BIM-Enabled Coordination and 5D Cost Estimation of Alternative Walling Systems for a Mixed-Use Building in Yaoundé, Cameroon

Résumé

Cost overruns remain endemic in construction, and unreliable quantities are among their most persistent causes. This paper reports a model-based estimating workflow applied to a real, not-yet-started four-storey (R+3) mixed-use building with an accessible roof terrace at Damas, Yaoundé, Cameroon. Four discipline models covering architecture, structure, electrical services and plumbing were authored in Autodesk Revit, verified against the Eurocodes in Robot Structural Analysis, federated and clash-tested in Navisworks Manage, and exported in IFC 4 to CostX for automated quantity take-off and bill production. Three walling variants sharing an identical reinforced-concrete frame were then generated by wall-type substitution: hollow sandcrete block (V1), stabilised interlocking compressed earth block (V2) and fired clay brick (V3). Coordination surfaced 233 interdisciplinary clashes, 44.6 % of them at the architecture–structure interface, and four iterations were needed to clear the federated model. Model-based quantities agreed with an independent manual take-off to within +1.8 % to +6.1 % on concrete, formwork, masonry and floor finishes, while take-off effort fell from 41 h to 6 h, a reduction of 85.4 %. The comparison also exposed a unit error in the manual reinforcement take-off, which returned 226 kg/m³ against 116 kg/m³ from the model and would have inflated the bill by about 29.0 million FCFA, or 11.2 % of the project cost. On the 5D bills, V2 returned the lowest total cost at 245,817,921 FCFA (157,576 FCFA/m²), 5.35 % below V1, together with 59 days (13.8 %) saved on the critical path, whereas V3 cost 0.47 % more than V1. Most of the V2 advantage arose not from the price of the block but from the elimination of rendering, a saving the model quantified automatically. Ten of the sixteen work packages were identical across the three variants and accounted for 57.4 % of direct cost, which bounds the achievable saving. The study demonstrates a reproducible 5D route to design-stage cost control in a low-maturity BIM market and shows that walling choices should be appraised on whole-project cost rather than on unit material price.

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