K Design Graphs and Charts for Raft Foundations Spanning Local Depressions The reader is advised to read the text in Chapter 13 before using these charts. The charts and figures are repeated here for quick reference and the following is an aide-mémoire for the experienced user of the procedure. (1) Select the required design span of the depression based on the engineer s experience and using Table K.1 and/or Fig. K.1 as a guide. (2) If there is no bottom reinforcement in the slab, use the chart on the right of Fig. K.1 to select the minimum effective depth required. (3) For each loading type on the area under consideration, calculate the total factored load on the design span from Table K.2 and multiply each by the appropriate moment factor K m which is also obtained from Table K.2. (4) Sum the results from (3) above to give (T u K m ) and use Fig. K.2 for slabs with top reinforcement only and Fig. K.3 for slabs with top and bottom reinforcement to determine the area of reinforcement required for the selected effective depth. Note that the area of reinforcement is required in both directions (i.e. a square mesh is needed). (5) If heavy point or line loads are present, a shear capacity check should also be undertaken. (6) A similar design process is adopted for designing raft beams, using Table K.3 and Figs K.4 and K.5.
35 Appendices slabs with top reinforcement only 4. 4. minimum allowable effective depth (mm) 225 assumed diameter of depression (m) 3. 2. soil classification D C B A 3. 2. design span of depression L (m) 2 175 15 125 1 75.5 depth of hardcore/compacted material h fill (m) Fig. K.1 Design span for local depression. For soil classifications see below. Table K.1 Design diameter for local depression Soil classification A Consistent firm sub-soil B Consistent soil type but variable density, i.e. loose to firm C Variable soil type but firm D Variable soil type and variable density Soil type One only of: clay, sand, gravel, sandy clay, clayey sand One only of: clay, sand, gravel, sandy clay, clayey sand Two or more of: clay, sand, gravel, sandy clay, clayey sand Two or more of: clay, sand, gravel, sandy clay, clayey sand Assumed diameter of depression (m) 1.5 1.5 2. 2. 2.5 2.5 3.5
Appendices 351 Table K.2 Load types and moment factors for raft slabs spanning over a depression of diameter L Top reinforcement only Top and bottom reinforcement K m T1 TB1 Uniformly distributed load f S (kn/m 2 ) F S = f S (πl 2 /4) F S = f S (πl 2 /4) T2 TB2 Parallel line 1.5 T3 TB3 Lateral line 1.5 T4 TB4 2 way line 1.5 P = 2PL P = 2PL T5 TB5 Point load P (kn) 2.
352 Appendices 5 d = 225 d = 2 45 d = 175 4 35 d = 15 3 d = 125 (K m T u ) 25 d = 1 2 15 d = 75 1 5 5 1 15 2 25 3 35 4 CONCRETE GRADE 35 REINFORCEMENT GRADE 46 AREA OF REINFORCEMENT EACH WAY = A S (mm 2 /m) Fig. K.2 Design chart for slabs with top reinforcement only.
Appendices 353 1 d = 225 d = 2 9 d = 175 8 7 d = 15 6 d = 125 (K m T u ) 5 d = 1 4 d = 75 3 2 1 5 1 15 2 25 3 35 4 CONCRETE GRADE 35 REINFORCEMENT GRADE 46 AREA OF REINFORCEMENT EACH WAY PER FACE = A S (mm 2 /m) Fig. K.3 Design chart for slabs with top and bottom reinforcement.
354 Appendices Table K.3 Load types and moment factors for raft beams Internal beam Edge beam Corner beam K m I1 E1 C1 Uniformly distributed load f S (kn/m 2 ) L 2.5 F S = f S (πl 2 /4) F S = f S (πl 2 /8) F S = f S (.64L 2 ) I2 E2 C2 Parallel line / 2 I3 E3 C3 Lateral line /2 / 2 I4 E4 C4 2 way line L 2 P = 2PL P = 3PL /2 P = 2PL / 2 I5 E5 C5 Point load P (kn) 2.
Appendices 355 5 45 (K m T u ) L/b (kn) 4 35 3 25 2 d = 55 d = 5 d = 45 d = 4 d = 35 d = 3 15 1 5 4 8 12 16 2 AREA OF REINFORCEMENT PER FACE/m WIDTH = A s /b (mm 2 /m) CONCRETE GRADE 35 REINFORCEMENT GRADE 46 Fig. K.4 Design chart for internal and edge beams.
356 Appendices (K m T u )L/b (kn) 25 225 2 175 15 125 1 d = 55 d = 5 d = 45 d = 4 d = 35 d = 3 75 5 25 4 8 12 16 2 AREA OF REINFORCEMENT IN TOP FACE/m WIDTH = A s /b (mm 2 /m) CONCRETE GRADE 35 REINFORCEMENT GRADE 46 Fig. K.5 Design chart for corner beams.