damage the flange in transit or disturb the sealant that sits in the groove of the flange. If the sealant becomes damaged then more will need to be applied to ensure a proper seal when assembled onto the end of a duct or fitting.
Spiralmate
Here is another manufacture of a flanged round joint; the Spiral Pipe E-Z Flange System.
Spiral Pipe EZ Flange System
And with this next video you can see how the roll former turns the end of a round duct until it is 90 perpendicular to its length thereby preventing the flange from slipping off, and allowing the two mating pieces to be clamped together.
Vanstone Flanges
Sheet Metal Combination Rotary
Preparing sheet metal for a wired edge, turning a burr, beading, and crimping are probably the most difficult of sheet metal forming operations to perform. Crimping the ends of round duct and fittings reduces the circumferences enough to allow the end to slip into its opposing duct or fitting.
Sheet Metal Rotary
This combination rotary is used for beading and crimping which in larger fabrication shops are produced by separate pieces of equipment.
Most low pressure round fittings will either have a crimped end or be coupling sized as shown below.
Crimped Ends
Crimped ends reduce the overall diameter just slightly so that the fitting or duct can slip into the next piece.
Round Crimped Joint
Round Couplings
Couplings slip inside the ends of two round ducts to be joined together and then sheet metal screws are fastened to hold it in place. Couplings could be required at a certain size such as connections 18” and larger. The shaded area in the image below is the coupling which has a beaded center so that each section of round ductwork slips over the coupling the same distance.
round coupling
Slip/Coupling Joint
Spiral duct can slip over the end of a fitting that is made coupling size, so that crimping isn’t required. The spiral duct will slip over the fitting until it hits a beaded section preventing it from going any further. The fitting has a bead similar to the one shown at the center of the coupling above.
Welded Connections
The requirement for a welded duct connection can be found for kitchen grease exhaust systems or some industrial processes and laboratory exhaust systems. Some of the more commonly welded duct materials are galvanized steel, stainless steel and black iron. Joints are often butt welded or flanged.
Often welded kitchen exhaust systems will use stainless steel where exposed to view and black iron for the hidden duct to the grease exhaust fan. Grease exhaust ducts need cleanout doors every so often as required by code for access for cleaning. (See chapter on Grease Exhaust Systems)
DuraFlange This is a proprietary system that uses light weight flanges that are inserted into the ends of ducts and fittings in order to join two pieces together with a neoprene or butyl gasket between them. Screws are then placed every 6 inches around the perimeter to fasten the DuraFlange to the end of the duct or fitting. This is a three piece joint, with two flanges (#1 & #3) and a gasket (#2).
DuraFlange
Watch the video below to see how this flange works.
Dura-Flange Ring
Rectangular Duct reinforcement
Intermediate reinforcement goes between the joints to strengthen the section of duct of fitting. The construction standards, like those established by SMACNA will dictate where and when reinforcement is required. The reinforcement could be top and bottom or on all four sides.
Intermediate Duct Reinforcement 2-Sided
Also, reinforcement can be around all four sides as shown below.
Duct Reinforcement 4 sided
Tie Rods
Another method of reinforcing ductwork and fittings is with the use of Tie-Rods. Reinforcement can be at the joint (JTR) or mid-way (MPT) between the joints. You can see by looking at the image below that you can have Joint Tie Rods (#1) or Mid Panel Tie Rods (#2) for additional strengthening of the duct section. The Tie-Rods are shown in yellow, with one at each end (#1) of the duct section and two mid-way (#2) between the joint ends.
EMT or solid rods are used for Tie-rods. Steel rods in sizes 1/4” & 5/16” are used or 1/2” to 1-1/4” EMT based on static pressure and distance to Mid Panel Tie Rod from joint. If the steel rod exceeds 3 feet in length, then a 3/8” steel rod must be used for added stability and strength.
Reinforcement Tie Rods
Mid Panel Tie Rods
The Static Press (#1), Sheet Metal Gauge (#2) and Mid Panel Tie Rod Distance (#4) will effect the Maximum Duct Width (#3) that can use a single toe rod for reinforcement when using a Mid Panel Tie Rod distance of 2-1/2’ (#4). As the table shows with the Red Arrow (#5) as the gauge of the duct gets thinner the maximum duct width (#3) decreases, and as the Static Pressure (#1) increases, the Maximum Duct Width (#3) decreases as shown by the red arrow (#6) although not as dramatically as with the reduction in sheet thickness.
Mid Panel Tie Rods Reinforcement
Relationship Between Width, Gauge and Reinforcement
Duct Width (#1), Thickness (gauge)(#2), Reinforcement Spacing (#3) and Reinforcement size (#4) all relate to each other and changing one usually affects the others. When you increase the Duct Width (#1) this can increase the Sheet Thickness (#2), Reinforcement Spacing (#3) & Reinforcement Sizing (#4). Likewise when you change anyone of the other items it can have an inverse relationship to the others, such as if you increase the reinforcement spacing (#3) you might be able to reduce the Duct Thickness (#2).
Duct Reinforcement Factors
Just remember that these factors are all related. Your Sheet Metal Fabricator will usually have a set of Duct Construction Standards that they use in most situation and only make adjustments in special situations.
Unreinforced Ductwork
Ductwork can be built without any additional reinforcement if it remains within SMACNA’s Table 2-47 “Unreinforced Duct (Wall Thickness)”. As can be seen in the image below, as the pressure class increases (#2), so does the thickness or gauge (#3) of the duct. The same applies to the dimension of the duct, so that as the duct gets larger, the thickness of the metal increases. In summary, if the pressure or the duct dimension increases, then the thickness of the material will also increase according to the below chart.
Unreinforced Ducts
Aluminum and Reinforcement
Aluminum doesn’t have the same strength and rigidity as Galvanized steel for the same gauge or thickness. Any construction using aluminum would require an increase in the thickness of the metal and reinforcement in order to match the equivalent in strength and rigidity of that of galvanized steel. For example a 1” x 1” x 16ga Steel Angle would require a 1-1/4” x 1-1/4” x 1/8” aluminum angle to be similar in strength and rigidity (see reinforcement table below). Aluminum is 6061-T Strength.
Galv Aluminum Duct Reinforcement
Round Duct Reinforcement
Just like rectangular duct, round duct will require reinforcement under certain conditions. As can be seen in SMACNA Table 3-10 below, the requirements for stiffeners is based on duct size (#1) and stiffener spacing (#2). This table is for 2” Negative Pressure Spiral duct. The process would be that you would look up the size (#1) of the spiral duct required per the projects engineered drawings and then decide which stiffener spacing (#2) the duct will be fabricated at. This will determine the required gage or thickness of the metal and the reinforcement or stiffener type.
As example if we look at 54” spiral duct as shown on row #3 we can see our options are as follows starting with the Unstiffened Column #4. Here are your options;
54” Spiral
Unstiffened Gage = 16 gage, Stiffener = NR (Not Required)
As you can see from the example above, as you increase the stiffener spacing (#2), you can reduce the thickness of the material, this is because the stiffener adds strength and rigidity to the duct allowing for a thinner material. Using an Unreinforced duct requires 16 gage material, while installing a stiffener every 5 feet allows you to use a much thinner and cheaper 28 gage duct.
The fabrication shop has to determine what their standard fabrication preference is and the cost for the various options based on stiffener spacing.
