CHAPTER 9 STAIR CASES 9.1 GENERAL FEATURES 9.2 TYPES OF STAIR CASES

Transcription

1 CHAPTER 9 STAIR CASES 9.1 GENERAL FEATURES Stair cases are provided for connecting successive floors. It is comprised with flights of steps with inter mediate landings which provides rest to the user and support for the flight. A passage is provided at the start of staircase then for the vertical rise a flight is provided with rise and tread. Rise provided in the steps is normally 6 inch which conforms with the comfort of the user. Tread provided is 9.5 inch which can be more if the number of user is more depending on the type of building. The width of the stair can be between 3.5ft to 5 ft depending on the use. Generally public buildings should be provided with larger width. Going is the horizontal projection of the inclined flight between the first and the last riser. A flight is generally consist of two landings with going in between of 10 to steps. Staircases can be designed in many forms as per the requirement of the user and the facility and space available in the construction. Design procedure of few types are discussed in this chapter. 9. TYPES OF STAIR CASES Stair cases can be of varying geometrical shapes and structural behavior. Some of the most common types of staircases are shown is subsequent discussion.

2 9..1 DOG LEGGED STAIR CASE LANDING PASSAGE Figure 9.1 : Dog legged stair case Most commonly used in buildings. It comprises with two flights and a landing or lobby in between. Normally the landing is provided at mid height. The landing acts as a support of the flight and landing is supported by beams or wall. 31

3 9.. OPEN WELL STAIR CASE Open well UP UP Figure 9. : Open well stair case 3

4 Generally adopted in public building where adequate space can be provided for staircases. It ahs quarter landings which provide more comfort to user. Moreover the open well provide adequate ventilation. The flights are consisted of lesser steps in comparison to dog legged staircases TREAD RISER STAIR CASE This type of staircase is normally used for aesthetic beautification. No support for landing is provided. The tread and riser is constructed as folded plates. The construction of this types of staircase is costly as reinforcement required is more. Tread Riser Figure 9.3 : Tread Riser Stair Case 33

5 9..4 CANTILEVER STAIR CASE Cantilever slab Rise Figure 9.4 : Cantilever Stair Case In this type of staircase cantilever horizontal tread are projected from a wall or an inclined beam. This type of staircase needs complicated formwork and normally used for aesthetic beautification. 34

6 9.3 DESIGN OF DOGLEGGED STAIR CASE Step 1: General arrangement LANDING PASSAGE Figure 9.5 : Dog legged stair case (general arrangement) 35

7 The figure above shows the plan of the stair hall. Let the rise be 6 inch and trade be 9.5 inch. The width of each flight is 3.5 inch. 10 Height of each flight 5 ft. 5 No of risers required 10 risers in each flight. 6 No of tread in each flight Space occupied be trades ft. Width of landing 4.5 ft. Width of passage 4.5 ft. Size of stair hall 7 ft 16.5 ft. Step : Design constants For steel f y And for concrete 40,000 psi f c 3000 psi Step 3: Determination of loading The landing slab acts together with the going as a single slab. The bearing of the slab into the wall may be considered 6.5 inch. 6.5 Then the effective span ft. l Considering one-way slab with both end continuous minimum thickness is. 8 l.17 So, t 5.inches 6 inches Self weight of the slab plf. 1 Tread Riser Tread Self weight of the steps plf. 36

8 Floor finish 0 plf. Total dead load Live load 100 plf. So, Design factored load plf. Step 4: Bending Moment Calculation Maximum Moment 1 M max wl lb-ft k-in. 8 8 Check for depth ρ ρ max b d M max f y φρf yb ρ f c d.9 inch And t inch (with 1 inch clear cover) t 3.9 inch < 6 inch (Ok) d available inch 37

9 Step 5: Reinforcement Calculation Distribution Bar. Minimum reinforcement is provided as temperature and shrinkage reinforcement. Temperature and shrinkage reinforcement, A st 0.00 b t in / ft # 3 bar can be used. The spacing will be, 0.11 S inch c/c. Longitudinal Steel. This is selected by trial. Trial No Trial-1 Trial- Assumed a (inch) a1.0 a0.6 M As f y Steel Area, As a a φf y d 0.85 f b c (inch) (inch ) A s 0.49 a A s 0.47 a Comments Not OK OK So, A s 0.47inch is provided. It can be furnished by using # 4 bar. 0. Spacing inch center to center

10 Step 6: Detailing The following points are to be remembered in detailing: The main reinforcement should be bent to follow the bottom profile of the stair. Near the landing the reinforcement should be taken straight up and then bent in the compression zone of landing. For tensile stress in the landing zone separate set of bars should be used as shown in the detailing. The length of each type of bar on either side of the crossing should be at least equal to ft inches. All the bars of the tensile reinforcement should be taken into the supports and anchorage and development length requirement must be fulfilled. Distribution bars should be used parallel to the width of the steps. LANDING # 3 9 inch c/c # 4 6 inch c/c PASSAGE # 3 9 inch c/c # 4 6 inch c/c Figure 9.6: Detailing of Stair Case 39

11 PASSAGE LANDING Figure 9.6.: Detailing of Stair Case (continued) 330

12 9.4 DESIGN OF OPEN WELL STAIR CASE 4.5 ft 7.67 ft 4.79 ft 4.5 ft 4.5 ft 4 ft ft Figure 9.7 : Plan View of Open Well Stair Case 331

13 Step 1: General Arrangements Width of steps 4.5 ft Height of first flight 4.5 ft Height of nd flight 3 ft Total height between the floors 15 ft First landing 4.5 ft nd landing 4.5 ft Riser raised in first flight 9 Riser 6 inch Tread 11.5 inch The size of stair hall ft ft Step : Design Constants Let for steel f y And for concrete f c psi 3000 psi Step 3: Design of First Flight The bearing of the landing slab into the wall is 6.5 inch. 6.5 Therefore the effective span ft l Considering one-way slab with both end continuous minimum thickness is 8 So, l.71 t 5.45inches 5. 5inches Self weight of the slab plf 1 tread riser tread Self weigh of the steps plf Floor finish0 plf Live load 100 plf [can be determined by table 1.1 of ACI code] 33

14 Total Dead load plf Design factored load plf Bending Moment Calculation: 1 M max wl lb-ft 84.0 k-in 8 8 Check for the depth 3 87 ρ max 0.75ρ b M max 84.0 d f y φρf yb ρ f c d.99 inch 3 inch Provide 1inch clear cover t3+14 inch<6 inch or 5.5 inch So, Design is OK. Available d inch 3 40 Reinforcement Calculation: Distribution Bar Only minimum reinforcement is provided as temperature and shrinkage reinforcement. Temperature and shrinkage reinforcement A st 0.00 bt in / ft So # 3 Bar can be used Spacing 10 inch c/c

15 Longitudinal bar This is selected through trials. Trial No Trial-1 Trial- Assumed a (inch) a1.0 a0.7 M As f y Steel Area, As a a φf y d 0.85 f b c (inch) (inch ) A s 0.58 a As 0.56 a Comments Not OK OK So, A s 0.56in can be provided. It can be furnished by using # 4 bar. 0. Spacing inch c/c Step 4: Design of Second Flight Let the bearing of the landing slab into the wall is 6.5 inch The effective span ft l Considering one way slab with both end continuous minimum thickness is 8 l So, t inch 6.5 Self weight of the slab plf 1 tread riser tread Self weight of the steps plf 334

16 Floor finish0 plf Total dead load Live load 100 plf So, designed factored load plf Bending Moment Calculation: 1 M max wl lb-ft 0.81 k-in. 8 8 Check for depth: 3 87 ρ max 0.75ρ b M max 0.81 d f y φρf yb ρ f c d inch Provide 1-inch clear cover t4+15 inch <6.5 inch So, design is Ok d available inch 3 40 Reinforcement Calculation: Distribution Bar Only minimum reinforcement is provided as temperature and shrinkage reinforcement. Temperature and shrinkage reinforcement A st in / ft If # 3 bar is used as distribution reinforcement 0.11 Spacing inch c/c

17 Longitudinal bar This is selected through trials. Trial No Trial-1 Trial- Assumed a (inch) a1.0 a0.85 M As f y Steel Area, As a a φf y d 0.85 f b c (inch) (inch ) A s 0.67 a As 0.66 a Comments Not OK OK So, A s 0.66in can be provided. It can be furnished by using # 4 bar. 0. Required spacing 4 inch c/c

18 Step 5: Detailing # 3 inch c/c # 4 inch c/c # 4 inch c\c 11.5 inch 6inch 4.5 ft 7.67 ft 4ft Figure 9.8: First Flight 337

19 # 4 4 inch c\c 6inch 11.5 inch # 4 4 inch c/c # 3 8 inch c/c 4.5 ft 7.67 ft 4.5 ft # 3 8 inch c/c Figure 9.9: Second Flight 338