Frequently Asked Questions
Hollow Core Slabs
1. How is differential camber removed from the plank?
Shimming the plank as required. Threaded Rods and Plates are also used to clamp the plank prior to grouting. A lightweight gypsum floor underlayment is often applied to the top of the plank to eliminate plank surface irregularities. All camber issues and concerns should be discussed with the plank supplier and erector.
2. Can Plank be used on ownership property and luxury apartments?
Yes, plank has been used on hundreds of high end properties. A high strength gypsum underlayment topping product is recommended. Thickness can vary depending on plank span length. A 2" concrete topping can be used for structural reasons or when there are large expanses of a brittle floor finish (i.e.: ceramic tile).
3. Is a topping needed for the UL fire rating (Design number K912) as shown on the 3 details? Or is there a separate fire rating for the no topping system?
The concrete topping is optional for the 2 hour unrestrained rating. A 1-1/8" concrete topping is required for the 3 hour restrained rating.
4. "Spancrete" (a hollow core manufacturer) is asking if every cell in their plank needs to be broken out to place grout.
Rebar is suggested to be #4 x 2'-0" long spaced at 24" 0/C max, and these reinforced cells may require block outs. As for intermediate cells, if they are not opened, grout has to be able to flow between the plank ends to create a monolithic system. Therefore, an inspection method is required to assure this. EOR is the sole determiner of how much reinforcing is required and how often.
5. Are weld plates needed in plank?
It depends upon the diaphragm loads and if they are needed for erection (see Girder-Slab typical details). Consult with your local hollow core supplier for availability of weld plates, alternative details may be suggested that will meet the engineer's requirements.
6. Your design-guide V2.0 says "Open the top of the slab core for proper grout placement and inspection" can you please describe the various methods for doing this and show some details?
One method is that the hollow core supplier provides these cut outs in the plant and then provides a core plug to stop the grout from flowing more than 12" into the cell.
Another method is the hollow core supplier uses a tool to put indentations in the top of the hollow core, these indentations are then broken out in the field and the debris left in the core.
Another method is the saw cutting of the top of the hollow core in the field and then removing the cut outs and placing them in the hollow core of the slab.
Another method is to install a 2" hole in the plant in the top of the plank above the cell. The plank core is plugged 12" back into the core with a soft insulation material. After the steel D-Beam® and the far side plank are erected a "center marked" rebar is placed from the far side of the D-Beam® thru its web into the void of the erected piece of plank. The rebar is left protruding about 1/2 inch on the far side of the D-Beam®. After the far side plank is installed the workers slide the rebar into the far side plank. The grout will flow thru the D-Beam® into the plank openings and flow up thru the 2" hole and assure proper grout penetration. This method will require additional on- site inspection.
Also, some suppliers cut the hollow-core precast slabs twice; First, to cut to length, then a second partial cut to remove the top part of the slab to allow enough room for the reinforcement bar and grout installation.
Note: If the EOR has specified the DB 8x42 the web opening is 1" deep. Consider this when you decide on the length of the precast slab cut out for rebar installation.
7. How many floors of steel and plank can be erected before we must grout?
This is a decision for the EOR and the erector. Each building is different. It involves stability; the amount of permanent bracing which is installed and the use of temporary bracing such as tie beams, cables, and or angles temporarily fastened to the columns and floor slabs. Some engineers and contractors will call for weld plates in the bottom of the plank, and field tack weld to the D-Beam. This technique is used so that in the event of freezing weather erection can continue while the grouting is postponed until weather conditions are more favorable.
8. Our job has weld plates specified. Should we weld or grout first?
If you have a choice, grout first then weld.
9. I am concerned about plank clearances and bearing around steel columns, has this ever been a problem?
Consult with your local plank supplier about his tolerances. Plank should be carefully detailed around steel columns to assure proper bearing and avoid interference. Clip angles are often added to columns for plank bearing. NOTE: Some engineers will stop steel bracing short of the "work point" to simplify plank & grout installation. A Plank detailer familiar with steel and plank projects is recommended.
10. What size mechanical holes or openings can I core in the hollow core slabs?
This is more a question for your precast supplier and we would advise you to check with them. We would suggest however, that no slots parallel to the D-Beam be any closer than 18" to the center line of the D-Beam.
11. I notice in your Design Guide you say that bay sizes of 20' x 28' are very efficient. The design tables from my local precast hollow core supplier say that 8" plank can span around 33'. Why are you suggesting 28' and they are saying 33'?
Please discuss vibration, deflection, and differential camber ramifications with your local hollow core supplier.
12. I am a structural engineer with previous experience using hollow-core planks on both masonry and steel structures. Typically the geometry of the building is rectangular with maybe a few skewed ends. How are curved and round areas best handled with Girder-Slab and hollow-core plank?
Various hollow core plank suppliers throughout the country will have different options. Generally speaking; for curves with radii of 10ft. or greater, the hollow-core plank can be factory cut in straight lines to approximate the curve. Some additional minor trimming in the field will also be required. For curves with smaller radii less than 10ft. it may be better to provide rectangular plank augmented with cast-in-place infill.
13. We are steel fabricators bidding our first Girder Slab project. A subcontractor is quoting us on the grouting work so we can offer a complete bid of; all the structural steel, the D Beams (which we intend to fabricate in our shop), hollow core precast slabs, all erected and grouted. Are there any “scope issues” we should watch out for with the grouting?
(We encourage steel fabricators to bid this “entire” scope. Our system is often an alternative to cast in place flat plate concrete. When builders compare the two, having an entire package price is beneficial.)
A certain amount of wood form work, bent plate, angles or light gauge pour stop is always required around columns, block outs or edges prior to grouting. Who is including this; you, your subcontractor or the builder? Filler material or mesh will be required for small openings; larger openings may require a temporary plywood form.
As you know the D Beam and hollow core plank must be reinforced and grouted together to develop the “composite action”. Be sure to discuss this and agree with the hollow core supplier how their plank will be prepared (opened) for reinforcing and grout. The FAQ’s on our website (these pages) discuss various methods.
Additional ask about “weep holes” in the plank. Are they required? You should also check to see what plank embeds are needed, curtain wall framing, wall panels etc. are these all shown? Will some have to be done in the field?
If your project is located in New England (or other Northern climates) you can minimize the cost/delays of “Winter Conditions”. If you anticipate erecting in the winter months ask the precaster to supply some weld plates for the bottom of the precast. Install the temporary and permanent bracing then weld the plank to the D Beam. This may be sufficient to stabilize the building and allow you to continue erecting. On a mild day you can go back and do the grouting. Each building is different so be sure to consult with the structural engineer.
For more information on this subject please read the article "Let's be Plank" from the September 2007 issue of Modern Steel Construction. You can find this article under the Published Articles section of our website.
14. Can any precast hollow core supplier be used on a Girder-Slab project?
Structurally, yes; after grouting is complete (SEE GIRDER-SLAB DESIGN GUIDE FOR SPECIFICS) the composite action will develop between the precast hollow core slabs and the D-Beam.
Note, if the architect is specifying UL K912, or ULC J500, for fire resistance, the precast hollow core suppliers should be sure they can comply, not all suppliers are specifically mentioned in these UL numbers.
15. I am an Architect considering your system of structural steel and hollow core slabs for a student housing project. Some of the rooms must accommodate showers for handicapped students. I want to avoid the cost of a 2" concrete topping, I prefer to specify a 1/2 or 3/4" light weight leveling material. Please describe how this can be done.
The use of a light weight material on top of the plank, ½” to ¾” will make it suitable for most all flooring applications; tile, carpet, parquet, etc. After the leveling coat is applied, the door sill is applied, and the shower base goes down. The difference in elevation from the top of the shower (tub) base to the sill can be 2” +/-. A Sloping mud bed is applied for the tile base, so the tile can remain ½” +/- below top of shower tub. We believe the ½” is code acceptable for Handicapped areas.
16. I have questions about the correct way to install various finished floor material over precast hollow core plank. Where can I get information?
All types of finished floors; carpet, tile, hardwood, "engineered" hardwood, can be installed over precast hollow core concrete slabs. Your architect will specify a proper leveling material over the precast hollow core slabs. Included on our "Design Team Resources" page we have provided a helpful publication called "Hollowcore 101 - Finished Floors" courtesy of Oldcastle Precast Building Systems.
17. I am an architect and have little experience using precast hollow core slabs. Can you give me some idea what I have to watch out for?
Great question. The Girder-Slab System works with all precast hollow core suppliers. Each supplier has minor but unique differences.
A great place to start would be the Pre and Post Installation FAQs supplied to us by Mack Industries.
1. Without using the tree column moment connection how far can the D-Beam® span?
Some Engineers have found that with camber and the heavier D-Beam®, a span up to 20' with no Tree Column connection can be achieved. The EOR is the sole determinant of the D-Beam® span.
2. I noticed your "tree" column connection, detail S16, is welded to the column flange. Can we design a bolted connection?
Yes, but consider that the bolts may interfere with the precast slabs.
3. How high of a building can be constructed using the Girder-Slab® System?
As high as any other steel building. The D-Beam® and the hollow core plank carry the gravity loads. The height of the building is a function of lateral resisting system
4. I have been on your website and looked thru your design guide, I cannot understand how the D-Beams® get manufactured in such a way that the openings in the web line up with the openings in the hollow core slabs?
The design guide and details are graphic illustrations; the web openings in the D-Beam® are not always going to line up with the openings in the hollow core slabs. When the web openings do not line up the rebar is placed on an angle or hooked to find its way into the openings of the hollow core.
Note: Design Guide dimension tables show the web opening sizes for various D-Beams®.
5. The D-Beam® top flange is specified in your Design Guide as a flat bar 3 x 1 or 3 x 1-1/2 (Grade 50). I would like to substitute flame cut plate, is this ok and what weld would be required when the plates butt together?
A572 gr 50 or A529 gr 50 flat bar are commonly rolled, you may also substitute rolled plate of equal or greater strength, if required splice welds must develop 100 kips for the 3 x 1 and 150 kips for the 3 x 1 1/2.
6. Can I splice the top flange flat bar?
Our system does not anticipate spliced bars just as any engineer's design would not anticipate spliced wide flanges. When a fabricator deviates from the norm, his procedure is controlled by AISC requirements and all of his deviations must be submitted to the SER for review.
7. When the D-Beam® is cut to size we may be cutting through the web opening, how does that affect the connection?
When using a parent beam W10 x 49, DB 8 x 35, DB 8 x 38, DB 8 x 40, DB 8 x 41, DB 8 x 43 and DB 8 x 46, there will be cases, where the cut will occur thru a web opening and the need to reinforce the web must be checked by the EOR.
When using other parent beams, although cutting lengths may fall at an opening in the web, there should be enough remaining web to transfer the shear to the end plate connection but this must be checked by the EOR.
When the EOR specifies the "TREE" Column connection he can adjust the length of the "tree" connections so that the connecting D-Beams® have at least 2" of solid web for the connections at each end. This should result in most of the projects D Beams being of identical lengths.
When checking these connections the EOR may determine the need for additional weld between the web and top flange. The fabricator can fill in the missing web with plate or a double plate can be used.
8. Can the Girder-Slab® System be used as a diaphragm in a high seismic area?
The Girder-Slab® System utilizes precast concrete slabs and steel components joined compositely by grout and steel reinforcing. Both precast and steel components have been used in seismic areas. The use of standard details allows the Girder-Slab® System to adapt to the high seismic environment. (Concrete topping and reinforcing may be required.)
9. Can a D-Beam be moment connected to a flange of the steel column?
NO. The D-Beam should not be used in a moment connection.
10. I am an engineer and want to specify an end plate connection to the D-Beam. I do not want to develop "fixity". What do you suggest?
This is intended to be an end plate shear connection, as shown in AISC Code on end plate shear connections.
In order to preclude Fixity, employ one or more of the following;
1) Use only enough bolts to transfer end reaction.
2) Use a relatively thin plate.
3) Limit the amount of welding between the top and bottom flanges and the end plate.
4) Consider using 2 bolts in the web and 2 bolts below the bottom flange, vertically spread about 6" apart.
11. I am a structural engineer and have a job where some floors will be topped with 2" of concrete and other floors will simply have a light weight leveling material over the plank. I am satisfied with my calculations and see that I can use the DB 8 x 42 in both conditions. I want the economy of specifying the same DB shape throughout the job. Your design guide suggests I use the DB 9s with 2" concrete topping. Must I use the DB 9?
No. As engineer of record it is your decision what is the most economical solution for your client. We offer the DB 9 for a topped job because it is deeper (stiffer). Notice; the DB 9 x 41 is one pound per foot lighter, and of course the bottom flange is minimal and the top flange is concealed in the topping. Remember, "When concrete topping is used, attain specified strength of grout prior to topping placement".
12. We are designing our third Girder-Slab job and this has never come up before. Can we use the D-Beams in braced frames?
Yes, we often see D-Beams used in braced frames. This is primarily done to facilitate mechanical systems. We suggest you look at using HSS (tube) sections as a single diagonal. The HSS is designed as both a tension and compression member, and the axial load on the D-Beam due to frame action is zero.
1. I am a structural engineer considering the Girder-Slab System for parking decks on my mixed use project. Can I do it?
Structurally, yes. The construction and subsequent maintenance must guarantee all of the embedded steel is protected from de-icing salt and corrosion. The engineer should advise the owner that regular maintenance will be required to guarantee the structural performance of the system and eliminate the possibility of any dripping of material through the slabs. There is no structural reason the Girder-Slab System cannot be used for parking decks. Check with the precast supplier.
2. What procedure do I use to calculate the horizontal shear transfer with the composite Girder-Slab System?
Chapter 4 of the "PCI Manual for the Design of Hollow Core Slabs", Second Edition is a standard reference that engineers use to calculate horizontal shear transfer elements, i.e. collectors, chords, and drag struts. When considering the connection at the D-Beam, typically, the hollow core shear strength is less than the grouted beam's shear strength.
3. What is the basis for the allowable stresses used in the design guide design examples?
The allowable stresses used are those required by the AISC in their specifications in the 9th Edition Code, Chapter I (Composite Construction), Page 57, Para 1-2
4. Can Girder-Slab be designed for point loads or non-uniform loading?
Since all section properties are available, the EOR is able to calculate stresses for all loading conditions.
5. Does Girder-Slab Technologies provide engineering assistance?
Yes. Please call for further information. Additionally, all standard details are provided on our website.
6. Under "Check Superimposed Compressive Stress on Concrete", section modulus of 513 is apparently (63.8 in3)8.04. If 63.8 is already the transformed section modulus from your table, why is it again multiplied by 8.04?
The 68.3 in3 is the section modulus of the transformed section represented in terms of steel. In order to check the compressive stress of the concrete, it is necessary to convert to a section modulus in terms of concrete. That is the reason for multiplying 63.8 by 8.04 (N) resulting in a section modulus of 513 in3
7. Reviewing the calculations in your Design Guide, I see no consideration for construction loads.
Loading of construction material is not permitted on non-grouted plank.
8. I am a structural engineer who uses RAM software. How do I design your D-Beams using this software?
For designing Girder-Slab in RAM engineers will let RAM select a standard WF shape but ignore the output. The engineer will then size the D-Beams manually or from the calculator spreadsheet located on our website under the chapter Design Team Resources. Since the D-Beams are not normally used in a braced frame this procedure works well. If the D-Beam were located in a braced frame, engineers would again model it as a WF beam to obtain the axial load in the beam due to the lateral load. Then the engineer will check the D-Beam, manually, for the combined axial and bending stresses.
9. Our structural engineering firm designs multi story residential projects throughout the country. Can you give me a brief summary of how competitive Girder-Slab will be compared to other alternatives?
Compared to low rise CMU or light gauge bearing wall Girder-Slab is not likely to be cost competitive. It will be very schedule and quality competitive. When comparing Girder-Slab to a CMU wall bearing system all structural CMU should be removed from the Girder-Slab design. The use of metal studs for exterior wall backup, interior walls, stairs/elevator shafts etc. (Depending on project specifics) may result in an overall savings to the project.
Compared to conventional structural steel and precast hollow core slabs, Girder-Slab is cost competitive because, the D-Beam, allows you to lower the floor to floor height of the building.
Compared to cast in place flat plate concrete Girder-Slab is very cost competitive. Girder-Slab is the only non proprietary, prefabricated, precast and steel system that offers significant savings while offering low floor to floor heights.
Quality steel fabricators can provide budgets and time tables from preliminary plans for your specific project so you can compare speed and cost economies. Every project is different. You must consider geographic location of your project, availability of skilled workers, building size and unique requirements. Girder-Slab is widely used in the Northeast, New England, and the Mid-west for high-rise (8 stories and above) or large square footage low rise buildings. Girder-Slab's lighter building weight can save foundation, column and lateral bracing costs, and our rapid assembly process can meet special schedule demands for occupancy.
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