Chapter 8 (cont..3) - Straight bars for one way continuous slabs

In the previous section we completed the arrangement of the required reinforcement using bent-up bars. In this section we will see how the same required reinforcement can be achieved by using 'straight bars'.

We have to begin our work from the point where we obtained table 8.3. This table gives us the spacing required at various points. When we used bent-up bars, we saw that the spacing in one span or support will have 'influence' on the adjacent span or support. We were able to give only two spacing: 140mm at all spans and interior supports, and 280 mm at the end supports. 


But while using straight bars, there is no such influence. This is because the bottom bars in the spans will not become top bars at supports. So we are free to give any convenient spacing at different points. The only points to note is that the spacing that we provide should be a multiple of 5 or 10mm, and it should be less than the required spacing. Based on this we will prepare the final table which will show the actual spacing given at various points. This is given as table 8.5 below:

Table 8.5

	BM	d req.	Ast req.	S req.	s pr.	Ast pr.
Supp. A			252.91	310.54	200	392.70
Span AB	28.60	90.76	505.82	155.27	150	523.60
Supp. B	-29.76	92.59	527.55	148.88	140	560.71
Span BC	17.75	71.79	309.32	253.78	200	392.70
Supp. C	-27.61	89.53	490.47	160.05	150	523.60
Span CD	24.92	85.06	440.32	178.28	170	462.00
Supp. D			220.16	356.56	200	392.70

The above table gives the final spacing that we intend to give while using 'straight bars'. Note that this table corresponds to the table 8.4 which we prepared for bent-up bars. In span BC, we could have given a spacing of 250 mm (because the required spacing is 253.75). But it is decided to give a spacing of 200 mm. This is based on a common design practice of not giving a spacing greater than 200 mm. Now we will see how the bars are arranged in these spacing using straight bars. The following figs. gives the details.

Fig.8.13
Midspan AB and support B

The above fig. shows all the bars required for the following purposes:
• Resisting the sagging moment at midspan AB
• Resisting the hogging moment at support B
• Resisting the hogging moment due to possible partial fixity at support A


Let us first see the sagging moment at midspan AB. For this, two types of bars are shown in the fig.: a' and b'. Among these, a' are short bars. They do not reach the supports. b' are longer bars. They extend into both the supports A and B. Both these bars should be provided together and alternately. They will work together to resist the sagging moment at midspan AB. This is shown in the plan view given below:

Fig.8.14
Plan view

From the plan view it is clear that, the distance between any two adjacent a' type bars will be equal. If we denote this distance as 2s₁, then the distance between any two b' type bars will also be the same 2s₁. So the 'net' distance between any two adjacent bars will be equal to s₁. From table 8.5 above, it can be seen that we have given a value of 150 mm for s₁. This is less than the required 155.27 mm.

Now we will see the hogging moment at support. For this, two types of bars are shown in the sectional view in the previous fig.8.13: b'' and c''. Among these two types, c'' are longer. Both these bars should be provided together and alternately. They will work together to resist the hogging moment at support B. This is shown in the plan view given below:

Fig.8.15
Plan view at support B

From the plan view it is clear that, the distance between any two adjacent b'' type bars will be equal. If we denote this distance as 2s₂, then the distance between any two c'' type bars will also be the same 2s₂. So the 'net' distance between any two adjacent bars will be equal to s₂. From table, it can be seen that we have given a value of 140 mm for s₂. This is less than the required 148.88 mm.

Next we will see the details at midspan BC. The arrangement is similar to midspan AB. The fig.8.16 below shows the sectional elevation:

Fig.8.16
Details at midspan BC

Two types of bars are shown in the fig.: c' and d'. Among these, c' are short bars. They do not reach the supports. d' are longer bars. They extend into both the supports B and C. Both these bars should be provided together and alternately. They will work together to resist the sagging moment at midspan BC.

So we have seen the following:
• A typical end support (support A)
• A typical end span (span AB)
• A typical intermediate support (support B)
• A typical intermediate span (span BC)

Based on this we can give a satisfactory arrangement of straight bars for a continuous one way slab with any number of spans. The complete arrangement for the whole of our slab ABCD is given below:

Fig.8.17
Sectional Elevation

So we have completed the arrangement using straight bars. In the next section, we will see the distribution bars and other details.

PREVIOUS

NEXT

                                                         

            Copyright ©2015 limitstatelessons.blogspot.com - All Rights Reserved