Kormit Roof Deck System- Kormit pars co.
Kormit Roof Deck System is a composite roof deck systems consist of a special type of Open Web Steel Joists and concrete. Kormit steel joists does not require any temporary shoring during construction . In this system, various types of filler blocks such as permanent or removable filler blocks are used in between the joists. This system with more than 40 years of experience has the Iranian Code and National Standard with patent number 22445 in the Patent Office of Iran.
This article covers the introduction, specification, design, manufacture and use of KORMIT Open Web Steel Joists floor system.
Kormit pars co.-Kormit Roof Deck System
Kormit Roof Deck System can be used as floor or deck in either Steel or Concrete structures.
This system is used with either Permanent filler Concrete blocks, hollow core burnt clay, removable steel or plastic formwork in between the joists.
The Kormit system employs special kind of open web steel joists consisting of a flat bar as the bottom cord, an angle as the top cord, a bended bar as the web and a set of reinforcements. Some of these Bottom and top cord shape and combination are shown below:
Filler between joists might be hollow burnt clay, concrete hollow blocks, polystyrene as permanent filler, plastic or steel removable formwork.
With 75 cm c/c (30 in.) of joist spacing, arranged to span in one direction with Concrete topping of 6 cm (approximate 2.5 in.).
In Kormit Roof Deck System , Joists are designed as simply self-supported at their both ends and with /or without shoring during the construction period.
Joists are designed in two stages. First stage where no temporary shoring is to be use, joists should be able to support its own weight, fresh concrete weight, filler weight and the weight of the work crews and the construction equipment. Otherwise these loads are almost carried by shoring, and not by joists.
In the second stage where concrete reaches up to 75% of its strength, it will act as a composite section to support any extra dead and live load imposed to it.
2.1 Steel joists:
ASTM A36/A36M-14 Standard Specification for Carbon Structural Steel
ASTM A242/242M-13, Standard Specification for High-Strength Low-Alloy Structural Steel
ASTM A529/A529M-14, Standard Specification for High-Strength Carbon-Manganese Steel of Structural Quality
ASTM A572/A572M-15, Standard Specification for High-Strength Low-Alloy Columbium-Vanadium Structural Steel
ASTM A588/A588M-15, Standard Specification for High-Strength Low-Alloy Structural Steel, up to 50 ksi [345 Minimum Yield Point, with Atmospheric Corrosion Resistance
ASTM A606/A606M-09a, Standard Specification for Steel, Sheet and Strip, High-Strength, Low-Alloy, Hot-Rolled and Cold-Rolled, with Improved Atmospheric Corrosion Resistance
Filler metals and fluxes shall conform to one of the following specifications of the American Welding Society:
- -AWS A5.25/A5.25M
- -AWS A5.1/A5.1M-2012, Specification for Carbon Steel Electrodes for Shielded Metal Arc -Welding
- -AWS A5.5/A5.5M:2006, Specification for Low-Alloy Steel Electrodes for Shielded Metal Arc Welding
- -AWS A5.28/A5.28M:2005, Specification for Low-Alloy Steel Electrodes and Rods for Gas Shielded Arc Welding
In order to prepare a flat area for concrete placement, a filler should be used in between the steel joists, in either Steel or Concrete Structure.
2.2. 1 Permanent filler blocks:
2.2.2 Removable filler blocks
Concrete and steel reinforcement should be designed according to ACI 318-14, Chapter 19&20 respectively.
The loads, nominal loads and load combinations shall be those stipulated by the applicable building code. In the absence of a building code, the loads, nominal loads and load combinations shall be those stipulated in Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE/SEI 7 chapter2), and/or SJI’s load combinations.
Per ASCE/SEI 7 (Chapter 2) and American National Standard SJI 200-2015 (section 4.1.2);
At a minimum, the required stress for LRFD designs shall be computed for the factored loads based on the factors and load combinations as follows; a and b:
a) Kormit Joists- Non-composite
1.2Dc + 1.6Lc (4.1-2)
Dc = construction dead load due to weight of the joist, the blocks, and the fresh concrete, lb/ft 2 (kPa)
Lc = construction live load due to the work crews and the construction equipment, lb/ft (kPa)
b) For Kormit composite Joists system after concrete curing:
1.2D + 1.6 (L, or Lr, or S, or R) (4.1-4)
Where: D = dead load due to the weight of the structural elements and the permanent features of the structure, lb/ft2 (kPa)
L = live load due to occupancy and movable equipment, lb/ft2 (kPa)
Lr = roof live load, lb/ft2 (kPa)
S = snow load, lb/ft2 (kPa)
R = load due to initial rainwater or ice exclusive of the ponding contribution, lb/ft2 (kPa)
3.2 DESIGN BASIS
Design basis should be according to applicable ANSI/AISC 360-16
- An American National Standard B3-1 to B3-13, whichever is applicable ANSI/AISC 360-16
- An American National Standard and SJI : ANSI /SJI 200 – 2015.
Compression members should be designed according to AISC 360-16 Sec.B4 & SJI Sec. 4.3
According to Sec.D1 of AISC360-16 There is no maximum slenderness limit for members in tension. For members designed on the basis of tension, the slenderness ratio,L/R, preferably should not exceed 300.
As components of joists will be embedded and engaged in cured concrete, control of slenderness of those members should be checked, under condition, according to SJI sec. 4.3 before concrete curing. With an exception of the bottom chord as it does not fully gets embedded in concrete. Although the lower cord is a tensile member, bottom cord gets attached to the concrete by diagonal members, therefore, its buckling length in weak direction is limited by their distance (distance between two adjacent welds of the web members on the bottom cord).
3.2.2 Concrete in Kormit Roof System:
3.2.3 Bridging (Ties)
The use of bridges on this roofing system is recommended. Especially where a plaster ceiling is attached or suspended. Although integration of the roof with the walls as presented by ASCE7-16 (the figure below) is an issue. The distance between these ties is a maximum of 8 feet. In a case where shoring is used during construction or integration of the roof with the walls is not the issue, it can be avoided with the approval of the Structural Design Engineer.
In the case of roof integrity, If space of these ties is further than 4’ o.c., then the wall must be designed for bending between ties, or sub diaphragms must be provided.( ASCE7-16)
Our Recommendation on bridges detailing shown in figures below:
The deflection due to the design live load shall not exceed the following:
Floors: 1/360 of span
Roofs: 1/360 of span where a plaster ceiling is attached or suspended, or 1/240 of span for all other cases. (sec. 5-10 American National Standard SJI 200 – 2015)
According to our experience in this roofing system we recommend Joists be cambered equal to 1/200 joist lengths.
Section 4-6 of ANSI SJI 200 – 2015 notes that joists shall be cambered. The approximate camber shall be based on the deflection associated with 100 percent of the non-composite factored dead load plus any additional loads defined by the specifying professional. In expels by SJI use this combination:
“Camber joist for: 100%XΔ non-Composite DL + 50%X Δ composite DL + 10% X Δ cmposite LL “
4.3 Floor Vibration: To prevent floors vibration, it is recommended to use Table 1 of ISIRI No.12977 for Minimum depth of roof (Joist concrete) / length of joists.
4.4 Special design tips from َACI 318-14:
-Nonprestressed one-way joist construction consists of a monolithic combination of regularly spaced ribs and a top slab designed to span in one direction. (22.214.171.124 ACI318-14)
-Width of ribs shall be at least 4 in. at any location along the depth. (126.96.36.199 ACI318-14)
– Overall depth of ribs shall not exceed 3.5 times the minimum width. (188.8.131.52 ACI318-14)
– Clear spacing between ribs shall not exceed 30 in. (184.108.40.206 ACI318-14)
– One-way joist construction not satisfying the limitations of 220.127.116.11 through 18.104.22.168 shall be designed as Slabs and beams. (22.214.171.124 ACI318-14)
– shall be permitted to be taken as 1.1 times the value calculated in 22.5. (126.96.36.199 ACI318-14) (22.5 ACI 318-14—One-way shear strength)
– For structural integrity, at least one bottom bar in each joist shall be continuous and shall be anchored to Develop fy at the face of supports. (188.8.131.52 ACI318-14)
– Reinforcement perpendicular to the ribs shall be provided in the slab as required for flexure, considering Load concentrations, and shall be at least that required for shrinkage and temperature in accordance with 24.4. (184.108.40.206 ACI318-14)
– One-way joist construction not satisfying the limitations of 220.127.116.11 through 18.104.22.168 shall be designed as Slabs and beams. (22.214.171.124 ACI318-14) 9.8.2
Joist systems with structural fillers (9.8.2 ACI318-14):
– If permanent burned clay or concrete tile fillers of material having a unit compressive strength at least equal to fc‘ are used, 126.96.36.199.1 and 188.8.131.52.2 shall apply. (9.82.1 ACI318-14)
– Slab thickness over fillers shall be at least the greater of one-twelfth the clear distance between ribs and 1.5 in. (184.108.40.206 ACI318-14)
– For calculation of shear and negative moment strength, it shall be permitted to include the vertical shells of fillers in contact with the ribs. Other portions of fillers shall not be included in strength calculations (220.127.116.11.2ACI318-14)
Joist systems with other fillers (9.8.3 ACI318-14)
– If fillers not complying with 18.104.22.168 or removable forms are used, slab thickness shall be at least the greater of one-twelfth the clear distance between ribs and 2 in. (22.214.171.124 ACI318-14)
4.5 Kormit steel joists-Temporary Shoring:
Kormit steel joists does not require any shoring during construction .However ,In a condition where temporary shorings are used during construction, especially in concrete structures, top cord’s lateral –torsional buckling will not be of concern. where no shoring is used during construction in steel structures, the top cord must be supported laterally. in this case, our recommendation is to weld or tie shrinkage reinforcement to the top cord of joists.
5.Recommended connection of kormit steel joists to structures:
The 1994 (Northridge) earthquake in Southern California showed structural design problems and led to changes in common ACI codes. Lack of structure integrity caused by weak connection of roofs (diaphragms) to boundary elements (boundary members) such as collectors and building bridges caused a series of breakdowns.
Lack of structure integrity caused by weak connection of roofs (diaphragms) to boundary elements & walls
In section 126.96.36.199 (roof with two way joists) and in section 188.8.131.52 (roof with one way joists), ACI 318-14 stipulates that the joists are subject to horizontal forces and should also be connected to the supports due earthquake. Therefore, the following connections are recommended for the Kormit roof deck system.
In concrete structures, it should be noted that the stirrup cover of concrete beam is not reliable to resist the horizontal and bearing forces. So joists should run inside the stirrup square. Otherwise, the joist shear force should be carried by shear bent-up bar, shear plate or angle. Similarly tension should be resisted by reinforcement.
A:Incondition where kormit joists is placed inside the stirrup:
B:In condition where kormit joists is not placed inside the stirrup:
C:In condition where kormit joists is not placed inside the stirrup:Flat
American Institute of Steel Construction (AISC) – ANSI/AISC 360-16
-An American National Standard.
-Building Code Requirements for Structural Concrete (ACI 318-14)
-Commentary on Building Code Requirements for Structural Concrete (ACI 318R-14)
-SJI : – ANSI /SJI 200 – 2015
-AWS :American Welding Society-ANSI
-ISIRI 12977: Open web joist for application in building floor-Specifiction and test methods.
-ASCE/SEI 7 –chapter2 – Minimum Design Loads for Buildings and Other Structures